A Guide to Grapevine Abnormalities in South Africa - P.G. GOUSSARD

A Guide to Grapevine Abnormalities in South Africa

PG GOUSSARD

FOREWORD

G rapevine abnormalities refer to the accession(s) of problematic situations, as induced over a broad spectrum by damaging interventions of either diseases and pests, environmental conditions, or unsound/incorrect cultivation practices, within affected vines. In this richly illustrated and wide-ranging publication the occurrence of specific symptoms are visually displayed, described and explained. This publication should be of huge benefit to learners, producers and viticulturists in general. Prof Pieter Goussard’s contribution as sole author of this publication is greatly appreciated. With his background in research and teaching, as well as his practical knowledge of viticulture, prof Goussard is the ideal author of such a publication. He obtained all his academic qualifications, including his PhD (Agric) degree, in 1984, from Stellenbosch University. He was appointed as a lecturer at the same institution in 1971 and later became a professor. During this period, he was also chairperson of the Department of Viticulture and Oenology for 14 years. He is a recognised researcher, who has published more than 100 articles in scientific and popular publications, has presented 25 papers and posters at national and international conferences and has received five awards, including the South African Society for Enology and Viticulture (SASEV) prize for research and innovation. He has served as a board member of the South African Society for Enology and Viticulture for 20 years and was editor of the South African Journal of Enology and Viticulture for 15 years. All of this highlights the exceptional service he has provided to the South African wine industry.

JH BOOYSEN Technical Advisor: Winetech

A Guide to Grapevine Abnormalities in South Africa • 1

MEDIA

First published in 2015 by Wineland Media in association with Winetech, Villa Academy and Stellenbosch University.

COPYRIGHT © WineLand Media/Winetech AUTHOR Prof PG Goussard ART DIRECTION AND DTP LAYOUT VR Graphics PRINTING AND BINDING ABC Blackshaws

All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, whether electronic, mechanical, photocopying, recording or as any information storage and retrieval system, without the prior written permission of the copyright holders. This book – also available in Afrikaans, entitled ’n Gids tot Wingerdabnormaliteite in Suid-Afrika – is not only for the vineyard manager, but for all those who love the vine, its culture and the noble fruit and drink it produces.

ISBN 978-0-620-65725-9

WINELAND MEDIA ) 021 276 0458 7 086 611 7877  PO Box 1411, Southern Paarl 7624 8 in@wineland.co.za

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INTRODUCTION

O ptimal in-vineyard performance of individual vines, associated with the sustainable and profitable cultivation thereof, shows a close kinship with the health of the plant material as well as the continuous and correct application of practices which best meet the cultivation preferences and needs of the relevant cultivars in different regions. The occurrence and further development of any abnormality(ies) – whether associated with either biotic (vineyard diseases and pests) or abiotic (environmental condition) factors, or with injudicious/faulty establishment, vine development and continuous cultivation practices – will with hardly any exception contribute to a negative influence on the productive life expectancy of vines over the short or longer term. In this book, vine abnormalities – those based on the occurrence and/or causes thereof in terms of specific problem situations – are divided into the following categories, namely: (i) plant material problems; (ii) fungal diseases; (iii) bacterial diseases; (iv) virus and virus-like diseases; (v) grapevine pests; (vi) abiotic abnormalities; (vii) genetic deviations; (viii) deficiencies and toxicity of nutritional elements; (ix) herbicide phytotoxicity; (x) injudicious/faulty cultivation practices/actions and (xi) varia. With the abovementioned as background, concise descriptions regarding the visual appearance of specific phenomena are based chiefly on related literature with the emphasis on textbooks (Perold, 1927; Du Plessis, 1947; Winkler et al., 1974; Bovey et al., 1980; De Klerk, 1981; Marais, 1981; Saayman, 1981; Smit, 1981; Galet, 1978, 1995; Ferreira & Venter, 1996; Bailey et al., 1998). In addition to this, abnormality interventions are illustrated with photographic material – to provide cryptic but basic captions with individual examples as elucidation. Although the focus is chiefly on the appearance and description of the most typical abnormalities, due to the extensive diversity thereof, a concerted effort was also made to include less known phenomena which occur less commonly, where applicable. As this presentation mainly concentrates on the visual appearance/observations of specific abnormalities, descriptions (except in unusual and applicable cases) of: (i) scientific designations of causal organisms and their life cycles: (ii) specific disease and pest control programmes and (iii) the mechanisms of operation of control, prevention and eradication substances, are not included. In addition it is of cardinal importance that, if there is any doubt about the identification of a problem on the grounds of visual appearance, an expert should be consulted in such cases for confirmation. The overarching objective is based on correct identification of the problem, to allow stakeholders in the industry to act timeously and effectively in terms of control and/or corrective strategies.

