Dirty rotten scoundrels - the pre-harvest disease scourge

Disease management during the veraison through harvest period is very challenging, especially in a wet year like 2008. Leaf stripping has been done rigorously to try to mature the crop and reduce the chance for fruit rot to set in. Hail-injured berries provided entry points for Botrytis or activated infections that occurred during bloom, setting the stage for infections to be enclosed within a cluster after bunch close. Heavy rains pump up berries, squeezing out some and splitting the skins to leak juice providing food for invading pathogens and attracting insects. In order to optimize yield and fruit quality in a season such as this, it is important to correctly identify the problem in the vineyard so that proper measures can be taken.

Berries infected with Botrytis turn pinkish-brown, tan to grey spores are produced and the skin easily slips off the flesh which remains firm ("slip skin") (Figure 1). If conditions are dry after infection occurs, infected berries will shrivel and the infection will not spread (noble rot) (Figure 2). However, if it is wet after infection occurs, and no fungicides have been applied, the infection can spread from berry to berry within the cluster. Tight-clustered varieties are highly susceptible to botrytis. Mechanical damage caused by birds, insects or vineyard operations and ripe berries that split due to internal pressure (berry squeeze) cause entry points for the fungus. Research by Dr. Wayne Wilcox (Cornell University) demonstrated that latent (symptomless) infections that occurred at bloom are activated by high humidity and soil water after veraison.

Figure 1 Botrytis bunch rot starting to develop in Riesling. Note sporulation.

Figure 2 Desiccated berries infected with Botrytis can develop into "noble rot"

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Recent research at the Cool Climate Oenology and Viticulture Institute (CCOVI), Brock University, showed that Acetobacter (bacterium) and Hanseniaspora and Candida (yeasts) were able to induce symptoms of sour rot when inoculated on berries. The name "sour rot" comes from the acetic acid (vinegar) and ethyl acetate (nail polish) odours present with the disease. Crop damage due to sour rot can be extensive since infections that begin in a single berry can rapidly spread to adjacent berries and destroy most or all of a cluster. Outbreaks of sour rot can cause heavy crop losses and reduce the quality of the vintage. In recent years sour rot has become a serious issue especially in thin-skinned varieties (Baco noir, Pinot noir, Pinot gris, Riesling and Gewurztraminer) that are prone to physiological breakdown as they approach maturity. Sour rot infection causes the levels of volatile acidity due to acetic acid in the grapes to be very high. Once acetic acid is present in the juice, it will carry over into the finished wine and it will cause a fault. The Vintners Quality Alliance (VQA) limits the amount of volatile acidity in finished wine to 1.3 g/L and if levels in the wine are above the limit it will be rejected by the VQA.

Sour rot may be confused with Botrytis bunch rot. Berries infected with Botrytis will turn pinkish-brown and the skin will slip easily off the flesh which remains firm ("slip skin") while in berries infected with sour rot the flesh disintegrates (Figure 3). While sour rot often follows a Botrytis bunch rot infection, it may develop in with or without it. Physical injury from berry squeeze, insect or bird feeding can also get a sour rot infection going. Fruit flies have been implicated with the spread of the sour rot pathogens; however, using insecticides to manage fruit flies is pointless since more fruit flies will fly in. There are currently no fungicides registered to control sour rot, although several treatments, including fungicides, calcium applied at pea-size berry and veraison and potassium metabisuphite (KMS), are being evaluated in field trials. Concerns regarding the impact of KMS on vinification have been addressed in a preliminary study at CCOVI which showed that treatment with KMS at 5 kg/1000 L as little as 1 day pre-harvest did not significantly increase SO2 content of juice. The studies at CCOVI were supported by funding by Niagara Peninsula Fruit and Vegetable Growers Association and donation of fruit by Vincor Canada.

Figure 3 Cluster infected with sour rot

The "baked berry" symptom developed in some vineyards, particularly Riesling, in early August. The berries turn the pinkish-brown of Botrytis bunch rot. However, they do not have the characteristic acetic acid odour of sour-rotted berries nor do they produce spores when incubated (Figure 4 a and b). When affected berries are cut open, the browning appears to develop from the centre of the berry outward, as opposed to sun burn, which develops as a brown, sunken area on the sun-exposed part of the cluster only (Figure 5). The cause of this phenomenon has not been determined but it always seems to follow a day or two of high temperatures. Berries affected by either "cooked berry" or sunburn will likely shrivel and dry up. Bunch rot fungicides applied at 2 wk post-veraison should keep potential Botrytis infections to a minimum even with this damage.

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Figure 4a Early symptoms of "baked berry"

Figure 4b Advanced symptoms of "baked berry". Note that affected berries have shriveled.

Figure 5 Sun burned berries.

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