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Spring is a time of transition for agriculture in the desert southwest. Cool season crop harvest has finished and spring and summer crops are now growing. Now is also the time of year when powdery mildew can develop on melons. Some environmental and cultural factors that encourage powdery mildew development on melons include a lack of rainfall, moderate temperature and relative humidity, dense plant canopy, fertile soil, and succulent plant growth. The overall risk of powdery mildew increases as more of these factors are present in a melon field. Lack of rainfall and fertile soil are givens in our desert melon production fields. Spores of the melon powdery mildew pathogen, Podosphaera xanthii, can germinate to initiate disease at temperatures ranging from 72 to 88°F, and optimally at about 82°F. These moderate temperatures as well as reduced light intensity within a dense leaf canopy and succulent plant growth all become increasingly prevalent as melon plantings grow rapidly during April, May, and June. Another factor to consider when determining powdery mildew risk is the inherent susceptibility of the planted melon variety. Those varieties known to be susceptible to powdery mildew will require implementation of a rigorous disease management program involving applications of fungicides with differing modes of action throughout the period of high disease risk. On the other hand, if the melon variety being grown has a moderate to high level of genetic resistance to the pathogen, less fungicide inputs will be required. To achieve the best level of disease control, powdery mildew fungicide application programs need to be initiated before visible detection of the fungus. Good levels of disease control can also be obtained by waiting to begin fungicide applications until no later than the very first sign of disease in the field. These initial infection sites are often on the underside of leaves, so frequent and comprehensive examination of the melon planting is required. 
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.
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