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Mildew model could save grape growers money

A few years back there was something called “new math.” No one understood it, and it disappeared.

There is a variation of that in the California grape industry. It is called “mildew math,” more specifically powdery mildew math. At first glance, it seems confusing as the old new math.

However, a pair of University of California Cooperative Extension farm advisors from Fresno and Madera counties put some serious, fully comprehensible numbers behind how mildew math can save some cash in a series of grower meetings designed to help growers reduce input cost for powdery mildew control.

Mildew math is the key element in using a UC-developed powdery mildew control model calls Powdery Mildew Risk Assessment Index (RAI) to determine when it the best time to control the most prevalent disease in California's 950,000 acres of grapevines.

The RAI could save San Joaquin Valley grape growers two to three powdery mildew treatments per season, according to Madera County Farm Advisor George Leavitt and his Fresno County counterpart Steve Vasquez.

That represents a savings of from $9 per acre for three dusting sulfur applications to almost $50 per acre for systemic fungicides.

And by being more judicious with fewer applications of sulfur, the most widely used powdery mildew control material, growers could go a long way in keeping sulfur available. Sulfur has come under increasing pressure over the past few years as a result of more stringent air pollution rules and complaints from the public about being inadvertently dusted with sulfur.

The model Leavitt and Vasquez explained to growers was developed by UC Davis plant pathologist Doug Gubler.

It is not new. It has been around for about eight years. It is widely used in the North Coast where Vasquez said is has saved growers an average of at least one powdery mildew application per year. It is also widely used on the central coast.

Money saver

However, it has not been widely adapted in the central valley, where Leavitt said it could be a big money saver at a time when grape prices are at their lowest levels in years.

Powdery mildew, explained Vasquez, overwinters in the grapevine buds and when triggered by temperatures, can infect grapevine leaves and wood from bud break to leaf fall. It can infect fruit from berry set to eight to 10 Brix. An existing powdery mildew colony can survive until berries reach 16 Brix.

There is a wide array of products to control powdery mildew, said Leavitt. A few like oils can be used as eradicants. Most are used in preventative programs.

“You want to practice a preventive program…not an eradication program,” Leavitt said.

The Madera county farm advisor said it is not as important to know the names of the compounds as it is to understand their modes of action because it is imperative to rotate chemistry to ward off powdery mildew resistance. Sulfur is the only material to which powdery mildew has never shown resistance, and it has been used in agriculture for 200 years.

Bayleton was the first systemic powdery mildew material introduced into the grape market in the early 1980s. It was heralded as the powdery mildew silver bullet with which growers shot themselves in the foot by overusing it and creating resistance almost overnight.

Quite effective

“It was so effective in the beginning you could almost grab a bag of it and hold it in front of a vineyard and powdery mildew would disappear,” chuckled Leavitt. “We had growers with horrible powdery mildew problems who used Bayleton and mildew disappeared.

“It was so good we threw away our grandfathers' old sulfur machines, which was probably a good thing because when resistance developed to Bayleton we had to buy new sulfur machines, which we should have bought a generation earlier,” said Leavitt.

Powdery mildew rapidly built up resistance to Bayleton and taught growers and chemical companies a costly, but valuable lesson: Repeated use of a single class fungicide only results in rendering that fungicide useless because of resistance.

Thus began the keystone of powdery mildew control in California grapevines; resistance management. Never use the same mode-of-action chemistry back-to-back. Many chemical companies now limit the use of their product to a maximum amount of product each season in a specific number of applications in an effort to ward off resistance.

Six product groups

There are six different product groups, each with a different mode of action. And there are different lengths of control ranging from sulfur which is only about five to seven days to some products that can hold powdery mildew at bay for 21 days or longer under light pressure.

Regardless of what is used when, Leavitt said the three most important things to remember are “coverage, coverage, coverage.

“Don Luvisi (retired Kern County grape farm advisor) has said most farmers would rather find out that their spouses were having affairs than to find out that their spray rig was not giving good coverage for powdery mildew control,” said Leavitt.

“I don't necessarily believe that about spouses, but I do know it is often difficult to get people to check to see if they are getting good coverage,” said Leavitt.

Material must thoroughly penetrate the canopy. “I do not know of any winery that will pay a grower for clean leaves. They pay for clean fruit inside. That is where you need the coverage…inside the canopy,” said Leavitt.

Growers often get overanxious to control powdery mildew and begin much too early with spray programs that are ineffective because there is no active powdery mildew, according to Leavitt and Vasquez.

The RAI model developed by Gubler is based on a temperature range of between 70 and 85 degrees, ideal weather conditions for PM development.

When there are three consecutive days with six or more continuous hours of temperatures between 70 and 85 within the vine canopy, the clock starts running for mildew control.

Weather reports

There are a series of weather stations throughout the state recording those temperatures. These daily reports are available through the UC IPM Web site. And there are inexpensive dataloggers growers can hand within vine canopies to record temperatures.

Leavitt said for each day those six hours are recorded, a grower records a score of 20. It takes three consecutive days of a score of 20 for mildew to begin developing, according to the model developed by Gubler.

“If you have one day with six hours in that temperature range followed by a second day of the same six hours and during third day there is a 15 minutes period when the temperature falls below that 70 to 85 range, you start back at 0 the next day,” said Leavitt, who admitted it took him four years to “get comfortable” with the model.

When the RAI reaches 60, a powdery mildew epidemic has begun.

Following that a grower or PCA evaluates the temperature range each day afterwards, and adjusts the index by adding 20 points for each day there are six continuous hours within that 70-85 range and subtracting 10 points whenever it falls below that six hours.

100-point limit

But, this is where mildew math kicks in. When the index reaches 100 points, you cannot add points above 100. When the index is at 0, you cannot subtract points.

These numbers following the 60-point starting point give growers an idea of the disease pressure, dictating the control product necessary for control.

These ranges are

  • 0-30 indicating low pressure, which would allow a grower to stretch control intervals.

  • 30 to 50 indicating intermediate pressure with the powdery mildew pathogen reproducing every 15 days. This would indicate an interval shorter than the low pressure intervals.

  • When the index is 60 and above (after the three consecutive trigger days), the pathogen is reproducing every five days.

When pressure is heavy, Leavitt said waiting even a day can create problems. “The pathogen does not care if you are irrigating, it's Sunday, or you want to go to the coast, it is reproducing every five days. If anything, start early with control in those situations. You don't want to be late,” he said.

Powdery mildew flourishes under mild temperature conditions. When temperatures climb quickly above that 70 to 85 range, Leavitt said the threat of mildew is over for the season.

This is a simplistic overview of the model and Vasquez and Leavitt encourage growers to contact them or go to the UC IPM Web site for a more thorough explanation of the system.

The idea behind the model is to minimize the cost of powdery mildew control. “For table grape growers you cannot push the envelope. You cannot tolerate any powdery mildew,” said Leavitt.

“However, for raisin and wine grape growers you can push the envelope and maybe not go to the expense of putting two or even three powdery mildew sprays in the valley in the spring,” said Leavitt.

“If your PCA is not using the model, tell him to start using it,” added Leavitt. “It can save you money. Most years, it can lengthen spray intervals.”

e-mail: [email protected]

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