A Guide to Grapevine Abnormalities in South Africa • 3

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Villa Academy educates on crop protection best practices

C rop protection, like any other industry, needs smart people. Companies tuned in to their do. Education is just as valued as practical experience as a way to achieve the professional success and status which workers strive to achieve. The Villa Academy identified a need to bring young people into the industry, and obviously bringing in a new generation of salespeople necessitates someone to train them. Previously there has been a lot of discussion in the industry about how it needs to be done, but not much action – it got to a point where Villa Crop Protection decided it should take the lead. Founded in 2011, the Villa Academy occupies two campuses in Cape Town and Johannesburg. A mix of entry-level and senior-level professionals benefit not only from attending classes but also from taking the opportunity to network. A blend of academic and hands-on experience is a theme underlying the Villa Academy’s workforce understand people want to succeed and be good at what they

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philosophy. Approximately 30 lecturers have built careers both in academia and in the field, a combination they leverage in diverse classrooms. Part of the Villa Academy’s uniqueness, is that students at different levels of experience are often grouped in the same classroom. At first that would seem like a challenge for anybody working in an academic environment. But when you have a mixture of people with 20 or 30 years of experience in the field with no tertiary education and then a graduate fresh out of university with zero experience, and they sit in the same classroom, they all come out better people, wanting to know more and realising that they actually need to learn more. Classes are of one to three days duration and cover diverse topics, including: basic toxicology, crop protection reference material, product labels, disease management, plant growth regulators, weed management, biotechnology, rodent control, insect management, biologicals, handling of complaints, adjuvants, application technology, viticulture, wheat and barley cultivation, fly control, etc.

The curriculum is characterised by strong emphasis on the problem-based method of learning in which students collaborate in groups to solve real-world examples of problems. The lecturer acts as a facilitator to encourage self-direction and active engagement among students to a greater extent than the passive, teacher-centric lecture model allows. Classes are intensive and students are held to high standards. In just the three-hour “Introduction to the Crop Protection Industry” class, students are expected to be able to demonstrate their understanding of key concepts including history of the industry, past and future industry trends, market segmentations, the role of Chinese and Indian suppliers of generic products, classification of suppliers and products, product lifecycle and post-patent technology, seeds and GM technology. Assessment criteria in addition to in-class work includes written tests and a final project in which students create a production plan for a field crop produced in their area by researching its significance, evaluating the suitability of climate and soil for production and applying key principles of plant production and protection.

The crop chemicals business is rapidly evolving, and it has become highly technical. We’ve been through a decade of experiential and self- training, which just isn’t good enough for the environment we operate in. The position of agrochemical sales professionals is such that if they make a mistake, the consequences can wreak havoc not only on customer relations, but also on local economies. Furthermore, because their customers often have agriculture degrees themselves, agents need the appropriate knowledge to be able to deal with customers and to be adequately equipped to talk with them on the same level. The crop chemical industry in South Africa is small. If everyone could understand and appreciate the big-picture benefits, we could all be winners. In order to also ensure that students, growers and technical advisors in local agriculture do have access to handbooks and relevant literature the Villa Academy embarked on a program to sponsor the compilation and publishing of handbooks earmarked for local agriculture.

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1 PLANT MATERIAL PROBLEMS________________________________________ 8 2 FUNGAL DISEASES ________________________________________________ 14 2.1 Oidium ____________________________________________________ 16 2.2 Downy mildew ______________________________________________ 28 2.3 Dead-arm disease____________________________________________ 40 2.4 Anthracnose ________________________________________________ 47 2.5 Botrytis (grey rot) ___________________________________________ 53 2.6 Sour rot and blue-green / black moulds _______________________________62 2.7.1 Fungal diseases associated with wood and root rotting/dieback _______ 70 2.7.2 Fungal diseases associated with wood and root rotting/dieback _______ 81 3 BACTERIAL DISEASES______________________________________________ 94 3.1 Bacterial blight ______________________________________________ 95 3.2 Crown gall ________________________________________________ 103 4 VIRUS AND VIRUS-LIKE DISEASES __________________________________ 110 4.1 Fanleaf____________________________________________________ 112 4.2 Leafroll ___________________________________________________ 120 4.3 Stem grooving and corky bark ( rugose wood complex) ______________ 136 4.4 Shiraz disease ______________________________________________ 146 4.5 Shiraz decline ______________________________________________ 157 4.6 Fleck and enation ___________________________________________ 161 4.7 Aster Yellows ______________________________________________ 169 5 GRAPEVINE PESTS________________________________________________ 177 5.1 Phylloxera, margarodes and nematodes __________________________ 178 5.2 Snout beetles and long-horned grasshoppers _____________________ 186

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CONTENTS

5.3 5.4 5.5 5.6 Mealybug and ants __________________________________________ 198 Erinose and bud mite ________________________________________ 208 Stem-borers, snails and fruit fly ________________________________ 215 Leafhoppers, boll-worm, leaf roller caterpillars, spring beetles, wilting beetles and thrips _________________________ 226 Birds, bagworms, rodents, white ants, dassies, antelopes and moths/butterflies ______________________________________________ 238 6 ABIOTIC ABNORMALITIES ___________________________________________ 249 6.1 Wind, heat, sunburn and frost damage __________________________ 250 6.2 Drought, drowning and salinity ________________________________ 262 6.3 Hail, lightning, sulphur burn, bunch stem necrosis and growth arrestment___________________________________________ 272 7 GENETIC DEVIATIONS ____________________________________________ 282 8 DEFICIENCIES AND TOXICITY OF NUTRITIONAL ELEMENTS __________ 296 9 HERBICIDE PHYTOTOXICITY______________________________________ 308 10 INJUDICIOUS/FAULTY CULTIVATION PRACTICES/ACTIONS___________ 322 11 VARIA __________________________________________________________ 335 5.7

