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

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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

14 • A Guide to Grapevine Abnormalities in South Africa