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

FUNGAL DISEASES Sour rot and blue-green / black moulds 2.6

Sour rot In addition to grey rot one can single out sour rot – within the group of fungal diseases/ moulds that cause rot in berries and bunches – as a pertinent role player in such situations. Similar to grey rot, sour rots occurs in practically all cultivation areas where it may cause considerable crop losses, if conditions are conducive to the disease. Considering that whole berries cannot be penetrated, the incidence and further development of sour rot depend on damage to the berry at some stage. Such wounds may be induced in several ways, the most important being initial botrytis infection, pecking of birds, berry burst due to oidium or otherwise, stinging by fruit flies and bursting/tearing apart of pedicels during swelling of berries in compact bunches. The first signs of sour rot occur during and after the ripening process and are associated with yellowish brown to almost orange skin discolourations accompanied by rotting, without any slip skin or cracked skin phenomena (characteristics of grey rot). The above- mentioned scenario is best reflected when grey and sour rot occur simultaneously on individual bunches and create the impression that the latter entails a further developmental phase of the former (Photos 88, 89 & 90). In actual fact botrytis infection causes initial wounding, whereafter sour rot is manifested (following the arrest of grey rot due to unfavourable conditions). In instances where wounding is caused inter alia by pecking of birds, fruit fly stings, berry burst or mechanical actions, sour rot would be able to achieve successful developmental cycles in the absence of grey rot (Photo 91). The acrid vinegary odour associated with sour rot derives from acetic acid formed by acetic acid bacteria. Such bacteria are distributed to rotting bunches by vinegar flies, thus exacerbating the situation. White larvae, not to be confused with those of the fruit fly, may be present in the tissue of rotten berries. As the rotting progresses, the berries dry out and become empty, leaving only dry skins with seeds (Photos 92, 93 & 94). Blue-green and black moulds Similar to sour rot, blue-green and black moulds are only able to infect broken (wounded) berries and are usually only found in ripe grapes. In contrast to black moulds associated with watery rotting and masses of black, odourless spores, blue-green moulds usually affect a few berries only – thereby causing soft berries with raised blue-green pads (Photo 95). In wine grapes such moulds are usually not considered a major threat and are usually encountered in bunches that are left on the vines until late in the growing season with the objective of achieving noble rot. In such instances one often finds that entire bunches, or parts thereof are subjected to the simultaneous occurrence of sour rot and blue-green moulds (Photos 96 & 97). In due course such bunches, including

62 • A Guide to Grapevine Abnormalities in South Africa