By Thomas Isakeit, Associate professor and
Extension Plant Pathologist, Texas A&M University,
College Station

Gummy stem blight, caused by the fungus, Didymella bryoniae, is one of the more important diseases of watermelon in the southeastern United States. During periods of rainy weather, the fungus can reproduce rapidly and infect leaves extensively, resulting in the death of leaves. In growing areas where the disease is common, fungicides are frequently (i.e. weekly) applied, even before symptoms are seen, to suppress development of the fungus on leaves.

Fungicides used for control of gummy stem blight include contact fungicides such as chlorothalonil (e.g. Bravo and Echo) and mancozeb (e.g. Dithane and Manzate), and systemic fungicides. Systemic fungicides can move within plant tissue, increasing the amount of protected tissue, in comparison with contact fungicides, which are active only where they are deposited. The achilles heel of systemic fungicides is that they are prone to resistance development by the pathogen. Benomyl used to be an effective systemic fungicide for gummy stem blight control, but eventually, there was widespread development of resistance and it, as well as related fungicides such as thiophanate-methyl, can no longer be used for this disease.

In recent years, a new group of systemic fungicides, the strobilurins, have become available for disease control. Two of them, Quadris (azoxystrobin) and Cabrio (pyraclostrobin), are labeled for gummy stem blight control on cucurbits. The potential for development of pathogen resistance to both of these fungicides was recognized even before they were marketed and specific recommendations were made for use of these chemicals to prevent resistance development.

Resistance of the gummy stem blight fungus to a strobilurin fungicide has been previously reported in 2001 in Georgia, Delaware, and Maryland, and was associated with decreased effectiveness of Quadris for disease control. In at least one of those cases, the use of Quadris in the greenhouse production of transplants was documented. University of Georgia researchers found resistant isolates in all of 26 Georgia fields surveyed in 2001. Resistance had been observed in fields in Maryland where strobilurin fungicides had not been previously used.

In the fall of 2002 and the spring of 2003, I collected isolates of the gummy stem blight fungus from three locations in Texas and most of the isolates were resistant to azoxystrobin, as determined by a laboratory test, courtesy of Syngenta Crop Protection. At two locations, watermelon transplants grown in Texas were used. In one field, the grower had never used a strobilurin fungicide, suggesting that the isolates were already resistant prior to the growing season. In another field, I had isolated resistant fungi from transplants before they were planted. In this particular field, gummy stem blight was suppressed for most of the season by a weekly program that included contact fungicides, although sustained rainy weather late in the season led to extensive vine death.

As far as I’ve seen, there has been no crop failure in Texas because of resistance of the gummy stem blight pathogen to strobilurin fungicides. However, the appearance of azoxystrobin-resistant isolates of the gummy stem blight fungus in three locations in Texas is troubling. I am concerned that improper use of these fungicides can lead to the rapid development and spread of fungicide-resistant isolates over a large area, with the likelihood of control failure in many fields. The use of strobilurin fungicides during the production of cucurbit transplants presents the greatest risk in developing widely-disseminated fungicide-resistant isolates.

If the fungus is resistant to azoxystrobin (Quadris), it is likely also resistant to other fungicides in the strobilurin group, which include Cabrio. For example, in watermelon field trials with an isolate of the gummy stem fungus resistant to strobilurins, Dr. Kate Everts of the University of Maryland showed that both Sovran and Cabrio were equally ineffective in controlling the disease.

The loss of strobilurin fungicides because of resistance would represent a critical loss in fungicides useful in a gummy stem blight management program. To prevent future catastrophic losses, it is imperative that all fungicide options be kept. This means preventing resistance development in the first place. Cooperative research between Texas A&M University and Syngenta will be started in the Fall of 2003 to address resistance management.

Recommendations

• Gummy stem blight is seedborne, but the incidence can greatly increase during propagation in transplant production facilities. Systemic fungicides (both strobilurin and DMI fungicides like Nova and Procure) must not be used in these facilities because of the potential to rapidly select for resistant isolates and spread them onto many transplants. Unfortunately, seed grown in transplant greenhouses may already be contaminated with strobilurin-resistant isolates and overhead watering can increase the incidence of infection of transplants.
  
• Growers should inspect transplants for disease symptoms and obtain a diagnosis if symptoms are found.
  
• When there is an elevated risk of gummy stem blight (e.g. use of transplants, planting on a field with a previous gummy stem blight history), a protectant fungicide (Bravo or Dithane) should be the first fungicide used.
  
• Alternate systemic fungicides with different modes of action (e.g. second spray is strobilurin, third spray is DMI fungicide)
  
• Spray a mixture of a systemic fungicide with a protectant fungicide.
  
• Use the full label rate of fungicide in at least the minimum recommended volume of water, at no more than the maximum recommended interval, and apply preventatively, rather than after the appearance of symptoms.
  
• Do not apply any one systemic fungicide more than four times per season
  
• If gummy stem blight occurs, rotate field out of cucurbits at least two years.

For photographs of gummy stem blight symptoms, see

http://plantpathology.tamu.edu/Texlab/Vegetables/wmelon/gsb.htm