Powdery Mildew (April 29, 2015)

Spring is a time of transition for agriculture in the desert southwest. Cool season crop harvest is wrapping up and spring and summer crops are being planted and grown. This is also powdery mildew season. Powdery mildew can develop on commercial crops, such as late-season lettuce, wheat or melons, as well as landscape plants. It is not too early to begin considering management options for powdery mildew on melons. The disease generally is favored by a lack of rainfall, moderate temperature and relative humidity levels, reduced light intensity for leaves within the plant canopy, fertile soil, and succulent plant growth. The overall risk of powdery mildew increases as more of these factors become established 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 the leaf canopy and succulent plant growth all become increasingly prevalent as the melon plantings grow rapidly during April and May. Another factor to consider when determining powdery mildew risk is the inherent susceptibility of the melon cultivar being grown. 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, melon varieties that have moderate to high levels of genetic resistance to the pathogen will require less fungicide inputs. To achieve maximum levels of disease control, powdery mildew fungicide application programs must be initiated before the visible detection of the fungus. Good levels of disease control can also be attained 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.

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