Powdery mildew

I've just been spraying our vines with sufur to control powdery mildew. This nasty disease, caused by the fungus Uncinula necator, infects green tissue on the vine, most commonly showing up on leaves, but also fruit and shoots.

Powdery mildew on a shoot pre-bloom. If the leaves are infected while young, you see this cupping of the leaf margins

Powdery mildew infection on a green shoot.

Powdery mildew infection on a dormant cane. The fungus cannot infect dormant vine material, so this problem came about during the growing season.

Control is through the application of sulfur, primarily, as this is a pretty soft compound for the environment. There are synthetic compounds available, the most common of which fall into the demethylation inhibitors (DMI) category. However, as long as the green tissue has a covering of sulfur, infection by the fungus is prevented. This requires repeated application through the season, to cover new growth, and also re-cover sulfur that has been washed off, or which has all volatilised. DMIs are great, but there is concern about resistance developing against them, so their use is limited to a certain number of applications per season.

It is the gaseous form of sulfur that is active against powdery mildew, so with heat, the solid sulfur is converted into sulfur dioxide (SO2). However, like with any disease spray, good coverage is essential!

Powdery mildew likes warm and dry conditions: in fact, heavy rain can help prevent infection by washing off the spores!

Careful monitoring for evidence of the disease, which usually starts in those places where the sulfur sprays can't reach (hence the need for good canopy management!), can delay the timing of the first and subsequent sprays, reducing their frequency. However, the disease is like an opportunist - miss one spray and it's likely to take over!