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

FUNGAL DISEASES Fungal diseases associated with wood and root rotting/dieback 2.7.1

Eutypa dieback (tandpyn) Eutypa-associated dieback (especially problematic in grapevines older than 10 years) is characterised by the development of shoots with short internodes and small, semi- transparent, pale green to pale yellow leaves (occasionally with necrotic leaf edges) in bush-like format on one or several arms (bush vines), or cordons (trellised grapevines) (Photos 100, 101, 102, 104 & 105). Normally no bunches are formed and flower clusters (should they occur) dry out and die back after flowering or, in extremely rare instances, contribute to berry development where both large and small berries occur in loose bunches. Eutypa is usually associated with a gradual internal distribution and deterioration of grapevines – which may eventually play a role in complete dieback (Photos 106, 107, 108 & 109). Cross sections of infected organs display characteristic rotting patterns which are mostly associated with the V-shaped occurrence of necrotic wood stretching from the bark to the core wood and reflecting an obvious boundary with healthy wood (Photo 110). This dead wood is hard, unlike other types of trunk rotting, which are usually soft. When eutypa- like traits are first noticed (Photo 111), they are easily confused with other causes and it is recommended that expert advice is sought – even if wood sections (Photo 112) are indicative of the onset of Eutypa dieback. Phytophthora root rotting/dieback Although Phytophthora root rotting may be found in fully grown grapevines, it usually occurs in young plantings, as well as in nurseries, where it is truly problematic. Seeing that fungal spores require free water for rapid multiplication (so as to be able to thoroughly infect the roots), the occurrence of this phenomenon is mostly associated with shallow soils that get wet easily or drown or remain wet for long periods during the growing season. The first visual observation in infected vines is associated with the leaves looking pale and sometimes wilted (especially when ungrafted in nurseries) (Photo 113). This pale colour gradually turns pale green, then yellow and finally brown when the vines die back (Photos 114, 115 & 116). In most instances complete dieback occurs within a period of only three weeks, whereafter dead leaves and bunches remain on the vine for a long time (Photo 117). It is worth noting that no characteristic burning of the leaf edges, as with drought or salinity, occurs at any stage. Roots of infected, dead grapevines look rotten – and in most instances dead – with a blue- black discolouration (Photo 118).

70 • A Guide to Grapevine Abnormalities in South Africa