REFERENCES _________________________________________________________ 348

INDEX_______________________________________________________________ 356

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PLANT MATERIAL PROBLEMS 1 I n South Africa a very high premium is placed on plant improvement – which entails that all the material, whether from imported or locally developed clones and cultivars, are subject to extremely strictly controlled improvement processes before they are made available to the industry. Currently the SA wine industry has mainly two plant improvement institutions, namely: (i) Vititec and (ii) Ernita Nur- sery (Distell), which continuously strive for a communal goal – based on the issuing of exclusively the best available viticulturally and oenologically selected clone material, which tests free of known harmful viruses and virus-like entities. Based on the fact that tissue culture techniques ( in vitro cultures) represent an essential link with plant material improvement, intensive and comprehensive research on this subject was launched (since 1977) by the Department of Viticulture and Oenology, Stellenbosch University – with specific objectives to develop such techniques for local conditions, to provide for among others the successful elimination of harmful viruses and related entities (Goussard, 1981, 1982, 1984, 1985, 1987). Arising from this pioneering work and later co-operation with abovementioned entities, technology as related to these techniques (Goussard & Wiid, 1989; Goussard et al., 1991; Goussard & Wiid, 1992, 1995) were developed and refined to world standards – and are currently still applied very successfully in plant material improvement processes. In terms of the Plant Improvement Act, the Vine Improvement Association (VIA) is the delegated authority which co-oridinates plant improvement in wine grapes in South Africa and administers the SA Certification Scheme for Wine Grapes. Clones which are developed and of which material has completed the total cycle of plant improvement (clone selection, virus detection and elimination, as well as local evalution in the field of viticulture as well as oenology), are registered with the VIA, after which grafting material is issued from approved sources as ‘SA Certified’.

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PART 1

In practice, where the productive and sustainable performance of any vine is associated with an expected life expectancy of at least 25 years, the quality of establishment material is very highly rated. Given the local situation regarding plant improvement, an urgent and important appeal is made to producers to at all times insist on certified nursery vines from registered nurserymen/growers. In this way one can ensure that only the best available material is planted. A further recommendation could also be to build a trustworthy relationship between producers and growers, in which case experience has shown that the latter would not be reluctant to act if problems are experienced with young plantings – especially regarding virus associated phenomena (Photo 1), as well as cases of problems with graft unions (Photo 2) and root formation (Photo 3). The same approach could apply where problems associated with delayed or absence of bud burst, yellowing of foliage, stunted shoot growth (Photo 4) and complete die-back (Photo 5) of young vines should occur. In this regard certainty must be obtained concerning among others the presence and contribution(s) of Petri’s disease (black goo) and blackfoot to the discolouration, decay and blockage of vascular tissue associated with the abovementioned situations (Photos 6, 7, 8 and 9). Besides plant material, correct and careful soil preparation, handling of grafted vines when collected from nurseries as well as correct and scrupulous planting procedures are of equal great importance so as to obtain a minimum life expectancy of 25 years and even more, but also to ensure continued sustainable and profitable returns. In this regard complete and richly illustrated guidelines regarding correct soil preparation practices, planting of vines and young vine development procedures have been set out in detail by Archer & Hunter (2010). The precise compliance with these recommendations then makes it easier to ascertain the cause(s) of specific problem situations as indicated above, should they occur.

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Virus associated phenomenon (1); thickened graft union (2).

PHOTO 1. The visual appearance of this young Pinotage grapevine reveals a typical closterovirus associated phenomenon. Similar situations emphasise the necessity to consistently use certified material for new plantings.

PHOTO 2. A typical thickened graft union (not to be confused with Shiraz decline or crown gall) associated with ineffective graft union healing. As a result of superfluous callus formation from the scion, with little or none of it from the rootstock component, the latter appears to be strangled. In such instances it is most likely that no cambial bridging takes place with a concomitant lack of vascular tissue differentiation and connection, thus causing the grafted combination to die back.

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Root formation problems (3); withered shoot growth (4); vascular tissue problems (5).

PHOTO 3. Root development on the one side of the rootstock only means that the wound on the opposite side never heals properly and consequently creates ideal entry channels for harmful secondary organisms. This kind of situation should be resolved before the establishment of one-year-old grafted vines.

PHOTO 4. This young vine clearly shows withered shoot growth combined with yellowing of the canopy shortly after establishment.

PHOTO 5. Complete dieback of the grapevine (Photo 4) indicating that sap flow has been hampered (vascular tissue problems).

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Black foot.

PHOTO 6. Longitudinal sections of the base (feet) of grapevines (Photos 4 & 5) reveal unmistakable signs of discolouration, rotting and blocking of the xylem. Note the condition of the roots. Laboratory investigations confirmed the role played by black foot.

