Downy mildew

By Mike Matheron, UA Extension Plant Pathologist

Rainfall amounts recorded Feb. 19 were 0.38, 0.46, and 0.14 inches at the Arizona Meteorological Network (AZMET) stations at the Yuma Agricultural Center, North Gila, and Roll locations respectively. High relative humidity levels in the morning exceeding 90 percent were recorded for at least three days following the rainfall event.

These environmental conditions, plus the current presence of lettuce downy mildew in the southwest desert vegetable production region, strongly suggest further development of the disease in the future.

To obtain the highest level of disease containment, fungicide applications should begin before the actual appearance of downy mildew lesions on lettuce plants. There is a lag time between infection by the pathogen (Bremia lactucae) and the appearance of visible symptoms.

This incubation period can range from three days to longer than a week, depending on the temperature, relative humidity, and susceptibility of the lettuce variety. By the time lettuce downy mildew lesions are observed, many more are likely present but have not matured to a sufficient extent to be visible.

Data from trials conducted on lettuce downy mildew for the past two years at the Yuma Agricultural Center demonstrate that disease severity could be reduced from 70 to 86 percent compared to untreated plants with treatment programs containing Acrobat, Aliette, Curzate, Presidio, Previcur Flex, Prophyt, Revus, and Tanos.

If a lettuce downy mildew fungicide trial now in progress is successful, we will have efficacy data for many of the individual compounds that can be used within disease management programs.

This will allow the creation of treatment programs containing the most effective component fungicides which should maximize overall disease control.

For effective resistance management, using compounds with different modes of action within treatment programs is essential.

Contact Matheron: (928) 726-6856 or