How can powdery mildew in melon plantings be managed most effectively? Maximum disease control requires initiation of a fungicide application program when environmental conditions favor disease development but before the first visible detection of disease. Less than optimal but substantial 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 rapid development and spread of powdery mildew from initial microscopic and invisible infection sites within the crop. Use of a newly registered novel active ingredient usually is effective on virtually all individual pathogen spores or colonies developing from spores. However, the very small number of individuals not killed by the fungicide will become an increasingly larger proportion of the pathogen population as the use of the same active ingredient continues over time, resulting in a potentially resistant pathogen population. The melon powdery mildew fungus Podosphaera xanthii has developed significant resistance to some fungicides in the past. An important strategy for delaying development of 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 lower 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 simultaneously using fungicide alternation programs containing different modes of action 1) of only highly effective chemistries or 2) products with high efficacy alternated with those that are less effective. Recent fungicide evaluation trials conducted at the University of Arizona Yuma Agricultural Center showed that, compared to nontreated Olympic Gold cantaloupe plants, at least a 90% reduction of powdery mildew was achieved by treating plants with Luna Sensation (fluopyram+trifloxystrobin), Mettle (tetraconazole), Procure (triflumizole), Quintec (quinoxyfen), Rally (myclobutanil), Rhyme (flutriafol), Torino (cyflufenamid), and Vivando (metrafenone). These findings should reflect expected efficacy on melons other than cantaloupe as well, since powdery mildew on all melons in the desert southwest is caused by the same pathogen. Some of the tested fungicides may not currently be registered for use on melons, so read labels carefully before considering their use.

To contact Mike Matheron go to: matheron@ag.arizona.edu.

For questions or comments on any of the topics please contact Marco Pena at the Yuma Agricultural Center.
College of Agriculture, The University of Arizona, Tucson, AZ.