Many people have been asking why powdery mildew in tomatoes was so difficult to control this past season of 2007.

We do know that the summer weather was very conducive to the disease and that some of the newer tomato varieties appear to be more susceptible. And fungicide resistance has been reported in a powdery mildew isolate from tomatoes growing on the central California coast.

We don't yet know how widespread resistant individuals might be or whether they are present in the Central Valley. Regardless of whether we have resistance in the valley or not, it is always wise to keep this risk in mind and apply fungicides in such a way that we lengthen their useful life.

Hopefully, we will find that mildew goes back to being less of a problem this coming season.

The powdery mildew (PM) fungi as a group are considered to have a high potential for resistance development. In fact, we have learned a lot about the development of resistance from the case of the powdery mildew pathogen of melons (and other cucurbits).

In the melon pathogen, resistance to several groups of fungicides has already occurred and, because of this, the DMI fungicide triadimefon (Bayleton) and the benzimidazole fungicide benomyl (Benlate) are no longer registered for use on cucurbits.

The first strobilurin fungicides, azoxystrobin (Quadris) and trifloxystrobin (Flint), were registered in 1999, and a new DMI fungicide, myclobutanil (Rally), was registered in 2000. By 2002, resistance to strobilurins fungicides had been detected in the melon PM pathogen in the U.S. And the two DMIs, myclobutaniland triflumizole (Procure), have recently exhibited poor control of melon mildew on the East Coast.

The newer fungicides, while effective, are at higher risk for developing resistance than the older protectants or contact fungicides. Part of the reason they are more effective is that they are systemic or translaminar. This helps us obtain adequate protection of the undersides of leaves where contact materials usually can't reach.

These newer fungicides have a high risk of developing resistance because they have a specific mode of action (acting at a single site in the metabolism of the fungus). The resistance that has been observed in the melon pathogen to DMI fungicides is categorized as “quantitative”, meaning that the fungus exhibits a range of sensitivity to the fungicide.

With quantitative resistance, we see a slow but ongoing loss of control over time as the population of the fungus becomes more tolerant of the chemical. Such erosion of disease control may sometimes be reversed by using higher rates or more frequent applications. However, eventually complete loss of disease control may occur. With quantitative resistance, failure out in the field may not be apparent until resistance has been developing for some time.

On the other hand, resistance to some other fungicide groups such as the benzimidazoles and strobilurins is qualitative, meaning that the fungicide goes from providing control to failing in a short period of time.

The fungicide resistance action committee (FRAC) has assigned codes to the various fungicide groups. Within a group, cross-resistance is likely, meaning that resistance to one member of the group likely means resistance to all, even to fungicides that have not been used on that particular disease.

These codes are also important to recognize when planning a spray program because rotation among the groups is important. The riskiest groups (i.e. group 11 fungicides) should not be used in consecutive fungicide applications, and their use within a season should be minimized. FRAC codes often appear on the front of the material label or in the label's resistance management section. Look for “GROUP # FUNGICIDE” on the label.

To minimize the risk of developing resistance to fungicides:

  • Systemic fungicides (Groups 11 & 3) should be applied prior to seeing symptoms or early in the development of powdery mildew. Once mildew is established, systemic fungicides should no longer be used, but contact materials (Groups ‘M’ & ‘not classified’) should be used. Rotate between products in different chemical groups.

  • At those stages of crop growth where you can tolerate some disease (late in the season closer to harvest), a contact fungicide should be used. Maximize control obtained with contact fungicides by getting good coverage of the crop.