By: Wendy McFadden-Smith, Cristina Huber and Debbie Inglis 

Ontario Grape and Wine Research Inc. and the Niagara Peninsula Fruit and Vegetable Growers’ Association have funded a research project on understanding and managing sour rot in vineyards.  The following summarizes the some of the results to date.

In some cases sour rot starts in the middle of the cluster and develops outward.  This is likely due to crushing of these internal berries as the berries in the cluster swell. However, sour rot can also start on a single berry on a shoulder of a cluster.  In this case, sour rot develops starting from the base of the berry where it attaches to the berry stem.  As berries take up water with rainfall (and possibly even with dew), they swell and pull away from the berry stem, creating a wound through which the acetic acid bacteria can enter and start to initiate sour rot.  This was replicated in lab inoculations. 

Botrytis does not have to be present in order for sour rot to infect.  Neither does diffuse infections by powdery mildew.

In lab tests, berries of Pinot noir and Riesling of a range of Brix were inoculated.  No disease developed until berries had reached 15 Brix.  We don’t know why this seems to be a trigger but it is consistent.

Potassium metabisulphite (KMS) was applied at 5 kg/1000 L and 10 kg/1000 L in 600 L/ha in the fruiting zone 5-6 times starting at veraison with the final application 1 day pre-harvest in Riesling.  This treatment significantly reduced both sour rot and the level of volatile acidity.  Treatments (5 kg/1000 L) started later (1 week preharvest) also reduced sour rot but not as much as earlier treatments.  We are trying 20 kg/1000 L this year.  No detectable SO₂or free SO₂was detected in samples treated even 1 day pre-harvest at 5 kg/1000 L and there was no effect on fermentation.  A trace of SO₂was detected in some of the 10 kg/1000 L juice.  There was no difference in the level of potassium in treated vs. untreated juice with as little as 1 day pre-harvest.  The volume of water in commercial vineyards may be increased.  With the development of sour rot from the inside of the cluster out, it may be necessary to wash the spray solution through the cluster instead of just depositing it on the surface.

Calcium treatments applied this late in the season are not effective in reducing berry split.

Milstop (potassium bicarbonate) did not reduce the severity of sour rot; however, it did seem to reduce botrytis severity.  This has been reported to me by growers in the past.

I have heard of growers turning on their wind machines in order to dry off the fruit.  The wind machines will NOT generate enough air movement in the fruiting zone to help dry off clusters adequately to reduce risk of sour rot.