Evaluation of New Fungicides for Management of Downy and Powdery Mildew of Lettuce in 1998

Michael E. Matheron and Martin Porchas


Bremia lactucae and Erysiphe cichoracearum, respectively, cause downy and powdery mildew of lettuce. Cool moist environmental conditions favor the development of downy mildew, whereas warm and dry weather is conducive for powdery mildew. Several potential new fungicides were evaluated for control of these diseases of lettuce in 1998. The final severity of downy mildew in this trial was low. In addition to the standard compounds maneb, Aliette and Trilogy, several fungicides currently in development significantly reduced the severity of downy mildew compared to nontreated plants. These chemistries included Acrobat, RH-7281, an Unknown, Actigard, EF1295, Curzate, Quadris, BAS 500, QST 153, BAS 505 and BAS 490. Untreated lettuce plants were heavily infected with powdery mildew. In addition to the standard materials Microthiol Special and Trilogy, powdery mildew was significantly reduced on plants treated with BAS 490, BAS 505, EF1295, BAS 500 and Quadris. The possible availability of one or more of these chemistries under development for lettuce could help in efforts to develop and maintain a fungicide resistance management program for plant medicines of importance for this crop.


Bremia lactucae, the fungus that causes downy mildew on lettuce, can cause serious losses on this crop when prolonged cool to mild and moist conditions occur, which favor disease development. On the other hand, powdery mildew caused by Erysiphe cichoracearum develops on spring lettuce in western Arizona, when warm dry environmental conditions prevail. Lettuce planted in November in western Arizona is subject to downy mildew during periods of moist mild weather and to powdery mildew as the crop nears maturity, when warm dry weather conditions conducive to this disease predominate.

When available, effective control of downy mildew on lettuce can be achieved by planting varieties that are tolerant or resistant to the races of the pathogen present in the area. If lettuce varieties susceptible to downy or powdery mildew are grown, it is extremely important to have fungicidal protection in place when environmental conditions become favorable for the development of each disease.

Downy mildew of lettuce can be significantly reduced by timely applications of compounds such as maneb, fosetyl-Al (Aliette) and mefenoxam (Ridomil Gold). Sulfur, which is an effective powdery mildew control agent, can be used to manage this disease on lettuce. Several new agrochemicals are in development that have activity on the pathogenic fungi that cause downy and powdery mildew. A fungicide trial was initiated during the 1997-98 vegetable season to test the potential efficacy of these new chemistries on downy and powdery mildew of lettuce.

Materials and Methods

This study was conducted at the Yuma Valley Agricultural Center. The soil was a silty clay loam (7-56-37 sand-silt-clay, pH 7.2, O.M. 0.7%). Lettuce ‘Barnburner’ was seeded and watered November 4, 1997 on double rows 12 inches apart on beds with 40 inches between bed centers. Treatments were replicated five times in a randomized complete block design. Each replicate consisted of 25 feet of bed, which contained two 25 foot rows of lettuce. Plants were thinned December 12 to a 12 inch spacing. Treatment beds were separated by single nontreated beds. Fungicide treatments were applied with a tractor-mounted boom sprayer (hollow-cone nozzles spaced 12 inches apart) that delivered 50 gallons/acre at 100 psi. Foliar applications of fungicides were made January 8 and 27, February 11, 20 and 26 and March 2, depending on fungicide treatment. Maximum and minimum ranges ( F) of air temperature were as follows: December 1997, 53-72, 34-51; January 1998, 62-77, 33-48; February, 63-73, 39-52; March 1-12, 66-83, 37-52. Total rainfall (inches) was as follows: December, 1.47; January, 0.04; February, 0.99; March 1-12, 0.00. Furrow irrigation was used for the duration of this trial. The severity of downy mildew caused by Bremia lactucae was determined at plant maturity (Mar 11-12) by counting the number of leaf lesions caused by the pathogen on 10 plants randomly selected from each of the five replicate plots per treatment. The severity of powdery mildew caused by Erysiphe cichoracearum was determined at plant maturity by rating 10 plants randomly selected from each of the five replicate plots per treatment using the following rating system: 0 = no powdery mildew present; 1 = some colonies of powdery mildew present, with light infection on a few leaves; 2 = moderate colonization of several leaves; 3 = heavy colonization of many leaves.

Results and Conclusions

A relatively low level of downy mildew had developed by crop maturity in this trial. The standard treatments Maneb, Aliette and Trilogy, as well as many of the new compounds tested, including Acrobat, RH-7281, Actigard, EF1295, Curzate, Quadris, BAS 500, QST 153, BAS 505 and BAS 490, significantly reduced the number of downy mildew lesions compared to nontreated plants (Table 1). On the other hand, the level of powdery mildew in the plots was very high at crop maturity. The standard treatments Microthiol Special and Trilogy, as well as some of the chemistries in development, including BAS 490, BAS 505, EF 1295, BAS 500 and Quadris, provided a significant level of powdery mildew control compared to nontreated plots. No phytotoxicity was evident in any plots treated with fungicides.

Bremia lactucae, which causes downy mildew of lettuce, and Erysiphe cichoracearum, the causal agent of powdery mildew, are very unrelated organisms. Most fungicides currently registered for use on lettuce are effective at inhibiting one or the other but not both of these pathogens. On the other hand, this study reveals that some of the new fungicides in development have activity against both mildew pathogens.

Fungicide resistance management, which seeks to minimize the risk of a plant pathogen population becoming resistant to one or more fungicides, is imperative for the preservation of fungicide effectiveness. Resistance management is achieved by applying mixtures of fungicides or alternating between different classes of chemistries to prevent or minimize a shift in the pathogen population toward tolerance or insensitivity to one or more disease control compounds. The future registration and subsequent availability of some of these new chemistries for lettuce could help in the implementation of an effective fungicide resistance management program for compounds of importance to this crop.

Table 1. 1998 Downy mildew and powdery mildew of lettuce fungicide trial Michael Matheron and Martin Porchas, Yuma Agricultural Center, University of Arizona.

Table 1 (continued)

This is a part of publication az1101: "1998 Vegetable Report," College of Agriculture, The University of Arizona, Tucson, Arizona, 85721.
This document located at http://ag.arizona.edu/pubs/crops/az1101/az1101_20.html
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