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

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

GRAPEVINE PESTS Stem-borers, snails and fruit fly 5.5

Stem-borers Once the moth-laid eggs (quince stem-borer) have hatched, deep tunnels are systematically bored by young stem-borer larvae in trunks and cordons that are more than a year old – whereafter feeding activities are initially characterised by the destruction of pith sections and subsequent burrowing of the wood. During the growing season, induced abnormalities range from no visible effect to grapevines, or individual cordons that manifest less vigour, or even the absence of bud burst – depending on the degree of infestation and the time lapse from the initial penetration of the wood. Initially the holes are very small and hardly noticeable, but in due course they grow larger and may reach a diameter of up to 10 mm – which in itself is characterised by obvious fine wood shavings which are worked outwards by the larvae – resulting from the maintenance/enlargement thereof to ports through which adult moths (after completion of the life cycle) may escape again. Characteristic, visually detectable damage associated with the feeding activities of stem-borers, is best exposed by sawing through affected wood sections, when adult larvae, cocoons and pupae (typical of quince stem-borers) may also often be observed. Although stem-borer damage is mostly encountered in wood of two years and older, one- and two-year-old canes may also be affected, in which case – due to the destruction of the pith – total dieback may ensue. Obviously the latter serves as an indication that stem-borer damage cannot be specifically ascribed to the well-known quince stem-borer and that other types also occur. In this regard harmful interventions by characteristic stem-borer types that are commonly encountered in grapevines are visually portrayed and elucidated by means of applicable cryptic captions (Photos 336 - 343). Snails Deformities caused by the feeding activities of snails are very characteristic and are usually associated with typical, visually-detectable eating patterns on affected organs of grapevines. Severe damage is wrought during spring and early summer especially, with the result that young, sprouting buds and subsequent new vegetative growth may be destroyed – a situation where even flower clusters may be compromised. In young shoots feeding actions are usually followed by shortening of internodia and delayed growth with smaller and misshapen leaves. Later in the season snail damage is associated with varying feeding patterns, which may range from young leaves that are practically eaten in their entirety, to older leaves with holes or half-moon-shaped sections eaten out of the edges. On the whole, based on severe damage that may be inflicted by snails within short periods (even overnight) in targeted grapevines, it is of cardinal importance to remain abreast of the incidence, as well as behaviour and feeding patterns of this pest (Photos 344 - 349).

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