The moist weather pattern in early January which affected Arizona and Yuma County in particular has provided the environmental conditions needed for downy mildew development on winter-grown vegetable crops susceptible to this plant disease. 

Rainfall in the vegetable production regions of Yuma County, plus the associated high relative humidity, provided ideal conditions for the rapid development of downy mildew.

Major vegetable crops in Yuma susceptible to downy mildew include lettuce, spinach, cauliflower, broccoli, cabbage, and onions.

Different pathogens cause the disease on lettuce, spinach, cole crops (cauliflower, broccoli, cabbage), and onions, but the general requirement of free moisture on plant surfaces is needed for disease development on all plant types.

Disease severity is a function of the duration of plant wetness, with more nights and following mornings with wet plants resulting in increasing disease.

Downy mildew is usually not lethal to infected plants, but the infected plants or plant parts can be unmarketable or have reduced quality.  The downy mildew lesions that grow on leaves of lettuce and spinach make infected plant parts unmarketable. 

Infections on cabbage heads render the head unmarketable. For cauliflower and broccoli, severely infected leaves can affect head size. Downy mildew on onion leaves can lead to reduced plant vigor and old downy mildew lesions can serve as infection sites for other onion plant pathogens. 

Preventative fungicide applications

If not already initiated, now is the time to start a preventative fungicide application program for downy mildew on susceptible crops. Waiting until the symptoms of downy mildew are visible before beginning fungicide applications can result in a less than satisfactory level of disease management.

By the time downy mildew lesions are observed, many more are likely present but have not developed to a sufficient extent to be visible.

Yuma fungicide trials

Fungicide evaluation trials at the Yuma Agricultural Center at Yuma, Ariz. and in other states have demonstrated statistically significant reduction in disease by application of fungicides including Actigard, Aliette, Cabrio, Curzate, Forum, Manzate, Orondis, Presidio, Previcur Flex, Prophyt, Ranman, Reason, Revus, and Tanos.

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

Periods of low relative humidity and little or no dew on leaves will help arrest downy mildew development. However, constant vigilance is needed, as future occurrences of dew and rainfall will favor further development and expansion of downy mildew activity.

(Editor’s note: Dr. Matheron’s comments are from the Jan. 6 issue of the University of Arizona’s Vegetable IPM Update e-newsletter and are reprinted with his permission. To subscribe to the free newsletter, send an e-mail to Marco Pena at [email protected] or call (928) 782-3836.)

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