A powdery mildew season to forget

It has been two decades or more since California grape growers have been pummeled by powdery mildew like this season.

Powdery mildew started early; never let up, and challenged grape growers and pest control advisers from April until July. University of California powdery milder control gurus, Cooperative Extension plant pathologist Doug Gubler at UC Davis and UCCE Madera County farm advisor George Leavitt say it is the worst they have experienced, and they have seen more than two decades of powdery mildew seasons.

It was so bad that in early July Gubler eradicated the powdery mildew in the Sacramento Delta vineyard where he annually rates a wide array of control treatments before he could rate treatments for mildew severity. He put out the raging disease fire with a 2 percent stylet oil application and came back later to rate treatments by berry damage.

The best treatment in Leavitt’s powdery mildew trial in the Carignane vineyard near Madera, Calif., where mildew was rampant still had 4 percent powdery mildew severity on July 6. Normally the Madera trial’s top performing treatment drives mildew to less than 1 percent. The fifth best treatment in the 31-treatment trial with the leading and experimental powdery mildew fungicides had almost 17 percent mildew. The untreated check was at 93.6 percent mildew; berries on those vines were half the sized of vines effectively treated for mildew and the vines were beginning to defoliate by mid-July.

“This has been a nightmare year for mildew. Mildew reined supreme,” said Leavitt. “Grower and PCAs were fighting mildew all season long.”

Worst year ever

“Growers and PCAs are telling me it is the worst year they have ever seen. From what I have seen in my trial, I would have to agree,” said Gubler.

As bad a mildew season as it was, the trials conducted by Leavitt and Gubler indicated a wide range of materials did work very well, compared to the untreated check, just not as well as in the past because it was such a bad mildew year. “The good materials we have relied upon like Quintec, Pristine and Flint still held up pretty good. What completely fell apart was the so-called soft chemistry,” said Gubler.

The only question remaining in the wake of the worst powdery mildew year since the new era of fungicides became available for the California grape market, is how will the disease-damaged fruit impact winery deliveries?

As the wine grape harvest begins, Gubler and Leavitt have not heard reports of wineries rejecting or discounting loads because of mildew damage.

As one person at the Madera field commented, when there is an oversupply of grapes, “one mildew berry can get a load rejected.” This year wineries need grapes, especially wine grapes from the San Joaquin Valley, so who knows what is going to happen.

There were valuable lessons learned in 2005, said Leavitt and Gubler.

“The three most important things to remember in powdery mildew control are coverage, coverage, coverage. Coverage is the name of the game, and everyone found that out this year,” said Leavitt. “The bigger, the thicker and denser the canopy the better coverage had to be to get any kind of control.”

High water volume

Leavitt has been preaching high water volumes for effective powdery mildew control. His message came home to roost this year. In the Madera trial conducted by Leavitt and staff research associate Tome Martin-Duvall, the first two treatments went on in 100 gallons of water the remainder in 150 gallons. “If you used low volumes of 20 to 30 gallons per acre, I would really be surprised if you had decent control,” Leavitt told growers and PCAs at this annual field day where treatment ratings were posted for the replicated trial. “Volumes had to definitely be up there this year.”

Leavitt said if control was poor this year, “go get some yellow spray cards and stick them in the deepest, darkest, most dense areas of your vineyard. Fill your spray rig with water and spray. If you are not covering those yellow cards with water, you need to do something about coverage.

“Change nozzles or change spray rigs but most of all slow down. Most spray rigs I have seen are moving too fast. It is amazing what happens when you change a spray rig speed from three or four miles per hour to one and a half miles per hour,” said Leavitt.

Coverage and timing were the two biggest culprits Gubler found in talking with growers and PCAs who called him at the close of the mildew season to discuss control successes and failures.

Stretching the fungicide intervals beyond recommended intervals was the most common culprit Gubler found.

“We know from past test that if you are three days late under heavy mildew pressure, you are doomed. Sporelation begins in three days and within five days you have a brand new batch of spores,” said Gubler. “You can go from millions of spores to billions in a matter of week. If you wait five days, there will be a powdery mildew fire you cannot put out.”

Leavitt’s trial provided clear evidence of that. The No. 1 treatment in the trial had only a 4 percent severity rating. The winning treatment started with an application of Microthiol (sulfur) on May 7 followed 7 days later with there first of two 21-day interval Pristine treatments. Pristine has topped the Madera trial every year it has been in it.

When the final Pristine treatment was stretched to 28 days, the severity rating jumped to almost 13 percent and that was with a product that was part of four of top five applications in the Madera trial.

Gubler echoed Leavitt’s comments about coverage. Leaf removal is common in many of the premium wine grape areas to improve air movement and fungicide coverage. However, Gubler said it must be done early in a high mildew pressure year.

“If you waited until berries were pea-size before removing leaves this year, you were in deep trouble,” said Gubler.

The Carignane vineyard near Madera was treated twice with copper sulfur in April. However, Leavitt believes mildew was active before then and therefore the products used in his trial struggled to gain control since most are preventive materials and not eradicants.

“We already had mildew before this trial started. That is why a lot of the treatments really looked bad. In 20 years of trials, I have never had one look this bad. Treatments that would normally control mildew did not eradicate mildew,” he said.

Needed weather data

Part of the problem stemmed form the fact Leavitt did not have access to the weather station at the vineyards to calculate the Risk Assessment Index (RAI) developed by the University of California. He had to rely on a weather station five miles away for temperatures to compute the RAI. At the end of the season, data from the station in the Carignane vineyard became available. That was when Leavitt discovered he had started treatments in the test vineyard two weeks late.

“Use weather stations with close proximity to your vineyard — at least within a mile or so,” he recommended. If there is not one close enough, Leavitt suggested buying a pocket-sized weather station. “It costs only $229 and it works. Leave it in the vineyard and take it out when you spray. It will tell you what is going on in your vineyard,” he said.

The second best result in the Madera trial featured a numbered compound from Valent. Remember the number, V 1018. “It looks excellent,” said Leavitt.

Three formulations of the compound were in the trial, according to Leavitt. The new compound, which is four to five years away from the market according to Leavitt, was tested in three formulations, EC, WD and Flowable. The EC formulation was rated at less than 6 percent powdery mildew. The severity of the mildew with the other two formulations ranged from 66 percent to more than 77 percent mildew. “Formulation obviously makes a difference with this compound,” said Leavitt, who has not been told the class of the new Valent experimental fungicide.

The No. 3 position in Leavitt’s trial was a combination of Microthiol, Procure 4SC and Pristine plus the sticker Latron B1956 (9.8 percent): No. 4 was the Microthiol plus Pristine plus Latron B1956 with the last Pristine application at a 28 day interval (12.8 percent); No. 5 was Microthiol, Elite, Flint and Quntec (16.7 percent mildew severity). These top five were not statistically different.

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