5.3 Suggestions for Managing Fungicide Resistance

Only certain pathogens are highly likely to develop resistance to fungicides. Powdery mildews, downy mildews, and Botrytis blight are the foliar diseases for which resistance management is most crucial. Resistance is often first seen in greenhouse culture, so greenhouse operators should be careful stewards of effective chemistry.

The key to resistance management is not using a single-site mode of action material over and over again, week after week. Many of the older fungicides are multi-site mode of action materials that are not subject to this problem. Newer fungicides are more likely to have the single-site mode of action.

5.3.1 Resistance in Botrytis

Thiophanate-methyl is now largely ineffective against greenhouse Botrytis populations, and iprodione is often only partially effective. Protectant materials for Botrytis control such as mancozeb, chlorothalonil, and copper are not at risk for resistance development. The new fungicide fenhexamid, however, is one to which strains of Botrytis could potentially become resistant. Hence the Decree label stipulates that no more than two consecutive applications of Decree should be made before rotating to another Botrytis fungicide. Respecting this label recommendation is important because it will guard the future effective use of this material. A similar usage statement appears on the label of strobilurin fungicides, some of which are labeled for control of Botrytis. Because these fungicides (*Compass, †Cygnus, Heritage and *Insignia) are single-site mode of action materials, they must be rotated with other materials according to label directions.

5.3.2 Resistance in Powdery Mildews

Some greenhouse flower crops are notoriously susceptible to a powdery mildew fungus. Using good resistance management practices when controlling powdery mildew on either potted mini-roses or cut flower roses is extremely important. The same is true for many other vegetatively produced crops, including poinsettias, verbenas, begonias, petunias, and African violets. Many powdery mildew control materials do not appear to be at risk for resistance – these include bicarbonates, coppers, chlorothalonil, and piperalin. Some of the newer materials, however, are very much at risk. The ergosterol biosynthesis inhibitor (EBI) materials (including Strike, *Rubigan, and *Terraguard) and the strobilurins (including *Compass, †Cygnus, and *Insignia) are two groups of materials that need to be used with resistance management in mind. The EBI materials may be rotated with the strobilurins, but do not “rotate” two materials from within the same group (e.g., Strike should not be rotated with *Terraguard, and *Compass should not be rotated with *Insignia). In addition, any nonsystemic protectant fungicide (e.g., a bicarbonate, a copper, or piperalin) can be rotated with either of these groups. The details for rotation are now spelled out on the labels of the strobilurin fungicides. For powdery mildew management, e.g., the labels direct the grower to use *Compass or †Cygnus only twice in a row before alternating to another compound that is not a strobilurin. Using the strobilurin every other application is another recommended strategy. It is feared that continuous hammering of a particular powdery mildew with any strobilurin will lead to the pathogen developing resistance to that entire class of compounds.

5.3.3 Resistance in Downy Mildews

Snapdragons and roses are the crops with the best-known downy mildew diseases, but pansy, coleus, alyssum, bacopa, impatiens, lamium, rosemary, coreopsis, veronica, aster, strawflower, and other crops also have susceptibility to various downy mildews. Any grower making applications for downy mildew control for several consecutive weeks should be practicing resistance management. For downy mildew, strobilurins are one of the important tools, but they are subject to the possibility that resistance will develop in the downy mildew. Rotation is important, but the rotation partners will be different from those used in powdery mildew management. Fungicides containing mancozeb can be rotated with a strobilurin, fosetyl-Al or dimethomorph for downy mildew control. Remember that although mancozeb has excellent efficacy against downy mildew, coverage is extremely important when using protectant materials.

5.3.4 Resistance in Pythium and Phytophthora

Fungicide resistance can also be a factor in the control of soilborne diseases. Currently researchers are finding that roughly half of greenhouse Pythium isolates are not sensitive to mefenoxam/metalaxyl (e.g., Subdue MAXX/Subdue). These insensitive isolates are being exchanged among greenhouses, complicating disease management in many facilities. Propamocarb (Banol) resistance has also been detected in greenhouse Pythium strains. Continuous use of either mefenoxam/metalaxyl or propamocarb is obviously a poor management strategy. Rotating among materials for water mold control is important for disease control as well as for resistance management. Rotate among products containing the active ingredients etridiazole, fosetyl-Al or other phosphorous acids, mefenoxam/metalaxyl, and propamocarb. Fungicide rotation is especially important if you are a propagator of geraniums, poinsettias, calibrachoas, or other Pythium-prone crops. If you are using strobilurins for Phytophthora crown rot management, remember that †Cygnus, *Compass, Heritage and *Insignia all share the same mode of action.