PHOTO 7. A typical example of black foot, browning from the bark section of this rootstock.

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Black foot (8); Petri disease (black goo) (9).

PHOTO 8. Black foot xylem discolouration is revealed by this longitudinal section through the foot of the rootstock.

PHOTO 9. Discolouration, blockage and “gumming” of xylem that may be ascribed to Petri disease (black goo).

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2

FUNGAL DISEASES

A lthough grape cultivars differ from one another regarding susceptibility/ resistance to the entry of important/seasonal fungal disease (Perold, 1927; Orffer, 1979; Marais, 1981; Goussard, 2008), one must keep in mind that especially moisture and temperature as climate parameters play a very important role in the occurrence thereof. Therefore decisions on slopes, locations, vine spacing and canopy systems – including sensible and appropriate procedures regarding vine development and canopy management – must be taken with the greatest circumspection, besides judicious application of spray programs. Practices which contribute to wet and humid conditions within vines make an overall contribution to promote fungal diseases. Abnormalities associated with this are in most cases a reflection of local damage to relevant organs, with in nearly all cases specific observable lesions to organ surfaces. Comprehensively it must be kept in mind that when fungal diseases occur to such an extent that the visual appearance thereof can/may be associated with specific abnormalites, serious damage has already been caused with a resultant impairment of control strategies. It goes without saying that control practices must at all times focus on a preventative base. In addition control must not be discontinued with the collection of the current harvests, but must be continued with the purpose of keeping leaves in as healthy a condition as possible for as long as possible. Such actions could, besides building up sufficient reserves, also make a valuable contribution to decreasing the inoculum pressure of relevant diseases regarding the next growth season. In contrast with the occurrence of seasonal fungal diseases, in which regard damaging interventions could be controlled/eradicated/prevented with judicious strategies, it is not the case where tissue of one-year or multi-annual wood (above ground) and roots (below ground) is exposed to the entry and further progress of decay/necrosis/blockage – with the orverarching implication that the vegetative and productive life of vines over the short

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PART 2

and long term are seriously undermined. Total or partial die-off can thus occur in all viticultural areas in young as well as older plants. With reference to studies above ground, such die-off is associated with specific fungal diseases – which can occur singly or within a complex – and pruning wounds can/may be singled out as prominent entry points regarding initial infections. Thus the development cycle of various pathogens is launched to contribute to decay and/or blockage of vascular tissue, by which normal sap flow is of course seriously hampered and can even be halted. In the abovementioned regard leading research is currently being undertaken by both the Department of Plant Pathology (Stellenbosch University) and ARC Infruitec-Nietvoorbij (Stellenbosch) in which a number of overall aspects of especially the vine trunk disease complex are being unravelled. Emerging from this research a number of informative and industry applicable actions have already appeared in published format in which among others specific protocols could be laid down regarding: (i) the management of Eutypa die- back in South African vineyards (Halleen, 2010a) and (ii) the application of sanitary measures at nurseries to combat vine trunk pathogens (Halleen, 2010b). Contributions regarding the protection of pruning wounds against trunk infections – with inclusion of the application of Trichoderma products for biological control – could be considered extremely valuable for application in the local industry (Mutawila et al., 2011a, b). As vine dieback – which often varies in visual appearance - can be ascribed to individual or various pathogens within the trunk disease complex, the in-vineyard identification thereof is accordingly more complicated. Against this background, producers are strongly encouraged to immediately consult with experts at the abovementioned institutions (who have effective functioning disease clinics) in all cases of uncertainty concerning specific fungal diseases/problem situations, even if not exclusively applicable to fungal diseases. In this way actual causes can by accurately revealed, to ensure that suitable control and/or combating/ prevention strategies are implemented as speedily as possible.

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FUNGAL DISEASES Oidium 2.1 O idium occurs in all viticultural regions of South Africa and depending on climatic conditions, intensive control programmes, drawn up in conjunction with consultants, are normally required to keep the disease in check. Fungal spores (do not require free water for subsequent germination) usually hibernate in winter buds and are subsequently released during bud burst, when vines are already susceptible to infection. Although all green parts of grapevines are affected, the first signs are usually observed on the upper surfaces of leaves as small, pale yellow spots (not oil spots) which are especially noticeable when holding the latter at a slant against the light (Photos 10 & 11). A white, powdery fungal growth develops in due course and keeps developing until the entire leaf surface displays a powdery appearance (Photos 12 & 13). It goes without saying that in such instances normal physiological functions of leaves are seriously hampered, thus impeding the normal ripening process. Intense leaf infection (Photos 14, 15 & 16) may result in early leaf fall, whereby bunch exposure to sunburn or heat damage may assume serious dimensions. In addition the essential accumulation of reserves for initial growth in the subsequent growing season is also compromised. As on the leaves, superficial powdery growth in the form of irregular spots occurs on green shoots (become darker in colour when rubbed) (Photos 17, 18, 19 & 20). These spots are retained with lignification and can therefore also be observed on canes during winter. In serious infections shoots may be covered entirely with this powdery growth, causing limited growth or even dieback in young shoots especially. Furthermore buds on such canes may experience problems with bud burst in the subsequent growing season. In bunches oidium can be observed over the entire period from flowering until just before ripening. Inflorescences may wilt with subsequent poor, or even no berry set. Small, green berries are rapidly covered in white powder, whereafter they turn brown and are usually either dropped or develop abnormally and unevenly (Photo 21). Bigger berries usually display, in addition to the white powderiness (Photo 22), a kind of brown (sometimes black) corking on the surface (Photo 23). If the powdery “dust” is rubbed off, darker colour scars appear on the berries, which will subsequently burst and expose the seeds (Photos 24, 25, 26 & 27). Pedicels may also be affected, which hampers the translocation of nutrients to such an extent that ripening cannot occur normally (Photo 28). As ripening progresses (increase of sugar in berries), a decrease in susceptibility to oidium is experienced (Photo 29), which causes the appearance thereof in such cases, when the bloom is rubbed off, to be associated with cobweb-like, superficial brown networks (Photo 30).

