System helps forecast strawberry diseases

08/06/2013 10:39:00 AM
Renee Stern

Florida's strawberry growers are embracing a forecasting tool to better manage their fungicide sprays against anthracnose and botrytis fruit rots.

The Strawberry Advisory System uses data from six weather stations in the state's main strawberry producing areas to predict when environmental conditions favor outbreaks of these two diseases, says Natalia Peres, associate professor of plant pathology at the University of Florida's Gulf Coast Research and Education Center in Wimauma.

Growers can check the website (agroclimate.org/tools/strawberry) for a color-coded update from their nearest weather station: green for low risk, red for high risk and yellow for moderate risk. For more immediate updates, they can opt to receive text or email alerts.

System gains popularity

So far, 55 growers representing about half the state's industry have signed up, Peres says.

The system also provides spray recommendations, based on when the field was last sprayed and with what fungicide. Other factors considered are whether plants have hit peak bloom and whether symptoms are present.

"Growers need to be very quick on their response" to anthracnose and botrytis, Peres says. "When the system says conditions are favorable, [disease development] is already started."

"You've got to get the materials on at the right time," says Carl Grooms, owner of Fancy Farms in Plant City. "You don't take aspirin this week to deal with next week's headache."

Over the past five years, Grooms has used the forecasting tool to manage fungicide applications on an increasing proportion of his farm. Now he's committed his entire 50 acres of strawberries.

"For years we've been putting out [fungicide sprays] on a time basis, once a week," he says. "Now through the model we understand sometimes we can get by without an entire spray."

"It's been a significant tool," says Dudley Calfee, general manager at Ferris Farms in Floral City. He estimates the company has cut the number of fungicide sprays by 30 percent to 50 percent.

Reducing applications not only saves money but also helps combat strawberries' image with consumers as a pesticide-intensive crop, he says.

Another benefit is extending fungicide usefulness through better resistance management, Grooms says.

Fungicide resistance is an ongoing concern, with many of the current materials—including Pristine (a mix of boscalid and pryaclostrobin), Elevate (fenhexamid), Scala (pyrimethanil) and Topsin M (thiophanate-methyl)—showing signs of reduced efficacy, Peres says.

"It's more important for growers to spray only when they need to," she says.

Improved disease control wasn't her goal. "I don't know if we could get that with the tools available," she says.

Instead, Peres aims to optimize fungicide use. Growers using the system have reduced their sprays by a third to a half "without compromising disease control."

"You have to have the courage to believe the model," Calfee says. That's not an easy proposition for new users, especially when faced with what looks like a serious botrytis event in the making—but trust comes with experience.

Conduct a small trial first

Calfee hesitated at first. "It's the kind of thing you don't want to bet the whole farm on," he says. Like Grooms, he experimented with small trials and added acreage each year as his confidence in its recommendations grew.

Last year he expanded fungicide management with the tool to 83 acres. "It did so well I put all of our strawberry acreage on it," he says.

Grooms recommends new users focus it on only part of their farm "and see it through" for a full season rather than a single spray cycle.

Once the season ends, compare the number of fungicide applications on that section to the rest of your strawberry fields, the amount of management time spent on disease control, and any differences in fruit quality and losses, he says.

The results should simplify the decision whether to commit fully to the advisory system, Grooms says.

Calfee also has done comparison tests, following the model's recommendations not to spray on some sections and pitting them against other sections following the old calendar-based schedule.

"The results bear out" in favor of the model, he says.

Growers may need to customize model

But growers may need to modify recommendations with their own site knowledge and experience.

"It's an accurate model," Calfee says. "But like any tool you have, you've got to use it when it makes sense."

Scheduled irrigation, for example, could increase disease risk—but fall outside the model's considered factors, he says.

At the same time, experience with the model also can filter into growers' considerations.

"I've been working with it long enough to have a sense of when conditions will trigger an alert,"

Grooms says.

The model measures the duration of leaf wetness, either dew at night or periods of rain, and the temperature during those wet periods, Peres says. Botrytis and anthracnose fungi need both moisture and temperature to hit threshold levels before they develop and colonize fruit.

Each of the system's weather stations transmit data every 15 minutes, prompting a new round of calculations against the model's thresholds.

In addition to ongoing refinements for spray recommendations, she hopes to add calculations for different strawberry cultivars based on their disease susceptibility.

The once-common Festival, which is less susceptible to both botrytis and anthracnose, has given way in recent years to Radiance as the industry's most important variety. But the characteristics that make it so popular with growers come with a greater susceptibility to botrytis, she says.

"The challenge is to find the right [fungicide] products for that," she says.

Peres also would like to expand the advisory system to include powdery mildew recommendations. Although not as prevalent as botrytis and anthracnose, powdery mildew can cause problems when conditions are right.



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