Although you can still use the fungicide Quadris (azoxystrobin) in areas where the melon disease gummy stem blight hasn’t developed resistance, Extension personnel urge strong resistance-management to prolong its usefulness.

Key to their program is starting with clean plants, and rotating and tank mixing fungicides with different modes of action. “We’ve talked about resistance management for a long time, but occasionally some growers overused the material, and this resistance happens. This is what we were trying to avoid,” says University of Florida Extension plant pathologist Tom Kucharek in Gainesville.

Regardless of the product you’re using or crop you’re growing, Extension personnel stress resistance management should always be practiced to slow the problem from developing.

HISTORY REPEATS
Quadris belongs to a relatively new fungicide class called strobilurins. The class also includes Flint, a Bayer CropSciences product also known as Gem in some markets; Sovran from BASF; and Headline and Cabrio, two fungicides from BASF. Powdery mildew on cereal grains throughout much of Europe became resistant to strobilurins after only two seasons’ use. In Japan, downy mildew on cucurbits is resistant to strobilurins as well as to famoxadone and fenamidone -- fungicides that aren’t strobilurins but are closely related. Japan also has powdery mildew that is resistant to strobilurins.

Based on that information, officials with Syngenta Crop Protection, which manufactures Quadris, believed the first cases of strobilurin resistance developing in the United States would involve mildew. They believed that gummy stem blight would follow a similar time frame as benomyl. It took only one year before cucurbit powdery mildew developed resistance to benomyl but 23 years before benomyl lost its effectiveness on stem blight. But the discovery of pockets of gummy stem blight resistant to Quadris in Delaware, Maryland, Georgia, and Florida has changed their minds.

Extension personnel blame overuse on the resistance problem developing so quickly. “We have a lot of growers who relied on it too heavily, and they sprayed Quadris without rotating to other chemicals,” says David Langston Jr., a University of Georgia Extension vegetable pathologist in Tifton. “I think a lot of Quadris was used in greenhouses, and it wasn’t rotated, and I think that led to a lot of resistance.”

Langston first started seeing gummy stem blight resistant to Quadris in 1999 -- the same year the Environmental Protection Agency registered the product. The fungicide received a section 18 for gummy stem blight control in some areas in 1997. Since 1999, the problem has become widespread in Georgia. Of the 272 isolates collected from 27 fields in 13 Georgia counties, 91 percent were resistant, Langston says. Kucharek also has seen gummy stem blight develop resistance to Quadris in Florida. Because he is still conducting tests on some of the isolates collected, Kucharek says he doesn’t know how widespread the problem is. “Not all of the isolates, but some of them, are less sensitive, that’s for sure,” Kucharek says. The problem seems more prevalent in south Florida, although Kucharek says he is also seeing it in central Florida. The strobilurins remain effective against other cucurbit diseases, including powdery mildew, Rhizoctonia belly rot, and anthracnose in the United States.

LABORATORY TESTING
Syngenta refers to the problem as loss of sensitivity, based on laboratory tests, says Jim Frank, Syngenta technical brand manager for fungicides. Scientists expose samples of an organism to various concentrations of a pesticide. They compare how well the pesticide controls the suspect samples to samples of the same organism that have never been exposed to the pesticide.

Resistance, on the other hand, is generally what you see in the field if you have a pest-control failure, Frank says. But just because the product may not perform as well as it used to in a petri dish doesn’t necessarily equate to resistance in the field, he says.

What growers may see with loss of sensitivity is more disease than they are used to after treatment under the same conditions. As a result of the loss of sensitivity seen with Quadris, Syngenta officials worked with university plant pathologists to develop best-use guidelines for its use on cucurbits. Regardless of whether you have resistant gummy stem blight, Kucharek says starting with disease-free plants is crucial. “You want to reduce the inoculum - then you don’t have as much disease to contend with,” Kucharek says.

The organism also can reside on plant debris left in the soil from the previous year or on volunteer melons, so field sanitation should be part of the strategy. Also imperative is early-season disease control. “You need to get good disease control early in the season with something like Bravo to suppress the disease as much as possible,” Kucharek says.

What Langston recommends for gummy stem blight control is chlorothalonil, such as Bravo, Echo, or Equus. But the product also can cause rind burn within two weeks of harvest. In the past, growers relied heavily on Quadris, since it controlled the disease without injuring the fruit.

If you have resistant gummy stem blight, you’re now in a quandary about late-season gummy stem blight control.