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New research on the use of sulfur in powdery mildew control

Sulfur is becoming more and more
important in powdery mildew control as we start getting resistance to
everything else,” says Wayne Wilcox, professor of Plant Pathology at
Cornell University in New York.

Wayne Wilcox

Sulfur is becoming more and more important in powdery mildew control as we start getting resistance to everything else,” says Wayne Wilcox, professor of Plant Pathology at Cornell University in New York.
 
Wilcox has been experimenting with the application of sulfur to control powdery mildew on Riesling seedlings. He reviewed the results of three experiments that looked at the effect of temperature and timing of sulfur application at a recent OMAF Integrated Pest Management seminar for crop consultants held in Guelph, Ont.

One of the presumed disadvantages of sulfur is that it is supposed to be temperature dependent – it was thought to be much less effective at temperatures of 18ºC or lower. This is presumably due to insufficient volatility, but Wilcox says that as of yet, we have no data to support this.

Wilcox started looking at the effect of temperature on sulfur activity last year, inoculating seedlings with powdery mildew at temperatures of 15ºC and 21ºC in closed containers. Researchers measured powdery mildew growth and spore formation eight days after inoculation and, says Wilcox, “the long and the short of it was that we weren’t getting control through the vapour phase at either temperature.”

He suspects the effect of temperature was on the fungus, not the sulfur. Wilcox emphasized these results are preliminary and obtained under laboratory conditions, but he is telling growers there is the possibility that researchers may have overstated this inability of sulfur to control powdery mildew at lower temperatures. Growers may want to reconsider this issue if using sulfur is important to them.

The conventional wisdom with using sulfur as a fungicide is that has to be present before infection in order to be effective. But how does the timing of the application prior to infection affect the activity of the fungicide?

In a study conducted with potted plants in a greenhouse, Riesling seedlings were sprayed with sulfur between three and fourteen days before inoculation with powdery mildew. It was thought that the new leaves produced after the spray application would be protected by the vapour activity, but under these experimental conditions they were not protected.

“We were hoping for 100 per cent control,” said Wilcox, but in the very top leaf, which was the furthest away from the sprayed leaves due to shoot elongation between the spray and inoculation dates, they got almost no control. There was significant protective activity only when there was residue on the leaves, which was on those plants inoculated three days after spraying.

Wilcox also sprayed Riesling seedlings with sulfur at one, three, seven and 10 days post-infection. For the first seven days after infection, they received virtually complete control and reasonable control even at ten days post-infection. When researchers looked at the amount of spores being produced, which indicates the capacity of the fungus to spread, even after the 10 day post-infection spray, they were obtaining 84 per cent less sporulation than on the unsprayed plants. Wilcox concludes that it does appear there is significant post-infection activity from sulfur.

“Knowing how the materials work helps us to use them most efficaciously,” stated Wilcox.