16 • A Guide to Grapevine Abnormalities in South Africa

Oidium.

PHOTO 10. Hardly visible yet, but the first signs of oidium can be recognised as small, pale green spots on the upper surfaces when the leaves are held at a slant to the light.

PHOTO 11. In this instance the spots (Photo 10) are more prominent which facilitates the visual observation of oidium.

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Oidium.

PHOTO 12. The characteristic white powdery fungal growth associated with oidium is established and rapidly distributed to cover entire leaf surfaces.

PHOTO 13. A typical example where control measures against oidium were not sustained during the post-harvest period. In addition to oidium, leafroll is another factor which hampers normal physiological processes in these leaves.

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Oidium.

PHOTO 14. In addition to the white powderiness, the development of purplish, brown-black discolourations may be observed here.

PHOTO 15. Intense leaf infection results in the rapid necrosis of leaf parts, whereby the processes involved in early leaf fall are initiated.

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Oidium.

PHOTO 16. Severe oidium infection later in the season may comfortably be associated with this example. Also note the typical powdery spots (already looking darker in colour) on the petioles.

PHOTO 17. A typical example of the occurrence of irregular, superficial oidium spots on young, green shoots.

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Oidium.

PHOTO 18. Spots as in Photo 17 become darker in colour when rubbed. A similar situation is encountered on petioles.

PHOTO 19. Oidium spots on slightly older shoots. One can almost be certain that when such infections are present, buds will also be affected.

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Oidium.

PHOTO 20. Oidium spots on shoots are retained with lignification and are therefore easily observed on cane parts.

PHOTO 21. A typical example of small, green berries being covered in a white powderiness. Note that some of the berries have already turned brown/black and are on the verge of dropping.

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Oidium.

PHOTO 22. A bunch of Chenin blanc displays typical coverage of practically all the berries with a white powderiness. At this stage it is already impossible to “doctor” or save the bunch – even if one uses the best and most expensive fungicides.

PHOTO 23. Apart from the white powderiness, bigger berries often display a brown (sometimes black) corkiness on the surface. This is not to be confused with sulphur burn or thrip damage.

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Oidium.

PHOTO 24. Darkly coloured scars appear on berries when rubbing off the powdery dust.

PHOTO 25. Scars (Photo 24) on berries of this Shiraz bunch indicate the onset of the development of cracks.

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Oidium.

PHOTO 26. Cracking of berries goes hand-in-hand with the exposure of grape seeds.

PHOTO 27. Large-scale cracking of berries with the accompanying exposure of grape seeds.

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Oidium.

PHOTO 28. The visual appearance of lateral branches of the bunch framework as well as pedicels subjected to oidium infection.

PHOTO 29. Decreased susceptibility to oidium is experienced with progressive ripening.

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Oidium.

PHOTO 30. When the bloom is rubbed off practically ripe berries, the appearance of oidium is associated with cobweb-like, superficial brown networks.

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FUNGAL DISEASES Downy mildew 2.2

J ust like oidium, downy mildew occurs in all viticultural regions in South Africa and although it does not necessarily occur each season (depending on the region), unlike oidium, it causes such crop losses that it may be singled out as one of the most important fungal diseases from an economic point of view. In contrast to oidium, fungal spores require free water to germinate after hibernating on dead plant material in vineyards. Under favourable climatic conditions and from approximately 10 cm shoot lengths, downy mildew is able to attack and spread quickly and, depending on the rainfall pattern in particular, it will remain sporadic or take on epidemic dimensions in the course of the growing season. Additional aspects that accentuate the comprehensive impact of downy mildew, include inter alia that all grapevine cultivars of Vitis vinifera are susceptible and that young bunches (even before the flowering stage) often succumb without any signs being visually detectable on the leaves. Such situations may therefore result in total crop losses. The first signs of downy mildew are usually visible on the leaves and are associated with the appearance of irregular, pale yellow spots on the upper parts – sometimes referred to as oil spots (Photos 31 & 32) – which turn red fairly rapidly in red juice cultivars such as Alicante Bouschet (Photo 33). During humid conditions a characteristic white, downy fungal growth develops on the undersides (Photo 34), which represents millions of spore- producing organs of the fungus and spores. In due course the yellow spots become brown and/or reddish brown and the white downiness disappears (Photo 35). In the absence of control, fungal growth will resume when subsequent conditions are favourable and cause the vicious circle to be perpetuated, eventually causing total leaf surfaces to appear brown and desiccated. Late in the season – usually after the harvest – the entire or partial undersides of leaves may be covered in a white fungal growth, without yellow oil spots being visible on the upper parts of the leaves, which is quite unlike the visual appearance early in the season (Photos 36 & 37). Although fairly rare, young, green shoots may be infected under very favourable climatic conditions whereafter they turn black, dry out and die (Photos 38 & 39). In such instances young vines (one to three years old) may be wiped out completely. The economic impact of downy mildew can be ascribed especially to the drastic effect it has on the crop. Young bunches especially are very susceptible – even before flowering – which causes entire bunches to be wiped out at that stage (Photos 40 - 44). Although the susceptibility of berries decreases in the course of their development, infection can still occur at a fairly advanced stage. Typical reactions include shrivelling and browning of berries, which is often accompanied by the development of purplish colour hues (Photos 45 - 48). At that stage, parts of bunches or individual berries succumb, whereas

