Downy mildew

By Mike Matheron, UA Extension Plant Pathologist

Two rainfall events during November in Yuma County have raised the relative humidity levels considerably. As of Nov. 14, the number of days this month with maximum relative humidity levels of 90 percent or more recorded at the Yuma Agricultural Center, South Yuma, North Gila, and Roll AZMET stations was 5 percent, 6 percent, 8 percent, and 9 percent, respectively.

These environmental conditions favor the development of downy mildew and suggest increased inspection of lettuce fields for the possible appearance of the disease.

Downy mildew is best managed by having a fungicide in place before disease symptoms become apparent. Good levels of disease suppression can also be obtained by the initiation of fungicide applications at the very first sign of the disease.

However, 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 in 2009 and 2010 at the Yuma Agricultural Center demonstrated that disease severity could be reduced from 70 percent to 86 percent - compared to untreated plants - with treatment programs containing Acrobat, Aliette, Curzate, Presidio, Previcur Flex, Prophyt, Revus, and Tanos.

In the latest field trial completed in 2011 where individual products were evaluated, the reduction in disease incidence was the highest on plants treated with Forum, Curzate, Presidio, Manzate, Actigard, and Reason.

Somewhat lower but still significant efficacy compared to non-treated plants was demonstrated by Revus, Tanos, Previcur Flex, and Cabrio.

Several different modes of action are represented by these compounds facilitating alternation among different chemistries for effective disease management plus pathogen resistance management.

Contact Matheron: (928) 726-6856 or