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Managing Powdery Mildew on Melons (May 13, 2015)
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What is the best way to combat powdery mildew in melon plantings? 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 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 the initial and usually undetected infection sites within the crop. Application of a newly registered novel active ingredient usually is effective on virtually all of the 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 develops within the pathogen population. The melon powdery mildew fungus Podosphaera xanthii has developed significant resistance to some fungicides in the past. An important strategy to delay 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 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 that are less effective. In the most recent (2014) fungicide evaluation trial conducted at The University of Arizona Yuma Agricultural Center, tested fungicides that reduced the leaf area covered with powdery mildew on Olympic Gold cantaloupe plants by at least 90% compared to nontreated plants included Luna Sensation (fluopyram+trifloxystrobin), Mettle (tetraconazole), Procure (triflumizole), Quintec (quinoxyfen), Rally (myclobutanil), Rhyme (flutriafol), Torino (cyflufenamid), and Vivando (metrafenone). 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. Some of these tested fungicides are not currently registered for use on melons, so read labels carefully before considering their use.

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


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