Making the Dust Settle with Sulfur
GrapeGrower Magazine, (August 2003)
by Devin Carroll
Grape growers know that sulfur is handy for controlling powdery mildew. At pennies per pound, it is much cheaper than other fungicides. Dusting machines are faster than sprayers, and dust carries into bunches better than water. No spray can match the coverage of dust after bunches start closing.
But we are learning that sulfur has detrimental side effects. Reducing or eliminating sulfur can dramatically reduce problems with mites and leafhoppers. Some people also suspect that sulfur is directly harmful to grape vines, presumably reducing yield or quality of harvest.
Dr. Michael Costello at Cal Poly San Luis Obispo has been studying sulfur and Pacific mites for several years. His studies show that sulfur applications both before and after bloom result in higher mite populations in July and August. Surprisingly, sulfur before bloom had the strongest effect. Vineyards with sulfur applied in April, but not after bloom, had more mites than vineyards with sulfur applied in May and June, but not before bloom. Vineyards with no sulfur had the lowest mite numbers.
One explanation is that the sulfur may be interfering with predator mites and thrips in April, allowing the mites to start increasing early. The result is higher summer populations. Another possibility is that the sulfur changes the vine in some way that is advantageous to mites. But the message to growers is, if you have mite problems, try reducing your sulfur use.
Several of my grower-clients have reduced their sulfur use in the last few years. Like Dr. Costello, we found that mite problems usually disappear when we apply little or no sulfur. We have also observed a major reduction in leafhopper populations.
In eight low-sulfur vineyards I watched in 2002, only one needed a hopper treatment, after an invasion from a neighboring block. One block, where late July populations have been over 50 nymphs per leaf for many years, had less than 1 per leaf in the low-sulfur program.
It is not necessary to eliminate sulfur. In the block I just mentioned, the grower applied wettable sulfur three times, but no dust. In another vineyard, we applied dust once in June, but still had no hopper problem.
I have observed much higher percentages of hopper eggs parasitized by Anagrus in the low-sulfur blocks. Nymphs also disappear faster after hatching, an indication of increased predation by lacewings, spiders, or other predators. Details are available on the AAIE website, www.aaie.net .
The first step in reducing sulfur is to watch the mildew index and not treat unnecessarily when temperatures are too cool or too hot. The index is based on the number of hours between 70o and 85o F each day. Two websites make using the index easier. The Gowan site has a page showing the index in map form. The UCIPM website explains the calculations and gives temperature data, geared to the index, for many sites around the state. Links to these resources can be found on the AAIE website, under “Phenology and Degree Days”.
The second step is to replace sulfur with fungicides. We now have a variety of good mildew materials, and more are coming on the market soon. Organic sprays such as Kaligreen, JMS Stylet Oil, and Trilogy are also available.
When the weather is on our side, mildew control is not hard. In 2002, with a cool April and a hot May-June, one grower achieved adequate control with just two sprays: Flint & Kaligreen at bloom in mid-May, and Elite & Kaligreen three weeks later. But in bad weather years, growers will need 3-4 sprays and may need to use a little sulfur, too.
Fungicides cost more than sulfur, but growers will save on the cost of miticides and hopper treatments. The biggest benefit will be vines that are not damaged by mites and hoppers.