Powdery mildew

By Mike Matheron, UA Extension Plant Pathologist

Spring is a time of transition for agriculture in the desert southwest. Cool-season crop harvest is wrapping up while spring and summer crops are planted and grown.

This is also powdery mildew season.

Powdery mildew can develop on commercial crops, including wheat, melons, and landscape plants.

It is not too early to consider management options for powdery mildew on melons. The disease generally is favored by dry weather conditions, moderate temperatures, reduced light intensity, fertile soil, and succulent plant growth.

The overall risk of powdery mildew increases as more of these factors become established in a melon field. Dry weather conditions and fertile soil are givens in desert melon production fields.

Spores of the powdery mildew pathogen, Podosphaera xanthii, can germinate to initiate disease at temperatures ranging from 72 degrees to 88 degrees Fahrenheit; optimally at about 82 degrees. These moderate temperatures, reduced light intensity, and succulent plant growth 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 grown. The varieties known to be very 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 with 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.

Less than optimal but good levels of disease control can also be achieved 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.

Contact Matheron: (928) 726-6856 or matheron@ag.arizona.edu.