Strawberry disease resistance making production tougher

Growing strawberriesin Florida just got tougher. This spring, growers discovered that most of the usual fungicides had little effect on an outbreak of Botrytis cinerea.

It is solid evidence of the fungus developing widespread resistance to the chemical controls, says Natalia Peres, University of Florida plant pathologist at the Gulf Coast Research and Education Center at Wimauma.

Botrytis, commonly called gray mold, causes fruit rot and sometimes results in considerable yield loss. The fungus can also remain dormant on strawberries, then attack fruit being stored, causing rejections at the retail level. Both situations make this fungicide resistance particularly distressing.

“Most fungicides growers have been using have been highly effective but they only had one target on the fungal cell. In that case, it only takes one single cell-based mutation to develop resistance,” says Peres, whose research had indicated that this situation could occur, given the right circumstances.

“Diseases are different from a weed problem, where you can see the herbicide-resistant weeds developing in the field. You can’t see this disease until you already have lesions that produce millions of spores. You only see the disease resistant problem when there is a major outbreak.

“Botrytis-resistant strains have been building up in fields over the past two or three years, but there has not been a big outbreak. This year, conditions for it were right. It needs to have all the stars aligned.”

That alignment unfortunately occurred in a major way this season. Fungicides affected by resistance included Topsin M, Abound, Cabrio, Flint, Elevate, Scala, Captevate, and Pristine. The only material not affected was Switch.

However, the fungus shows resistance to one of the two compounds in Switch, cyprodinil, and is only being controlled by its other half, fludioxonil.

“Another factor with Switch was that it is the most expensive fungicide, so growers have not been using it as much during this season when prices were generally low as they did the others,” Peres says.

Cyprodinil is also a component of Inspire Super, a fungicide recently cleared for use on strawberries. Peres says the fungus will most likely have what she terms cross-resistance to the new material.

Looking for new materials

Other new fungicides being tested may eventually bring some relief. Two old materials, Captan and Thiram, can control the disease, but much higher rates are necessary, compared to the newer fungicides.

Resistance to multiple fungicides got growers’ attention fast.

“If we’d had resistance only to one product, they would probably not even notice because most of them use more than one fungicide,” Peres says. “The problem is that some of the fungicides they’ve been using have the same mode of action.”

She thinks the possibility of resistance increased because growers tended to use fungicides on a set schedule rather than applying them when they were truly needed.

“In most cases, the label states the limited number of applications you can use. If you use more than that, and sometimes growers do, you are more likely to get resistant Botrytis,” she says.

“The entire Botrytis population does not have resistance to all fungicides. Some have multiple resistance to two fungicides, some to three, some to four. But what we do know is that if you spray more, you select for more resistant strains.”

University of Florida scientists have been evaluating Botrytis isolates collected since 2001. That research shows that 15 percent to 20 percent of Botrytis isolates may have resistance to all fungicides except Switch and fludioxonil, while some may be resistant to only one or two.

Still, growers should not let this push them to overuse Switch, Peres says, because, in that case, the fungus will very likely develop resistance to it, too.

“It is overuse that causedthe problem,” she says. Changing cultural practices also might have had something to do with it, too.

“Some growers have been leaving the plastic row cover on the ground to re-use the following season. They kill the plants and just leave them on the cover. That saves the cost of the plastic.

“But our hypothesis is that it keeps the inoculum and the resistant strains there from one year to another. In the past, when they removed the old plants and the plastic cover, then worked the soil and planted again, that may not have been the case,” she says.

Botrytis can be a particularly virulent foe. It has a number of other hosts including tomatoes, some ornamental plants and some weeds. The fungus quickly evolves and can change from one year to the next.

It produces a lot of spores, which get blown up into the air. If even a few Botrytis fungi in a field are resistant to fungicides, those few can still spread material a great distance.

Fungicide resistance should motivate growers to use the Strawberry Advisory System Peres devised to alert growers when conditions are right for Botrytis and anthracnose, two major diseases affecting the crop.

Information available on internet

Based on weather station reports from six locations in Florida strawberry growing areas, the information is available on the Internet at It uses sensors to measure the length of the most recent wetness period, either rainfall or dew, along with temperature.

Designed to be simple to use, when there is no danger of disease, the weather stations on the map are green. In moderate risk periods, they turn to yellow. When disease is most likely to develop, they’re red.

Graphs showing conditions for disease development over the past month are available as well. Growers can also sign up to get e-mail and text message alerts when conditions change to moderate and high. There is no fee to use the system.

“The idea is to help growerssave money by not spraying unless there’s a real possibility of the disease developing,” Peres says.

“The standard has been to spray once a week as a preventive spray because strawberries are a high-value crop and growers don’t want to risk it. Sometimes they might even spray twice a week.

“I want them to spray only when conditions are favorable for disease in order to help them reduce cost and pesticide use and also to help save the fungicides we do have.”

Results from trials testing the disease modeling over four seasons showed that growers could reduce fungicide use by half, on average, when following the system. Peres determined the environmental variables for disease development and Clyde Fraisse, a University of Florida agricultural engineer, designed the Web program.

“The key was to develop the disease model,” Peres says. “Clyde is the computer expert. That is not my expertise.

“For Botrytis to develop,there needs to be above 12 hours of leaf wetness. Fourteen hours is ideal. About 16 to 20 degrees Celsius is the temperature Botrytis likes best.

“Anthracnose needs warmer temperatures to really develop, up in the 20 to 25 degrees Celsius range. That means a field can have one disease, but not the other.”

The system also recommends products to apply, based on specific climatic conditions. “If the strawberries have blooms, it recommends certain products to protect the flowers against Botrytis. If there are no blooms, it recommends different products,” she says.

Whether conditions are prime for Botrytis and anthracnose to develop next season is anybody’s guess.

“This season when everything was right for it, boom, we had one week of a big Botrytis outbreak. Growers were getting a lot of rejections of strawberries from markets up north because the gray mold developed on the way,” Peres says.

“Growers didn’t understand what was happening. It was clearly fungicide resistance. It now puts us in a whole different situation.”

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