For the second year in a row, the fungicide Pristine was the winner in the mother-of-all grape powdery mildew reality challenges. However, it was a photo finish in a very crowded photo.

A microscopic mycelium separated eight top performing treatments in the 2004 powdery mildew trial of veteran Madera County, Calif., University of California Cooperative Extension viticulture farm advisor George Leavitt and staff research associate Tome Martin-Duval.

None of the eight treatments in the photo finish had more than 3 percent powdery mildew in a Carignane vineyard where the untreated check had more than 90 percent mildew in what Leavitt called a “moderately heavy” powdery mildew year.

Leavitt told pest control advisers and growers that 5 percent mildew control is “great” and 15 percent is acceptable for raisin and wine grapes. All but five of the 31 treatments had less than 15 percent powdery mildew and in those cases the failures were not the product, but admittedly poor coverage, according to Leavitt.

Basically, Leavitt started the trial treatments with three applications of Microthiol (wettable sulfur). From late April on, he tested a host of registered and unregistered fungicides at a variety of intervals and rates.

Old test vineyard

The test vineyard is a 30-year-old Carignane vineyard near Madera farmed by veteran grower Joe Lilles. He and Leavitt call it one of the most challenging vineyards to control powdery mildew in the state.

Carignane is highly susceptible to powdery mildew. That is exasperating in Lilles 16-acre vineyard because it is bordered on two sides by large irrigation canals and trees prevent wind movement. Leavitt has been conducting powdery mildew control trials on the vineyard for 18 years. It is one of the toughest challenges possible for mildew control programs.

Resistance management has become the foundation of any powdery mildew control program and in Leavitt's trial, there were at least two if not more modes of action in each control treatment.

“Some people call powdery mildew a cosmetic disease,” said Leavitt. “I do not believe that. All you have to do is look at the untreated Carignane check in this trial and you will see defoliation, cracked berries, shriveled berries and berries half the size of berries in treated checks. It is a lot more than a cosmetic disease in highly sensitive varieties.”

Leavitt continues to harp on separating out different modes of action in mapping a powdery mildew control strategy.

“Sulfur is broad spectrum material, but it is a preventive material. It does not control existing infections,” he said. “When you wash with sulfur to control an existing infection, it is the water that is eradicating the existing infection. The sulfur is there to prevent future infections.”

The only materials that will knock out existing infections after 10 days are stylet oil, Neem oil or jojoba. “The three most important things in using contact materials is coverage, coverage, coverage,” he said.

Contact or organic control measures did not perform well in Leavitt's trial because he admitted coverage was poor. He hopes to correct that in future trials.

“Never use sterol inhibitors (Rally, Rubigan, Procure, Elite, Bayleton) or strobilurns (Flint, Abound, Sovran, Pristine) as eradicants,” he said. One reason is that they will not eradicate powdery mildew, but more importantly, resistance could develop from using then against an existing infection.

Resistance chances

“If there is a huge population of spores when you use one of these fungicides, there is a good chance of developing resistance spores,” Leavitt warned.

Leavitt's trial this year did not include SAR (Systemic Acquired Resistance) materials.

One of the older fungicides on the market is Abound, and Leavitt reported last year that “it look like resistance had developed with Abound.

“I take back that statement. This year we put it out by itself and if resistance was developing, we would have seen it. We did not see resistance this year,” he said. “However, Abound is not as strong on powdery mildew as it is on downy mildew.”

However, Leavitt reported that viticulturist in New York have recorded resistance not only to Abound, but most strobilurins. “I do not know the whys and wherefores, but it may be because we use significant amounts of sulfur and they cannot use sulfur on many of the varieties they grow in New York State. That maybe a factor in the strobilurns resistance there,” he said.

Also out of New York comes research that says grape berries may develop natural resistance to powdery mildew earlier than believed.

Once verasion begins or Brix reaches about 9, berries are considered resistant to powdery mildew.

Leavitt tested the New York theory, stopping fungicide (Sovran) treatments as early as 3.3 Brix on May 12, leaving the grapes vulnerable at 14 days after that treatment date or from May 26 until the end of the season, a month or more after the last treatment in Leavitt's regular tests.

He said Brix did not reach 4 by May 26, yet mildew control was excellent, less than 9 percent infection.

He treated with wettable sulfur three times before starting the first Sovran applications.

Leavitt said he was not surprised that grapes developed natural resistance earlier than thought previously, but he was surprised to see the resistance start as such low Brix readings.

“We are talking about mildew on berries, not on stems. I have not evaluated that yet,” he said.

Here are the treatments that basically were equal in the 30-treatment trial. All had three applications of five pounds of Microthiol per acre in 100 gallons of water per acre at seven-day intervals starting April 6. Fungicide treatments started on April 28. These are the fungicides only with rates and intervals applied in 150 gallons of water:

  • Pristine plus Latron B1956 (surfactant); 10.5 ounces/3 ounces/100 gallons; 21 days.

  • Pristine plus Latron B1956; 10.5 ounces/3 ounces/100; 21-35 days.

  • Elite 4 ounces/14 days; Pristine plus Latron, 10.5 ounces/3 ounces/100/21 days; Spiroxamine (unregistered) 18 ounces/14 days.

  • Rally 4 and 5 ounces/21 days; Quintec 6.6 ounces 21 days; Pristine plus Latron, 10.5 ounces plus 3 ounces/100, 21 days.

  • Rally, 4 ounces/21 days; Pristine (6 ounces) plus Elevate (1 pound) plus Latron 3 ounces/100.

  • Rally 4 ounces/21 days; Quintec plus Elevate, 4 ounces plus 1 pound, 21 days.

  • Rubigan 3 and 4 ounces/14 days; Pristine plus Latron 10.5 ounces/3 ounces/100, 21 days.

  • Sovran 3.2 ounces/14 days (applied 6-9 at 4.2 Brix)

e-mail: hcline@primediabusiness.com