By Mike Matheron, UA Extension Plant Pathologist

How does one successfully combat powdery mildew in melon plantings?

Maximum disease control requires the initiation of a fungicide application program when environmental conditions favor disease development but before the first visible detection of disease.

Less than optimal but good levels of disease control can also be achieved by beginning fungicide applications at the very first sign of disease in the field. Early initiation of fungicide treatment on susceptible melon varieties is essential due to the rapid development and spread of powdery mildew from initial invisible infection sites within the crop.

The application of a newly registered novel active ingredient usually is effective on most individual pathogen spores or colonies developing from spores. However, the very small number of individuals not killed or inhibited by the fungicide will become an increasingly larger proportion of the pathogen population as the use of the same active ingredient increases.

This is how resistance to a particular fungicide becomes established.

The melon powdery mildew fungus, Podosphaera xanthii, has developed significant resistance to some fungicides in the past. An important strategy to delay fungicide resistance is to alternate among or mix products with different modes of action.

Previous research demonstrated that fungicide application sequences containing a highly efficacious fungicide alternated with a product of moderate to low efficacy provided a final level of disease control not significantly different to that achieved by continuous application of highly effective compounds.

Data from these trials support the notion that high levels of disease control and resistance management can be realized with fungicide alternation programs containing different modes of action of only highly effective chemistries as well as application programs incorporating products with high efficacy along with those of moderate and low effectiveness.

Results are available from the 2012 cantaloupe powdery mildew fungicide evaluation trial conducted at the University of Arizona’s Yuma Agricultural Center. These findings should reflect efficacy on melons other than cantaloupe as well since powdery mildew on all melons in the Desert Southwest is caused by the same pathogen.

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Contact Matheron: (928) 726-6856 or