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Downy mildew.

entire bunches are usually destroyed before and until pea berry stage and even later than that (Photos 49 & 50). Early signs of oidium on leaves and young bunches may sometimes be confused with those of downy mildew. It is nevertheless simple to distinguish between the respective diseases by wetting the plant matter using a spray canister and keeping it overnight in a moist, inflated plastic bag at a mild temperature (20 - 30°C). In the event of downy mildew, vigorous fungal growth will be noticeable the next day (Photo 51), whereas no visible fungal growth will occur if oidium is present.

PHOTO 31. The first signs of downy mildew are associated with irregular, pale yellow spots on the upper parts of leaves. Note the oily, shiny appearance – hence the name “oil spots”.

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Downy mildew.

PHOTO 32. The irregularity in extent and size of the spots (Photo 31) on individual leaves is demonstrated here. This is not to be confused with oidium or herbicide damage.

PHOTO 33. In red juice cultivars such as Alicante Bouschet downy mildew spots turn red rapidly.

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Downy mildew.

PHOTO 34. Under favourable conditions characteristic white, downy fungal growth (representing millions of spore-producing organs of the fungus and spores) quickly develops on the undersides of leaves.

PHOTO 35. In time the yellow spots turn brown/reddish brown, which is accompanied by the disappearance of the white downiness on the undersides of leaves.

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Downy mildew.

PHOTO 36. A typical example where white fungal growth occurs on the undersides of leaves without the characteristic yellow oil spots on top. This phenomenon generally occurs late in the season – usually after the harvest.

PHOTO 37. Downy mildew on leaves late in the season. Note the absence of yellow oil spots.

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Downy mildew.

PHOTO 38. Infection of young, green shoots with downy mildew results in the shoots’ blackening, whereafter they turn brown, burst and dry out.

PHOTO 39. Young vines can be wiped out entirely following downy mildew infection of young, green shoots.

PHOTO 40. A typical example of flower clusters being wiped out as a result of downy mildew infection.

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Downy mildew.

PHOTO 41. Active, vigorous fungal growth which is representative of millions of spore-producing organs of the fungus and spores. Such situations are usually solicited after suspect downy mildew infected organs are subjected to plastic bag “incubation”.

PHOTO 42. Downy mildew shortly before and during the flowering stage.

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Downy mildew.

PHOTO 43. The occurrence of downy mildew after berry set.

PHOTO 44. In conjunction with young berries being wiped out (Photo 43), obvious browning of the bunch framework is visible here.

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Downy mildew.

PHOTO 45. The first signs of shrivelling of slightly larger berries are visually perceptible while fungal growth continues unabated.

PHOTO 46. Downy mildew is often associated with purplish colour hues (without prominent shrivelling) of berries at a fairly advanced growing stage.

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Downy mildew.

PHOTO 47. Very typical wrinkling and shrivelling of berries in reaction to downy mildew infection. This situation is usually followed by the berries’ drying out.

PHOTO 48. A typical example of berries which have come into contact with downy mildew at a fairly advanced stage. Note the brown discolouration, which later becomes black.

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Downy mildew.

PHOTO 49. A bunch of table grapes which has succumbed almost entirely to downy mildew at an advanced berry growth stage.

PHOTO 50. This bunch was entirely destroyed by downy mildew. Note the typical purplish colouration. Not to be confused with sunburn or sour rot.

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Downy mildew.

PHOTO 51. Vigorous fungal growth can be seen on this flower cluster after being kept overnight in a moist plastic bag at a mild temperature. In the case of oidium such “tests” will not result in visible fungal growth.

