Winetech Technical Yearbook 2022

JANUARY/FEBRUARY

Petri disease – WHEN AND HOW DO WESTERN CAPE VINEYARDS GET INFECTED?

RESULTS AND DISCUSSION During the study Pm. minimum perithecia and Pa. chlamydospora pycnidia were found on trunks and cordons of vines in several vineyards, as well as the “heads” of mother vines in rootstock mother blocks (Baloyi et al ., 2013, 2016). Perithecia were found in wood crevices and old pruning wounds, while pycnidia were found in crevices and cracks, as well as on bark and old pruning wounds. The spore trapping studies , which were conduct - ed from mid-March to the beginning of December over two seasons, revealed for the first time the presence of ae rial spores associated with Petri disease in South Africa (Hal leen et al ., 2020). Phaeomoniella chlamydospora and a total of 14 Phaeoacremonium species were identified, including Pm. australiense , Pm. griseo-olivaceum , Pm. griseorubrum , Pm. inflatipes , Pm. iranianum , Pm. italicum , Pm. minimum , Pm. parasiticum , Pm. prunicola , Pm. scolyti , Pm. sicilianum , Pm. subulatum , Pm. venezuelense and Pm. viticola. Of these, only Pa. chlamydospora, Pm. minimum and Pm. inflatipes have been reported as aerial inoculum within vineyards in other countries. This is by far the greatest diversity in Petri disease pathogens ever recorded in vineyards worldwide. The highest number of pathogen species found in a single vineyard was eight and six in a rootstock mother block. Two pathogen species, Pa. chlamydospora and Pm. minimum , were trapped in all the vineyards and rootstock mother vine nurseries during both seasons of the study. One of these two pathogens was the predominant species detected in all vineyards, except in Rawsonville where Pm. sicilianum was the predominant species. Similar to previous isolation studies conducted in South Africa, Pm. parasiticum appears to be the most common Phaeoac remonium species associated with grapevine after Pm. minimum , as this species was detected in four and six vineyards, respectively, during the two spore trapping seasons. Petri disease pathogens were detected throughout the trapping periods in all the vineyards. Spore release coincided with winter and spring (i.e. suckering) pruning activities. Phaeomoniella chlam ydospora and/or Pm. minimum spore release events occurred during the week of pruning or within four weeks after pruning, and rootstock cane harvesting in all the vineyards and rootstock mother blocks. Grapevine pruning wounds remain susceptible to Petri disease pathogens for four to sixteen weeks after pruning. Phaeoacremonium sicilianum and Pm. parasiticum spores were also detected after pruning in Rawsonville and one vineyard in Paarl, respectively, in both seasons. Phaeoacremonium subulatum , Pm. scolyti (Rawsonvile), Pm. prunicola (Stellenbosch), Pm. irani anum (Paarl) and Pm. inflatipes (Durbanville) were also detected during the susceptible period, but during only one of the seasons

THE AIM OF THE CURRENT STUDY WAS TO IDENTIFY PETRI DISEASE PATHOGEN INOCULUM SOURCES IN VINEYARDS AND ROOTSTOCK MOTHER BLOCKS AND TO DETERMINE IF, AND WHEN, SPORES ARE RELEASED AND WHETHER SPORE RELEASE EVENTS COINCIDE WITH SUSCEPTIBLE PERIODS IN VINEYARDS AND MOTHER BLOCKS.

BY FRANCOIS HALLEEN, ANNABELLA BALOYI & LIZEL MOSTERT

INTRODUCTION Petri disease is a serious problem in grapevines affecting grape production worldwide including South Africa. The disease is caused by xylem inhabiting fungi Phaeomoniella ( Pa. ) chlamydospora , Phaeoacremonium ( Pm. ) minimum and several Phaeoacremonium species and usually affects young vines in newly established vineyards. Symptoms include stunted growth, shortened internodes and dieback. Internal symptoms include black to brown spots (in cross-sections) and brown streaking in the xylem tissues (in lengthwise cuts). Petri disease pathogens spread within vineyards as aerial inoculum, which originates from fruiting bodies. Pruning wounds are known host ports of entry for aerial spores of these pathogens. However, knowledge is lacking on the occurrence of these fruiting structures within South African vineyards, as well as its possible contribution towards aerial inoculum. At the onset of the current project, only limited information was available from studies conducted in Grapevine wood pieces were investigated for fruiting bodies. Spore trapping studies were conducted in six 24 - 40 year-old vineyards (two each in Stellenbosch and Paarl, and one each in Rawsonville and Durbanville) and two rootstock mother blocks (19 year-old in Slanghoek and 17 year-old in Wellington) over two seasons. Pathogenicity studies with newly found Phaeoacremonium spp., or species that have not been subjected to pathogenicity studies in local vineyards, were conducted on field-grown Cabernet Sauvignon vines by inoculating pruning wounds. France, California and Australia. MATERIALS AND METHODS

Black vascu lar streaking

associated with Phaeomoniella chlamydospora wound infection in a grapevine rootstock (left). In a cross-section, the occluded xylem vessels can be seen as black spots (right).

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WINETECH TECHNICAL YEARBOOK 2022