The fall rains have initiated sour rot breakdown of thin-skinned varieties. As berries swell, they start to pull away from the rachis or become squeezed by their neighbours and split, allowing the acetic acid bacteria and yeast to grow and wreak havoc on the crop. Add wasps to the mix, with mouthparts capable of puncturing berry skins to extract the sweet insides. To confound the problem, vinegar flies can transmit the bacteria and yeast from infected berries whether they are on the vine or on the ground in the vineyard.
While dropping infected fruit on the ground below the vines will reduce the amount of VA (volatile acidity in g acetic acid/L), unless you do this right before the fruit are harvested, you are leaving a source of contaminating organisms that are still attractive to vinegar flies. If you cut sour rot out of bunches, you’re actually causing more injury sites in the remaining berries where the bacteria and yeast can enter plus you carry it on your pruners from one bunch to the next.
As part of the sour rot project funded by Ontario Grape and Wine Research Inc., options for managing sour rot have been investigated. One of the most effective treatments in 2015 trials was applications of Delegate with or without KMS (5-10 kg/1000 L) to the fruiting zone. The insecticide reduced vinegar fly (and wasp) infestations while KMS helped to kill off any bacteria on the surface of the grapes. Because often sour rot doesn’t show up until a few days before harvest and Delegate has a 7 day PHI, this year we are also investigating products with shorter PHI’s registered for MALB and wasps (Mako and Malathion) for their efficacy. We also have observed that bicarbonate (Sirocco/MilStop) at 5.6 kg/1000 L appears to dry up Botrytis infected berries.
Opening up the canopy by at least thinning out the foliage in the fruiting zone can help by promoting faster drying of the fruit so infection is less favoured.
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