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.

Five different fungicides, including azoxystrobin, dimethomorph, fluazinam, fosetyl-Al, and mefenozem (metalaxyl), were evaluated for their ability to inhibit the development of root and crown rot and increase the survival of chile pepper plants grown in soil naturally infested with Phytophthora capsici. For chile pepper plants grown in field soil naturally infested with P. capsici and subjected to a 48 h flood period every 2 weeks, growth and survival of plants receiving one treatment of dimethomorph at 100 μg/ml or fluazinam at 1,000 μg/ml were significantly greater than that for plants treated once with azoxystrobin at 1,000 μg/ml or fosetyl-Al at 3,000 μg/ml. For each tested fungicide, values for duration of plant survival and shoot and root fresh weight usually were numerically larger but not significantly different for chile peppers receiving water as needed compared to those flooded for 48 h every 2 weeks. The potential and relative value of azoxystrobin, dimethomorph, fosetyl-Al, and fluazinam as chemical management tools for Phytophthora root and stem rot on chile pepper, in addition to mefenozem (metalaxyl), has been demonstrated.

Sclerotinia minor and S. sclerotiorum are the two soil-borne pathogenic fungi that cause Sclerotinia leaf drop in Arizona. Moist soils and moderate temperature favor this disease. Some new fungicides in development were evaluated for control of leaf drop on lettuce during the winter vegetable growing season of 1999-2000. Sclerotia of each pathogen were applied to plots after thinning and just before the first of two applications of test compounds. In this trial, the final count of dead lettuce plants usually was numerically reduced, compared to nontreated plots, in plots infested with either pathogen that were treated with the standard compounds Ronilan or Rovral as well as the experimental compound Medallion; however, the reduction was significant only in plots infested with Sclerotinia sclerotiorum. The number of dead plants in plots infested with either Sclerotinia minor or S. sclerotiorum was significantly reduced by another experimental chemistry, Fluazinam. Finally, a biological control product, Serenade, significantly reduced disease in plots containing S. minor, but not S. sclerotiorum. Continued demonstration of efficacy by Serenade may provide the opportunity to utilize a biological control product to reduce the incidence of Sclerotinia leaf drop of lettuce caused by S. minor.

Nine different cultivars of lettuce were planted at the Yuma Valley Agricultural Center. Plants were treated four times with two different rates of Actigard. Control plants were not treated. Near plant maturity (Mar. 23, 2000) the severity of powdery mildew was recorded. Among nontreated plants, Cibola, Conquistador, and Coolguard had the lowest levels of powdery mildew. On the other hand, Bos 9003 and Mohawk had significantly higher levels of powdery mildew than all other tested cultivars. Four applications of Actigard at a rate of 14 g. of active ingredient per acre significantly reduced the level of powdery mildew on all nine tested cultivars of lettuce. On two lettuce cultivars, powdery mildew was significantly lower when treated with Actigard at the 28 g. a.i./A rate when compared to the 14 g. a.i./A rate. No evidence of phytotoxicity was apparent on plants treated with Actigard. This study suggests that lettuce cultivars differ in susceptibility to powdery mildew. Also, Actigard was able to provide added protection against powdery mildew for all tested lettuce cultivars.

A field test was established in 1999 to determine the effect of preplant soil fumigation on yield of chile pepper in southeastern Arizona in order to give growers data on which to base management decisions. Replicated plots within a nematode-infested field planted with New Mex 6-4 chile in March 1999 were either treated with Telone II fumigant at 7 gal/A two weeks before planting or not treated. In a mid-season assay in July 1999, the effects of fumigation were evident in plant canopy growth although numbers of J2/cc soil were not significant between treatments (p=0.058). Differences in yields between fumigated plots and untreated plots were significant (p=0.014). The average yield in fumigated plots was 12.4% higher than that in untreated plots.

Powdery mildew on lettuce is caused by the fungus 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 2000. Powdery mildew appeared in our plots by Feb 9 and reached high levels by plant maturity on Mar 2. Nontreated lettuce plants were heavily infected with powdery mildew at plant maturity, whereas the disease ranged from low to virtually nonexistent levels in plots treated with BAS 500, Quadris+Actigard, Flint, Flint+Actigard, Flint alternated (alt.) with Trilogy, Rally, Microthiol, EksPunge alt. with Microthiol, KHHUBF-99-001, Quinoxyfen, Flint alt.with Serenade, Rally alt. with Serenade, and Serenade alt. with Microthiol. These compounds have various modes of action, and some could be available for “organic” production. The future availability of one or more of these chemistries under development could help in efforts to control powdery mildew of lettuce and to establish and maintain a fungicide resistance management program for plant disease control products of importance for this crop.

Powdery mildew on melons is an annual disease problem in Arizona. Sphaerotheca fuliginea is the plant pathogenic fungus that causes powdery mildew of cucurbits, which include cantaloupe, honeydew, watermelon, cucumber and squash. When environmental conditions are favorable, disease incidence and severity can reach economically significant levels. Factors that favor development of powdery mildew on melons include 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 1999 at the Yuma Agricultural Center. A high level of disease had developed by crop maturity (June 29). On nontreated plants 43% of the upper leaf surface was covered by powdery mildew, whereas the level on the underside of leaves was 78%.. All of the 34 different treatments significantly reduced the level of powdery mildew on both sides of leaves, compared to nontreated plants. The best treatments among those tested with respect to disease control on the underside of leaves, where disease control is more difficult than on the tops of leaves, included Topsin+Trilogy, Benlate, Benlate+Trilogy, Quadris, A815, Topsin+Microthiol, and Topsin. The potential availability of new chemistries for management of powdery mildew of cantaloupe and other cucurbits could help improve overall control of powdery mildew as well as the implementation of fungicide resistance management strategies, which strive to minimize the risk of resistance development by the pathogen to these compounds.