George Leavitt’s closely watched powdery mildew control trial this year was a photo finish for first place. However, the finish line needed to be as wide as a football field is long because nearly every commercial and some experimental entrants were bunched up tighter than the cars on the Los Angeles 405 freeway in rush hour traffic.
It was totally unlike last season when just finishing without breaking down was a victory. This year’s test at Madera, Calif., grape grower Joe Lilles Carignane’s vineyard, infamous for unrelenting powdery mildew, was a stroll around the track.
Leavitt, newly retired University of California Cooperative Extension farm advisor from Madera County, said the differences between standard powdery mildew fungicide treatments in his final powdery mildew trial were like splitting a horse whisker.
Last year’s powdery mildew pressure was the heaviest in decades and it separated the thoroughbreds from the Kentucky Derby wannabes in a vineyard considered one of the worst in the state for powdery mildew.
Leavitt has been testing fungicides in the same vineyard on the outskirts of Madera for about 20 years. If a product does well there, it can control the No. 1 disease in grape production anywhere.
This season almost every fungicide treatment worked at lowest label rates and for the advertised 14 to 28 days because disease pressure was inexplicable low. Even organic treatments with soybean oil were nearly acceptable.
The reason organics did not work as well as commercial fungicides season-long was poor spray coverage in the heavy Carignane foliage. However, Leavitt believes fungicides as benign as soybean oil may have a fit in an overall powdery mildew program as a replacement for early season sulfur.
Fewer than 12 of the 36 different treatments were rated less than acceptable. However, disease severity was far less. This year’s untreated check, which has not been treated for powdery mildew in almost two decades, had a PM severity rating of about 76 percent. Last year it was near 100 percent PM severity.
“Pressure was not nearly as heavy as last year, and I do not understand why because this year we had more optimum days for mildew than last year, yet mildew was not bad,” Leavitt said.
“We also had the largest number of overwintering bud infections from last year to this year that I have ever seen in this trial. We had crews go through the vineyard three times breaking out overwintering bud infections.”
Leavitt said these bud infections are a major source for new springtime powdery mildew infections. Carignane clusters are particularly bad in this area, he added.
While conditions were ideal longer this year, the disease came to a screeching half about June 15, when the temperatures quickly soared toward the century mark.
For the first time in about five years, the fungicide Pristine did not finish first. A numbered compound from Valent (V10118) that looked very good in its first year in the trial last season edged out the BASF product by the smallest of margins. Disease severity for the numbered compound was just 0.2; for Pristine it was 0.8: a dead heat in anyone’s race book.
Valent representatives at the field day where results of the trial were released, said V10118 not only does not have a name, they did not know the compound’s active ingredient. However, it is not a triazole compound. It may be a new mode of action that the Valent representatives said could be available in California in 2008.
Bayer CropScience also had a new numbered compound in the trial, USF 2010, that also performed well.
Not all new entries were numbered. Topguard from Chemnova was among the top performers. It is a sterol inhibitor developed for small grains in the EU. Cheminova acquired it from Syngenta two years ago. It should be registered for grapes in California in two years.
A fungicide named Mettle from an Italian agrichemical company, Isagro, also performed well, but there was no one at the trial to indicate if or when it would be registered for powdery mildew control on grapes. Isagro’s Web site does not list any chemicals registered in the United States, although it lists a wide array of products used elsewhere in the world.
A new organic compound, Bionatrol, was in the trial this year. It is a soybean oil product from a company called Doosan. It provided some control versus the untreated check, but Leavitt said spray coverage was lacking due to the heavy foliage in the Carignane vineyard. The best Bionatrol rating was 58.3 severity versus the untreated check of 76.
“Any cluster on the outside of the vine had very good coverage and good control. That was not the case inside the vine. Using this product season long would be a problem,” said Leavitt.
However, it is a very inexpensive product and could become a replacement for sulfur to control powdery mildew early and followed by a systemic fungicide.
“There are reasons to keep sulfur, and there are reasons to get rid of sulfur. Products like Bionatrol offer a different mode of action than sulfur early that you could follow with systemic materials,” explained Leavitt.
The lack of control due to poor bunch coverage with so-called organic or natural products like Bionatrol is also convincing Leavitt systemic fungicides are moving within bunches by means other than vapor.
“From the fact we see one side of the bunch clean with products like soybean oil shows me systemic materials are moving from action other than vapor. We are not getting 100 percent coverage with systemics, yet we are getting excellent control,” he said.
The value of the array of fungicides now available to control powdery mildew versus the decades-old sulfur was evident in this year’s trial when 3 to 5 pounds of Microthiol (wettable powder) on a seven day schedule still resulted in a mildew severity rating of 20.5, far more than any of the standard fungicide treatments that all gained control for at least 14 days.
Regardless of how systemics move, they all moved well in 2006 in one of the most susceptible powdery mildew vineyards in California.
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