Resistance Management is Essential with Strobilurin Fungicides
Margaret Tuttle McGrath
Associate Professor, Department of Plant Pathology,
Long Island Horticultural Research and Extension Center, Cornell University
3059 Sound Avenue, Riverhead, NY 11901
Strobilurin fungicides have become a very valuable tool for managing diseases. They are effective against several different plant pathogenic fungi. Also, they have translaminar activity, which means they can move through treated leaves thereby providing control on both leaf surfaces. This group is unique in that these fungicides are the first synthetic, site-specific compounds to provide significant control of plant diseases caused by pathogens from all three major groups of fungi: Oomycota, Ascomycota, Basidiomycota. Azoxystrobin, formulated as Quadris and Abound, was the first fungicide in this group registered by EPA. Trifloxystrobin, formulated as Flint, Stratego, and Compass, was the next. Pyraclostrobin, formulated as Cabrio EG and Headline, is expected to be registered soon. These products have been designated as reduced risk by EPA. Quadris is now registered for use on almost all vegetable crops. Quadris is labeled for managing Cladosporium leaf blotch, purple blotch, rust, downy mildew, and Rhizoctonia damping-off in bulb crops (onion, garlic, etc.); early blight, late blight, and Rhizoctonia damping-off in carrot and in celery; rust, gray leaf spot, Northern corn leaf blight, Northern corn leaf spot, and Rhizoctonia root and stalk rot in corn; anthracnose, belly rot, downy mildew, gummy stem blight/black rot, Alternaria and Cercospora leaf spots, Myrothecium canker, powdery mildew and Rhizoctonia root rot in cucurbits; Alternaria, Cercospora, and Septoria leaf spots, anthracnose, downy mildew, powdery mildew, white rust, and Rhizoctonia diseases (e.g. bottom rot) in leafy vegetables; early blight, late blight, black dot, powdery mildew, black scurf and silver scurf in potato; anthracnose, early blight, late blight, black mold, powdery mildew, buckeye rot, Septoria leaf spot, and target spot in tomato; and Alternaria, Cercospora, and Ascochyta leaf spots, powdery mildew, rust, white rust, Aphanomyces root rot, southern blight, Pythium root rot, and Rhizoctonia crown rot in the root and tuber subgroup (beets, carrots, ginseng, radishes, etc.) and in the tuberous and corm subgroup (artichokes, sweet potatoes, etc.). Considering the value of this group of fungicides for managing diseases in vegetable crops, it is essential to use them in a manner that manages resistance.
Resistance risk with strobilurins has proved to be higher than expected and more difficult to predict. They are site-specific compounds, which has often indicated a high resistance risk; however, they have a new mode of action (inhibition of mitochondrial respiration) which was thought to be difficult for fungi to overcome. Resistance developed much more quickly than expected, but resistance did develop first in a pathogen with a past history of developing resistance to site-specific compounds. Resistant strains of the cucurbit powdery mildew fungus were found in 1999 after just two years of commercial use in four countries. Unexplainably, resistance has not developed yet in North America despite the size of the pathogen population. Just as unexpectedly, resistance has developed in the United States in the fungus causing gummy stem blight and black rot in cucurbits. This was not expected based on past history with benomyl resistance in these pathogens. In the United States, control failure associated with resistance to benomyl was observed in the cucurbit powdery mildew fungus just 1 year after its registration for use on cucurbits, but it was not observed in the gummy stem blight fungus until 23 years after its registration. The cucurbit downy mildew fungus has also developed resistance to strobilurins, but not yet in North America.
Manage resistance to strobilurins by limiting their use and by using them as a component of an integrated program with other fungicides as well as with non-chemical management practices, such as resistant varieties and rotation. This is the standard approach for managing resistance with all fungicides that have potential for resistance development. It is critical to manage resistance before it develops. The goal is not to manage resistance once it has developed, but rather to prevent or delay development of resistance. Apply strobilurins in alternation with other systemic fungicides that have a different mode of action when these are registered for the target disease. Do not alternate among strobilurins because they all have the same mode of action. The fungicide program should also include multi-site contact fungicides that have a low risk of resistance, such as chlorothalonil and copper hydroxide. For example, the recommended program for cucurbit powdery mildew is a strobilurin applied in alternation with myclobutanil tank-mixed with a contact fungicide. No more than half of the applications in a season should include strobilurins. In other areas of the world a limit of one-third is recommended. Strobilurins have outstanding ability to inhibit spore germination, thus they should be most useful early in disease development. Use a disease threshold or a disease forecasting system such as TOM-CAST, when available for the target disease, so that the first application is made at the most critical time. Consider using contact fungicides alone at the end of the season. Resistance in the gummy stem blight fungus developed where strobilurins are believed to have been used exclusively. Using a resistance management program will also minimize yield loss if resistance develops. Resistant strains have exhibited a high degree of resistance. In fungicide evaluation experiments conducted in areas where strobilurin resistance developed, gummy stem blight was more severe on plants sprayed with just strobilurins than on plants that were not sprayed with any fungicide! It is extremely important to evaluate disease control when using any fungicide that is at risk for resistance development and to contact extension staff or the fungicide manufacturer promptly, while the crop is still there, if control is inadequate and there are no other feasible explanations, such as poor application timing.