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Virus associated phenomenon (1); thickened graft union (2). FUNGAL DISEASES Dead-arm disease 2.3

U nlike oidium and downy mildew, the occurrence of dead-arm disease in susceptible cultivars is restricted mainly to the Western Cape, where outbreaks usually occur at the start and during spring – often in a sporadic manner and under wet conditions. In the summer rainfall regions dead-arm disease is usually not considered a serious problem because of spring normally being dry. During hibernation in winter fruiting bodies are formed in the bark sections of affected one-year-old or older wood, or on dead plant material lying on the ground. When these fruiting bodies become wet due to rain, irrigation, dew or foggy weather, spores are released that attach themselves to susceptible grapevine organs and successful germination takes place under conditions of high relative humidity. The significance of dead-arm disease is often underrated. It may, however, rapidly turn into an acute problem due to damage caused under conditions that are beneficial to infection (inter alia dwarfed shoot growth and die-back of buds and shoots). It is therefore essential to obtain expert advice with regard to successful long-term control programmes. The first signs of dead-arm disease are usually observed on the basal internodes of young shoots and are associated with dark brown, lens-shaped spots which expand to become longitudinal cracks, black discolouration and withering of the tissue (Photos 52, 53 & 54). Normally such cracks are fairly shallow and only as deep as the bark. Sometimes, especially during prolonged damp conditions, lesions are not quite visible as individual entities, with practically the entire circumference of the shoot, or large parts thereof, being covered by dark brown to black tissue discolouration (Photo 55). When climatic conditions favour infection until late in the season, shoots may be severely affected and dwarfed, with the accompanying die-back of buds and even shoots (Photo 56), hence the name “dead-arm”. Similar lesions also occur on peduncles, spreading to the rachis and other parts of the bunch framework (Photo 57). Such situations usually cause young berries in certain bunch zones to turn black and shrivel. Infected petioles and main veins are also subjected to having lesions forming on them, often affecting vascular bundles and causing certain parts of the leaf to wither (Photo 58). The occurrence of dead-arm disease on leaves can be identified by small, black spots (surrounded by pale yellow/white circles) which, if numerous, cause a wrinkled and abnormal appearance to the above (Photos 59, 60 & 61). Leaves on the basal internodes are the most severely affected with a decrease in intensity towards the growing shoot tip (Photo 62). The phenomenon that apical leaves appear to be healthy leads to the misconception that infected vines get cured in the course of the season. Should conditions that favour and promote infection re-emerge at a later stage, these characteristic signs of the disease resume on primary and lateral shoots, which may contribute to die-back of pertinent organs.

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Dead-arm.

PHOTO 52. The occurrence of longitudinal lens-shaped cracks is characteristic of dead-arm disease, so too brownish-black discolouration and tissue withering on the basal internodes of young shoots.

PHOTO 53. In highly susceptible cultivars, longitudinal cracks are often more numerous and longer and not restricted to the most basal internodes of shoots.

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Dead-arm.

PHOTO 54. A typical example of black tissue discolouration which accompanies the longitudinal cracks on basal internodes of Gamay noir shoots.

PHOTO 55. In this instance practically the entire circumference of the shoot has been invaded by dark brown to black tissue discolouration, while longitudinal cracks appear less prominent.

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Dead-arm.

PHOTO 56. Total or partial die-back of shoots may occur as a result of severe dead-arm infection when conducive conditions prevail until late in the season.

PHOTO 57. The occurrence of lesions (similar to those on shoots) on peduncles and other parts of the bunch framework. In reaction young berries may blacken and shrivel, as seen in this photo. Not to be confused with bacterial blight.

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Dead-arm.

PHOTO 58. Petioles infected with dead-arm disease may cause vascular bundles to be affected, to the extent that certain parts of the leaf wither.

PHOTO 59. Dead-arm disease on leaves is associated with the occurrence of small, black spots (surrounded by pale yellow/white circles) which, in numerous quantities, cause abnormal characteristics of the leaves.

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Dead-arm.

PHOTO 60. A typical example where young leaves begin to have a wrinkled appearance, in conjunction with typical dark brown, lens-shaped spots on the shoot section and black discolouration of the leaf parts (below left). Note that holes in the leaf surface (as on the photo) are not associated with dead- arm disease.

PHOTO 61. Severely wrinkled leaves caused by dead-arm infection.

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Dead-arm.

PHOTO 62. A characteristic example of severely affected basal shoot parts and leaves with a significant decrease in the intensity of disease symptoms in the apical direction.

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Root formation problems (3); withered shoot growth (4); vascular tissue problems (5).

