Sour rot is caused by a number of undesirable yeasts and bacteria — often in association with other fungal rot diseases — including Botrytis, Penicillium and Rhizopus.
Sour rot has not been a commonly recognized problem in the Okanagan-Similkameen, but it was a serious issue in many vineyards during the 2004 growing season. Varieties that were affected included Pinot Blanc, Pinot Gris, Riesling, Pinot Noir and Chardonnay. Causal agents of the 2004 Okanagan outbreak of sour rot included several bacterial species (Gluconobacter cerinus, G. albidus and G. frateurii) and at least three yeast species (Pichia kluyveri, Candida spp. and Hanseniaspora uvarum).
It is likely that sour rot has always been present, but it may have been confused with botrytis bunch rot in the past. Sour rot affects both crop yield and wine quality. Infected fruit can give an unpleasant flavour to the finished wine and push volatile acid beyond acceptable levels.
Symptoms include a soft watery rot with leakage of berry juice and a distinctive vinegar smell.
Large numbers of fruit flies (also known as vinegar flies) and fruit fly larvae are generally present. The fruit flies produce ethyl acetate, a common fault in wine. There is very little tolerance of fruit contaminated with ethyl acetate.
Sour rot appears similar to botrytis bunch rot. Affected berries are brick-coloured in white cultivars and purplish-brown in red cultivars.
Sour rot is caused by a number of undesirable yeasts and bacteria, often in association with other fungal rot diseases including Botrytis, Penicillium and Rhizopus.
Sour rot pathogens gain entry into grape berries through cracks and wounds caused by wasps, hail, birds, powdery mildew, botrytis or by berry splitting caused by excessive vigour in tight clusters. Such injuries attract fruit flies which also effectively carry and spread the disease-causing organisms.
During favourable conditions (warm moist weather, sugar accumulation in berries), the fruit fly will lay hundreds of eggs and start a new generation every 10-12 days. Under the right conditions, this can lead to explosive disease outbreaks.
Fruit left on the ground from post-veraison bunch thinning can also be a contributing factor to buildup of fruit fly populations.
Cultivars with tight clusters and thin skins are generally more susceptible to sour rot. Very vigorous vines seem to be more prone to the disease due to tighter bunches with berries more likely to burst and also due to a higher moisture environment.
The onset of hot, dry weather can lead to the drying of the infected and damaged berries, thereby slowing the spread of the disease. Cool weather will considerably slow fruit fly population growth, in turn leading to a reduction in the spread of the disease.
Some local and Californian observations indicate that the use of botrytis fungicides during periods favourable to sour rot development may increase damage caused by sour rot. Some experts speculate that the fungicide may kill the micro-flora present on the berry surface which may have some protective effect against the yeasts and bacteria responsible for sour rot.
However, research done in Italy in the early '80s found that powdery mildew and botrytis control, together with fruit fly control, significantly reduced the incidence of sour rot.
No insecticides are registered for the control of fruit flies on grapes. Fruit flies quickly become resistant to most chemical control due to their fast rate of multiplication.
Some French sources recommend applications of Bordeaux mixture around the time of veraison, as copper applied to the fruiting zone has the effect of thickening the grape berry skin, hence reducing the risk of lesions.
For more information about fungicides, see the Pesticides page.
