Powdery mildew, caused by the fungus Eryisphe cichoracearum, in years past has appeared on spring lettuce during March and April in western Arizona, when moderate to warm temperatures and dry environmental conditions prevail. However, during the last two seasons, initial detection of powdery mildew occurred as early as December, then became increasingly widespread and severe from February through April.
Successful management of powdery mildew requires the presence of an effective fungicide on plants before disease onset. Additional applications of fungicides are required to maintain disease control until harvest. Sulfur can provide a significant degree of protection against powdery mildew if applied early and often; however, possible burning of leaves may occur if this material is applied at temperatures at or above 90 to 95 degrees.
Several new plant disease control compounds are in development that have activity on the fungi that cause powdery mildew on lettuce and other vegetable crops. A field trial was conducted during the last desert lettuce production season at the University of Arizona Yuma Agricultural Center to test and compare the efficacy of these new chemistries for control of powdery mildew of lettuce.
A planting of Winterhaven lettuce was divided into many 25-foot long plots. Treatment plots were established in a randomized manner and treatments were applied Jan. 18 and Feb. 2, 9, and 16, depending upon the treatment. The severity of disease was determined at plant maturity (March 1) by rating plants within each of the five replicate plots per treatment. The disease rating scale ranged from 0 to 5, with a value of 5 indicating powdery mildew covering the entire lettuce plant.
Yield loss due to rejected lettuce heads normally would begin to occur on plants with a rating above 2.0. In this field trial, nontreated lettuce plants were heavily infected with powdery mildew at plant maturity with a disease rating of 3.7. On the other hand, powdery mildew was highly suppressed (disease rating of 0. I to 0.5) in lettuce plots treated with Kaligreen alternated with Kaligreen plus Rally, Microthiol, Rally, Rally alternated with Serenade, Serenade alternated with Microthiol, EksPunge alternated with Microthiol, Quinoxyfen, Flint, Actigard plus Flint, and KHHUBF-99-001.
Less but still highly significant disease suppression (disease rating 0.7 to 1. 1) was observed on plants treated with BAS 500, Flint, Actigard plus Quadris, Flint alternated with Serenade, Flint alternated with Trilogy, EksPunge, and Kaligreen. No phytotoxicity was evident in any plots treated with fungicides.
The compounds tested in this field study have various modes of action. Some of the materials could be available for "organic" production systems. If continued efficacy is demonstrated and registration is granted for use on lettuce, then the destructive effects of powdery mildew on lettuce could be minimized by application of these materials in effective disease management programs, both for conventional and organic production.
Fungicide resistance management, which seeks to minimize the risk of a plant pathogen population becoming resistant to a fungicide, is imperative for the preservation of fungicide effectiveness. Resistance management can be achieved by applying mixtures of fungicides or alternating between chemistries with different modes of action to prevent or minimize a shift in the pathogen population toward tolerance or insensitivity to one or more disease control compounds.
The future availability of currently nonregistered compounds for lettuce powdery mildew control will help to both maximize the level of disease control and maintain an effective fungicide resistance management program.