Fungicide treatment and varietal effects on Alternaria leaf spot of Pima cotton

Abstract

The effect of foliar treatments for prevention of Alternaria leaf spot was evaluated in the field on six varieties of Pima cotton. Disease was significantly reduced by protective sprays of mancozeb and micronized sulfur but not by foliar applications of urea in trials at the University of Arizona Safford Agricultural Center in Safford, AZ.. Treatments had no significant effects on yields. Significantly fewer lesions developed on Pima variety UA 4 than on the other varieties. Disease pressure was relatively light, and even though scheduled preventive sprays with mancozeb were effective, fungicide applications probably would not increase yields under the environmental conditions of this experiment.

Introduction

Alternaria leaf spot of cotton, caused by the fungus Alternaria macrospora, causes lesions on leaves, bracts, and bolls of cotton. Disease is common in Arizona only under very humid conditions, and is usually associated with the onset of rains in the summer months at higher elevations. In the Safford Valley and other cotton growing areas of Graham, Greenlee, Cochise, and Pima Counties, disease can be severe on Pima cotton, causing defoliation if rain and high humidity are persistent in July, August and September. Variations in susceptibility among cotton cultivars have been reported (Cotty, 1987). Pima variety S5 was more susceptible than S6, and both Pima varieties were more susceptible than DP90 or other upland varieties. There is no data available comparing susceptibility of newer Pima varieties. Although older leaves are believed to be more susceptible to infection, disease development is not related to plant age (Shtienberg, 1993). Late season infections usually are not considered a problem since yields are probably not affected.

There are currently no fungicides registered for use on cotton for control of Alternaria leaf spot in Arizona. In other cotton growing regions of the world where disease is a problem and causes yield losses, fungicides such as maneb, mancozeb, difenoconazole and tebuconazole are used as protectant sprays (Shtienberg, 1991, 1992). Fungicides may be applied as often as every ten days to two weeks and initiated before flowering. Because of the restricted occurrence ofAlternaria leaf spot in the United States, especially in the Southwest, it is unlikely that new fungicide labels will be forthcoming for disease control. Therefore, it is important to determine the efficacy of foliar treatments that have current labels on cotton for the control of Alternaria leaf spot.

The objectives of this study were to (1) determine if preventive sprays of candidate foliar treatments would reduce disease incidence significantly; (2) demonstrate varietal susceptibility of Pima cotton to Alternaria leaf spot; (3) determine the effect of disease on yield; and (4) generate data for effective foliar treatments that would lead to a label for use on cotton for control of Alternaria leaf spot of cotton.

Materials and Methods

This study was conducted at the University of Arizona Safford Agricultural Center. Six varieties of Pima cotton were planted according to standard practices in 4 row plots 15 m long with 3 replications in a randomized block design. Varieties were OA 361, UA 4, OA 312, OA 325, S6 and S7. Varieties S6 and S7 are currently available for commercial use and are planted in the Safford Valley; varieties OA 361, OA 312 and OA 325 are short season varieties developed by Olvey and Associates and are well suited to the Safford Valley; UA 4 is a short season variety developed by the University of Arizona Pima Breeding group.

Foliar treatments for disease control - mancozeb, sulfur, and urea - were selected on the basis of their potential availability for use and their cost. Mancozeb already has a registration on cotton for prevention of cotton rust and is registered for use on other crops for control ofAlternaria diseases; micronized sulfur was applied because of its combined fungicidal and insecticidal potential and current registration on cotton for mite control; and urea, a known greening agent, was evaluated since disease has been shown to be related to leaf age (Shtienberg, 1993). Mancozeb was applied as 1.75 lb/ac Penncozeb 75df, sulfur as 7.5 lb/ac Microthiol Special, and urea as 5 lb/ac foliar urea by ground spray application on August 15, September 1, and September 15, 1997. These three application dates at two week intervals were chosen since more than two or three applications are considered economically unfeasible at current cotton prices and the six week interval was considered reasonable for disease control based on other studies (Shtienberg, 1992 ). Control plots were not treated. Treatments were applied along the varietal plots in a split-plot design.

Disease was assessed on September 19, 1997 by counting the number of lesions on leaves at the fifth node down from the terminal node. Ten leaves were sampled from each plot and taken to the laboratory where Alternaria lesions were counted. Lint yields were determined at harvest by mechanical harvesting of plots and assuming 35% lint. Data were analyzed using the General Linear Models Procedure and Duncan's Multiple Range Test of SAS.

Results and Discussion

As shown in Table 1, the number of lesions per leaf was reduced significantly by the application of mancozeb and sulfur compared to the urea treatment or untreated control. Mancozeb was the most effective treatment and reduced lesions by more than 50%. However, the average number of lesions per leaf, even in the untreated controls, was low. Environmental conditions were not favorable for disease in 1997, and disease pressure was relatively light compared to years with higher humidity and more rain in July and August. Treatments had no significant effect on yields (Table 1). There was a varietal effect on the number of lesions per leaf (Table 2). Variety OA 325 had significantly higher numbers of lesions per leaf than S-6 or UA 4, with UA 4 having significantly fewer lesions per leaf than all other varieties. OA 312 had a significantly higher yield than UA 4, but was not different from the other varieties.

Results indicate that Alternaria leaf spot will not reduce yields of Pima cotton under the environmental conditions of these trials, and foliar applications would not be warranted. However, treatments of Pima cotton should be repeated in years of higher disease pressure and at different application dates in order to compare higher lesion numbers on yields. Trials carried out when weather conditions are more conducive to disease development would give growers the information needed to decide if and when to make foliar treatments.

Literature cited

1. Cotty, P. J. 1987. Evaluation of cotton cultivar susceptibility to Alternaria leaf spot. Plant Disese 71:61082-1084.
2. Shtienberg, D. and J. Dreishpoun. 1991. Suppression of Alternaria leaf spot in Pima cotton by systemic fungicides. Crop Protection 10: 381-385.
3. Shtienberg, D. 1992. Development and evaluation of guidelines for the initiation of chemical control of Alternaria leaf spot in Pima cotton in Israel. Plant Disease 76: 1164-1168.
4. Shtienberg, D., Y. Kremer and A. Dinoor. 1993. Influence of physiological age of Pima cotton on the need for fungicide treatment to suppress Alternaria leaf spot. Phytopathology 83: 1235-1239.