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Beginner

Scientific Name
Hanseniaspora spp.,  Candida spp., Acetobacter spp., Gluconobacter spp.
           
Identification
Berries

• soft watery rot
• leaking juice and a distinctive vinegar and/or nail polish smell
• white varieties turn a light chestnut brown and red varieties turn brick coloured or purplish red
• pulp is completely broken down
• the skin becomes thin, fragile and breaks down
• frequently associated with vinegar flies

Often Confused With
Botrytis bunch rot – slip skin; infected berries may shrivel; greyish-tan sporulation

Grape berry moth – berries hollowed out and frass and possibly larvae present

Sun burn – primarily on exposed side of cluster; skin and flesh often sink down to the seed

Cooked berry – berries turn light brown, usually from the seed out; smell and taste like cooked fruit

Period of Activity
2-3 weeks pre-harvest.

Scouting Notes
Focus on tight-clustered, thin-skinned early varieties or blocks that have a history of grape berry moth or botrytis bunch rot problems, especially if the period within 2 weeks of harvest is warm and wet.  If warm weather extends through the maturation of later varieties, infection may spread to those as well.  Infected clusters will have a distinct vinegar/nail polish odour.

Threshold
There is no threshold for control since the majority of management practices are completed before sour rot develops.

Advanced

Scientific Name
Hanseniaspora spp.,  Candida spp., Acetobacter spp., Gluconobacter spp.
           
Identification
Berries

• soft watery rot
• leaking juice and a distinctive vinegar and/or nail polish smell
• white varieties turn a light chestnut brown and red varieties turn brick coloured or purplish red
• pulp is completely broken down
• the skin becomes thin, fragile and breaks down
• frequently associated with vinegar flies

Often Confused With
Botrytis bunch rot – slip skin; infected berries may shrivel; greyish-tan sporulation

Grape berry moth – berries hollowed out and frass and possibly larvae present

Sun burn – primarily on exposed side of cluster; skin and flesh often sink down to the seed

Cooked berry – berries turn light brown, usually from the seed out; smell and taste like cooked fruit

Biology
Under wet pre-harvest conditions in Ontario, sour bunch rot is caused by acetic acid bacteria and undesirable yeasts.  The pathogens break down the flesh of the berries and produce acetic acid (vinegar odour) and ethyl acetate (nail polish odour) as byproducts of metabolism. Infection from a single infected berry can rapidly spread to destroy entire clusters.

Sour rot is most severe when fruit are injured by other diseases (Botrytis bunch rot, powdery mildew), insects (grape berry moth, wasps) or physical causes (berry squeeze, leaf removal injury, hail). Disease development is promoted by warm temperatures and rain during the pre-harvest period. Fruit flies are common and help spread the disease. Although the pathogens are not very mobile on their own, fruit flies can spread them to uninfected berries and other vineyard blocks.

Wineries do not want fruit with these microbes because they can rapidly spread in the winery, contaminating equipment and spoiling wine.  Contamination of fruit is determined by measuring the acetic acid content.

Period of Activity
2-3 weeks pre-harvest.

Scouting Notes
Focus on tight-clustered, thin-skinned early varieties or blocks that have a history of grape berry moth or botrytis bunch rot problems, especially if the period within 2 weeks of harvest is warm and wet.  If warm weather extends through the maturation of later varieties, infection may spread to those as well.  Infected clusters will have a distinct vinegar/nail polish odour.

Threshold
There is no threshold for control since the majority of management practices are completed before sour rot develops.

Management Notes
Minimize all causes of wounding to the berries by controlling powdery mildew, Botrytis bunch rot and grape berry moth. 

Pruning, training and leaf-removal  to reduce shading and allow rapid air movement can reduce the frequency and duration of periods of high humidity and berry surface wetness which favour the disease. Canopy management can also enhance penetration of fungicide sprays. 

Thin clusters so that they are not in close contact to reduce cluster-to-cluster spread.  If fruit is removed close to harvest for purposes of crop thinning or to eliminate infected clusters, take the clusters from the vineyard and bury them to prevent spread of the disease.

There are no products labeled for control of sour rot caused by yeasts and bacteria.

Since the pathogens can be dispersed rapidly and widely by vinegar flies, remove infected clusters from the vineyard as they are noticed.

Different cultivars all have different susceptibilities to diseases.  See Table 5-5 Relative Susceptibility of Grape Cultivars to Diseases.

Fungicides have different modes of action, and differ in their activity on grape diseases. See Table 5-6 Activity of fungicides on grape diseases.