Fungal Disease Management Strategies

This article was reviewed by the editor of The Tender Fruit Grape Vine for technical accuracy and appropriateness.

Powdery and Downy

As expected, powdery and downy mildew have shown up in varying degrees in most vineyards this year. The wet spring and summer so far have been ideal for the development of these diseases. Based on research plot and commercial vineyard observations over the past few years, I have been harping on the importance of the immediate pre-bloom through fruit set period with regard to fruit infections by powdery mildew, downy mildew and black rot. Although we are pretty much through that period in most varieties in most locations, we are not beyond the effects of these diseases. While leaves are most susceptible to infection by downy and powdery mildew when they are young and succulent, they can be infected at any age. It just takes longer for symptoms to develop. Fruit become almost completely resistant to infection by 6 weeks after bloom but rachises continue to be susceptible a lot longer into the season.

The infections that have been observed so far are just the tip of the iceberg since the chances of finding a few isolated lesions in a vineyard are small. However, these same hard-to-find lesions can produce millions of spores each. The potential is still there for an epidemic that could defoliate the vineyard during this season when we may be struggling to mature some varieties and need every square cm of leaf area available.

So what can you do? Make sure that you don’t stretch your spray intervals beyond 14 days with the protectant materials (sulphur, Dikar) we have available. Make sure you’re getting good coverage of both the upper and lower surfaces of the leaf and the rachis. If you are renewing with suckers, it is very important to make sure that the wood stays disease-free to ensure the most winter-hardy and vigorous trunk possible for next year. If there are a lot of suckers clustered together, try doing some leaf removal to make sure sprays can penetrate. If a rain of greater than 2.5 cm (1 inch) follows your fungicide spray, shorten the interval to 10 days. Do not spray Sovran or Nova to eradicate sporulating lesions. This is a sure road to resistance. If you have sporulating lesions present, your only recourse is to protect the remaining healthy tissue and try to keep the epidemic under control.

When to stop spraying for powdery mildew is somewhat up for debate, but mostly driven by processor limitations. Therefore, the best strategy is to achieve good control of powdery mildew early in the season to reduce the damage caused by powdery mildew and avoid the band-aid solution of later season applications.

And Botrytis too…

Many varieties are either at or approaching berry touch. It is very important to make sure to cover all surfaces of the fruit (inside and outside the cluster) now while you still can, especially in tight-clustered, thin skinned varieties like Pinot noir and Pinot gris. This is the last chance to kill any of the Botrytis spores that may have landed on the inside of the cluster on berries. Once the cluster closes, the conditions inside the cluster are humid and protected and perfect for Botrytis to germinate and infect ripening berries. Be very careful to avoid damaging berries when performing canopy management practices. Leaf removal will speed up the drying of cluster surfaces and improve spray penetration. As the sugar content of berries increases during ripening, the fruit become susceptible to direct infection by Botrytis spores. Apply sprays to control Botrytis during veraison, as special sprays, possibly in conjunction with your Grape Berry Moth insecticide spray, directed to the fruiting zone only using full volumes of water and product. The products registered and available for Botrytis control are Rovral, Vangard and Elevate. All of these products are prone to the development of resistance and should not be applied in back-to-back sprays. Where possible limit the applications of each of these materials to no more than once per season. Rovral should be applied well before harvest to avoid potential residue issues

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