Fungicide treatment and varietal effects on Alternaria leaf spot of Pima cotton
Mary W. Olsen, Plant Pathology Department
Lee Clark, Safford Agricultural Center
Hal Moser, Maricopa Agricultural Center
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.
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
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
treatment or untreated control. Mancozeb was the most effective treatment and
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
higher numbers of lesions per leaf than S-6 or UA 4, with UA 4 having
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
at different application dates in order to compare higher lesion numbers on
Trials carried out when weather conditions are more conducive to disease
give growers the information needed to decide if and when to make foliar
- Cotty, P. J. 1987. Evaluation of cotton cultivar susceptibility to Alternaria leaf spot. Plant Disese 71:61082-1084.
- Shtienberg, D. and J. Dreishpoun. 1991. Suppression of Alternaria leaf spot in Pima cotton by systemic fungicides. Crop Protection 10: 381-385.
- 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.
- 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.
This is a part of publication AZ1006:
"Cotton: A College of Agriculture Report," 1998, College of Agriculture, The University of
Arizona, Tucson, Arizona, 85721.
Any products, services, or organizations that are mentioned, shown, or indirectly
implied in this publication do not imply endorsement by The University of Arizona.
The University is an Equal Opportunity/Affirmative Action Employer.
This document located at http://ag.arizona.edu/pubs/crops/az1006/az100610a.html
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