Powdery mildew, caused by the fungus Golovinomyces cichoracearum (formerly known as Erysiphe cichoracearum), can cause economic losses in lettuce fields harvested in March and April, the last months of the production season in western Arizona. A field study was conducted to evaluate 14 different cultivars of lettuce for potential resistance to this disease. Cultivars Two Star and Big Green COS were very resistant to powdery mildew. All other tested cultivars would have required application of fungicides to reduce the amount of powdery mildew to acceptable levels. On the other hand, planting lettuce cultivars with some disease tolerance may require less fungicide inputs to achieve acceptable disease control compared to planting very susceptible cultivars. Among tested cultivars, Slugger was most susceptible to powdery mildew.

Sclerotinia leaf drop in Arizona is caused by two soil-borne fungi, Sclerotinia minor and S. sclerotiorum. Moist soil and moderate temperature favor this disease. Some registered products as well as new chemistries in development were evaluated for control of leaf drop on lettuce during the winter vegetable growing season in 2002-2003. Sclerotia of each pathogen were applied to plots after lettuce thinning and just before the first application of test compounds. In plots infested with Sclerotinia minor, all materials tested at an appropriate rate significantly reduced disease. The best treatments included an application of Contans followed by an application of Endura (BAS 510), as well as two applications of an experimental compound or the standard materials Ronilan and Rovral. Other useful products included Endura, Serenade, Pristine (BAS 516), Botran, Switch and Contans. In plots infested with S. sclerotiorum, two applications of Contans provided the best level of disease reduction among tested materials. Three applications of Endura or Pristine also were very efficacious. Other compounds that provided some reduction in disease caused by S. sclerotiorum included Botran, Serenade and Switch. Two of the products tested, Contans and Serenade, are biological control materials. For a valid comparison of products for control of Sclerotinia drop of lettuce, it is important to compare the results obtained from more than one field study. The reader is urged to review previous studies in addition to this report to get a true picture of the relative efficacy of tested compounds for control of Sclerotinia drop.

Powdery mildew can occur on melons annually in Arizona. Podosphaera xanthii (Sphaerotheca fuliginea) is the plant pathogenic fungus that causes powdery mildew of cucurbits, such as cantaloupe, honeydew, watermelon, cucumber and squash. When environmental conditions are favorable, disease incidence and severity can reach economically significant levels. Development of powdery mildew on melons is favored by moderate temperatures and relative humidity, succulent plant growth and reduced light intensity brought about by a dense plant canopy. Potential new fungicides were evaluated and compared to existing chemicals for control of powdery mildew of cantaloupe in a field trial conducted during the spring of 2002 at the Yuma Agricultural Center. Among treatments, the degree of powdery mildew control ranged from minimal to essentially complete. One notable observation was the relative decrease in performance of Flint compared to earlier field trials. An isolate of the fungus from this trial was tested at Cornell University and found to be less sensitive to Flint compared to other isolates of the pathogen not previously exposed to this fungicide. This potential development of resistance by the pathogen to Flint will be examined in further studies. A moderately high level of disease had developed by crop maturity (Jun 25) on non-treated plants. The better performing treatments included Cabrio, Flint+pHortress, Foliar Supreme, Microthiol Disperss, Pristine, Procure, Quadris+Latron B-1956, Quadris+LatronB- 1956+Actigard, Quinoxyfen, Rally, Topsin M+Trilogy, and UCC-A1639. The potential availability of chemistries with new modes of action could help improve overall control of powdery mildew as well as facilitate the implementation of effective fungicide resistance management strategies.

Powdery mildew on lettuce is caused by the fungus Golovinomyces cichoracearum (Erysiphe cichoracearum). This disease is favored by moderate to warm temperatures and dry weather conditions. Several potential new fungicides were evaluated for control of powdery mildew on lettuce in 2003. Powdery mildew appeared in our plots by Jan 9 and reached high levels by plant maturity on Feb 19. Compared to non-treated plants, all treatments significantly reduced the final severity of powdery mildew on lettuce statistically. However, only a limited number of compounds, such as Rally, Microthiol Disperss, Quinoxyfen, Flint, Zoxamide, Maneb, Pristine and Cabrio, provided the degree of disease control that would be of value to growers. The trial was intended to be a downy and powdery mildew trial; therefore, some of the treatments within this study were specifically included for downy mildew. No downy mildew developed; however, the downy mildew test products did offer some protection against powdery mildew.