FUNGAL DISEASES Anthracnose 2.4

A lthough the onslaught of anthracnose on susceptible grape cultivars in the Western Cape has practically ceased since 1968 (with the onset of regular spraying against downy mildew), it remains important to recognise its characteristic traits – especially because of the destructive impact it may have on grapevines and the accompanying crop losses. It goes without saying that anthracnose remains problematic in the summer rainfall areas especially. Hibernation takes place on infected wood – which once again emphasises that it is imperative to remove such material from vineyards during pruning. In spring when moist, warmer conditions prevail, spores are released which are able to infect all green parts of grapevines. According to the available literature, the distribution of anthracnose from one grapevine to another is slow because spore distribution occurs mainly through water. As with downy mildew, spores require free water to germinate, which is mostly enabled by rain, irrigation, dew and foggy weather. Although the importance of anthracnose (just like dead-arm disease) is often underestimated, serious damage may nevertheless be inflicted – especially when spring is exceptionally wet and in instances where it has already occurred in the past. Consequently it is important that expert advice be sought in terms of successful prevention and control practices. Typical of anthracnose infection, is the irregular appearance on leaves of pale grey spots (bigger than in dead-arm disease) with reddish brown to purple edges (Photo 63), which may assume significant sizes, often with a characteristic round appearance in the case of older leaves (Photo 64). In petioles, peduncles and veins, the spots are considerably smaller and more elongated (Photo 65). Tearing and shedding of dead tissue from the spots is very characteristic, with the result that the leaves assume a broken (tattered) appearance – especially where spots occur close or adjacent to each other (Photo 66). On shoots spots are initially small and dark brown to black in colour, sunken in the middle with slightly raised edges (Photo 67). In due course the spots become bigger, join each other and cause shrivelling and destruction of tissue so that shoots die back (Photos 68 & 69). Such shoots eventually appear black and dry with a hard texture. Although typical anthracnose spots are less accentuated on peduncles and pedicels, young bunches (even before flowering) may become entirely shrivelled and die back (Photo 70). After set and berry enlargement typical dark brown to black spots occur on the peduncles (Photo 71), which may later result in the dieback of the entire bunch or parts thereof (which were meant to get nutrients from the infected peduncle). The occurrence of round spots with pale grey centres surrounded by darker edges is very characteristic as far as the infection of older berries is concerned (Photo 72). This phenomenon has given the disease the appropriate name “birds-eye rot”.

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Anthracnose.

PHOTO 63. A typical indication of the irregular appearance of characteristic anthracnose spots on leaves. Not to be confused with herbicide damage.

PHOTO 64. In older leaves (Photo 63) spots may assume considerable sizes, often with a characteristic round appearance.

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Anthracnose.

PHOTO 65. In contrast with the grotesque intravenous appearance, anthracnose spots on petioles and primary veins are considerably smaller and more elongated.

PHOTO 66. Tearing and shedding of dead tissue from spots is very characteristic of anthracnose, imparting a tattered look to the leaves.

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Anthracnose.

PHOTO 67. The initial occurrence of anthracnose spots on shoots.

PHOTO 68. Spots (Photo 67) become bigger, join each other and result in shrivelling and destruction of tissue.

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Anthracnose.

PHOTO 69. A typical example whereby shoots die back because of anthracnose infection. Not to be confused with bacterial blight.

PHOTO 70. The total shrivelling and dieback of young bunches (before flowering).

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Anthracnose.

PHOTO 71. The occurrence of anthracnose spots on young peduncles.

PHOTO 72. On older berries anthracnose occurs as round spots with pale grey centre parts, surrounded by dark edges (“birds-eye rot”).

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FUNGAL DISEASES Botrytis (grey rot) 2.5

A lthough grey or botrytis rot occurs in practically all areas where grapevines are cultivated, the severity of the disease is far less in warm, dry areas – mainly due to the restrictive effect of prevailing low relative humidity on infection. Apart from favourable climatic conditions, the onset and course of the infection is determined by several factors, the most important being cultivar resistance and cultivation practices such as vine and bearer spacing, trellis systems and canopy management (suckering and leaf thinning). Following infection, the disease can progress in several ways – regulated mainly by climatic conditions – to cause the development of noble rot (cool conditions, high relative humidity, but not wet) or grey rot (hot, moist conditions). In the former instance it is possible to make special late harvest or noble wines, while in the latter case tissue degradation (rotting) of berries is the order of the day, causing considerable crop losses. Hibernation occurs through the formation of specific structures (sclerotia) in infected shoots and berries as well as on dead organic material in vineyards. Under favourable conditions sporulation takes place, whereafter spores land on berries and undergo successful germination in the presence of free run-off, subsequently penetrating the skin. In view of the fact that infection can even take place during the flowering stage whereafter the pathogens will remain latent until the onset of ripening, chemical control (in areas with conducive climatic conditions) should be carried out based on expert advice. In warmer and drier areas, meticulous and correct application of canopy management practices in particular should suffice to control botrytis rot. Although germinating spores are not dependent on wounds to penetrate the berry skin, the latter offers additional entry ports, which are liberally used, as a result of berry burst and bird pecks. The first signs of grey rot occur during ripening and are associated with light brown (white berries) and dull (coloured berries) skin discolourations, where the slightest pressure causes the skin to slip away from the underlying tissue – hence the names “glyskil” and “kraakdop” (Photos 73 & 74). This phenomenon is very typical of grey rot in contrast with sour rot, inter alia, where it does not occur. The rot slowly penetrates the berry tissue causing total browning, which results in tissue degradation under favourable conditions (Photos 75 & 76). Grey spores are formed on infected berries where they may be observed either in bunch format or in streaked development format (Photos 77, 78 & 79). In the event of dry, hot periods after infection, this is usually limited to a few berries per bunch, whereafter they either shrivel, turn brown and toughen (Photo 80), or the infection is followed by the development of other harmful secondary organisms (Photo 81). Noble rot is totally dependent on natural, conducive climatic conditions (as mentioned above) at which time infection causes fine cracks in berry skins in bunches that are left on

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