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Bunch rot diseases may be a challenge again in 2009

After a couple of sunny weeks in September, growers were hoping that we'd seen the last of the rain that plagued us throughout the 2009 season. But the skies have opened up and botrytis and sour rot are starting to make an appearance. Many of the botrytis infections appear to be initiated within the cluster. These infections likely occurred before bunch close. The infections are activated as the fruit matures and Botrytis can then spread from one berry to another within a cluster, rapidly rotting the entire cluster. Berry splitting due to the rapid influx of water during rains or any physical injury that causes juice to leak, will promote the development of bunch rot. However, once berries reach veraison, they are increasingly susceptible to infection by Botrytis and no visible injury is necessary for infection to occur.

Fungicide options for botrytis control include Scala, Vangard, Elevate, Rovral and Serenade Max or ASO. Table 1 summarizes the use of each of these fungicides. Note that Scala and Vangard are from the same fungicide family so they should not be used in rotation with each other. While the labels for these products state a maximum number of applications per season, for resistance management, it is recommended that no fungicide family be used more than twice in a season if at all possible and that you rotate among families for consecutive sprays. The label for Rovral indicates a 7 day PHI; however, residue problems with the LCBO in the past indicated that this fungicide should be used only earlier in the growing season. Bayer CropScience recommends using Scala no later than 14 days pre-harvest. Your winery may have different recommendations so be sure to check with them.

Table 2 shows the relative activity of the fungicides registered for Botrytis bunch rot as well as their mode of activity. + indicates slight activity, +++ indicates the best level of activity. Rovral is rated as "++" due to historical wide-spread resistance to this fungicide.

Most of these products, with the exception of Serenade, have post-infection activity and in some cases, anti-sporulant activity. If a product has post-infection activity, it moves into the berry so it is therefore not prone to removal by rains (about an hour after drying, but check labels for more information). Fungicides with anti-sporulant activity will reduce spore production but will not eradicate infections that are actively sporulating. Do not expect any of these fungicides to completely "dry up" actively sporulating infections. They will help to reduce spread of bunch rot and protect healthy clusters.

Botrytis sprays should have been applied in bunch rot-prone varieties starting at bloom, again just before bunch closure, then veraison and 2 weeks after veraison. If Botrytis continues to be a problem, additional sprays should be applied but at no shorter than 2-week intervals, keeping product rotation, maximum number of applications and pre-harvest intervals in mind.

Sour rot is also a threat with the wet fall weather and extended period until harvest. There are no products registered for control of the sour rot organisms that are present in Ontario. Infection of clusters with Botrytis may promote the development of sour rot; however, in the past 2 years, there hasn't been a strong relationship between the severity or incidence of these two diseases. Research studies suggest that treatment of clusters with certain compounds around bloom may "stretch" clusters, reducing physical injury caused by berry squeeze as well as the time during which berries are wet, which in turn reduces sour rot. These are being explored in Ontario research trials. Various treatments applied starting at veraison are currently being evaluated to determine efficacy against sour rot and elevated volatile acidity (VA). Potassium metabisulphite (KMS) has shown promise in research trials but the optimum timing and rate are still unclear. We are also exploring the relationship between brix accumulation and development of sour rot and the optimum timing and rate of products. This research is part of a collaborative project among OMAFRA (McFadden-Smith), CCOVI Brock University (Inglis) and University of Guelph (Fisher), funded by NPF&VGA and Ontario Grape and Wine Research Inc., to address the sour rot problem.

Clusters with Botrytis bunch rot or sour rot should be cut out and removed from the vineyard. Rotting clusters on the vineyard floor will produce more spores to infect healthy clusters and also attract fruit flies which can transmit both Botrytis bunch rot and sour rot-causing organisms. Improving air movement by leaf removal will help clusters to dry, reducing the opportunity for infection. Wind machines will not provide adequate air movement to dry off the clusters.

The take-home points: don't expect more from fungicides than they are capable of doing - they will not eliminate sporulating infections; maximize cluster exposure to reduce wetting times; remove diseased clusters from the vineyard. And hope for an end to the showers.