The most widespread and destructive disease of watermelon in Georgia — gummy stem blight — has shown possible resistance to a popular fungicide.

Azoxystrobin, sold as Quadris by Syngenta, showed reduced efficacy on gummy stem blight in several grower fields and in watermelon fungicide trials conducted in 1999 and 2000 in Cordele, Ga., according to David Langston, University of Georgia Extension vegetable pathologist. Quadris is in the new strobilurin class of chemistry.

“In 2001, six applications of azoxystrobin (Quadris 2.08SC at 12.3 fluid ounces) resulted in a 14 percent increase in gummy stem blight severity compared to plots not receiving fungicide treatment,” says Langston.

“Reduced efficacy may be a result of a possible fungicide resistance, which has been observed in strobilurins in Europe.”

Gummy stem blight, he explains, is caused by the fungus Didymella bryoniae. Although watermelon suffers the greatest losses from gummy stem blight, severe epidemics are seen each year in cucumber and cantaloupe, he adds.

“Management options for this disease include rotation, deep turning diseased tissue, avoiding irrigating that prolongs leaf wetness and preventive fungicide applications. Of these management options, preventive fungicide applications is the most effective,” says Langston.

Fungicides labeled for control of gummy stem blight — in addition to azoxystorbin — include EBDCs such as Dithane, Maneb, Manzate, Penncozeb, etc.; chlorothalonil (Bravo, Echo, Equus); benomyl (Benlate); and thiophanate methyl (Topsin M).

Benomyl or thiophanate methyl tank-mixed with EBCDs and alternated with chlorothalonil products had proven to offer good control of gummy stem blight until resistance to the benzimidazoles (benomyl and thiophanate methyl) was observed in the early 1990s, notes Langston.

“Chlorothalonil products have shown good efficacy on gummy stem blight but are not used because they have been implicated in causing phytotoxicity to mature watermelon rinds,” says the pathologist.

Azoxystrobin was shown to have excellent efficacy on gummy stem blight by several researchers in the early 1990s and was granted Section 18 emergency exemption status in Georgia in 1997 and 1998, specifically for gummy stem blight, says Langston. A full Section 3 national label was granted for azoxystrobin use on the cucurbit crop grouping in March of 1999, which led to the widespread and routine use of the fungicide to control a broad spectrum of foliar cucurbit diseases, he says.

Isolates of the gummy stem blight pathogen collected in 2000 from watermelon fields in Delaware, Maryland and Georgia — where disease control was unsatisfactory — recently were confirmed by Syngenta to be resistant to azoxystrobin, says Langston. In 2001, an extensive survey was conducted to determine the frequency of azoxystrobin-resistant isolates in commercial watermelon fields in Georgia.

“Isolates of the fungus were obtained from samples of infected watermelon from 25 commercial watermelon fields and research sites in Georgia. Preliminary results from eight of the 25 locations provide evidence of widespread resistance in the gummy stem blight pathogen to azoxystrobin in Georgia.

“Of the 65 isolates tested to date, 54 or 83 percent were found to be resistant to azoxystrobin based on the spore germination assay. Isolates from the remaining 17 locations currently are being tested, and extensive sampling for the 2002 season is planned.”

It's difficult to determine at this time, says Langston, exactly where the azoxystrobin-resistant isolates originated. However, he adds, over-use of the product both in the greenhouse and in the field are the primary suspects.

“When a fungicide of this type is used repeatedly, without rotating to fungicides with a different mode of action, the chance of selecting for a fungicide-resistant population of the target fungus is very great.

“Rotating to different fungicide chemistries hopefully will control the resistant populations before they can reproduce and spread. Growers now should look towards the more traditional fungicides for protecting their watermelon crops.”

The mancozeb and chlorothalonil products both suppress gummy stem blight to some degree, he says. Mancozeb products alone usually are marginally effective at best, and chlorothalonil products have been implicated in rind burn when applied within two weeks of harvest, he says.

“However, chlorothalonil remains our most effective labeled material for gummy stem blight suppression.”