1. Target Spot, An Emerging Disease of Cotton
A. K. Hagan and K. L. Bowen, Auburn University, Auburn, AL; R. C. Kemeriat, University of Georgia, Tifton, GA; P. T. Price, Louisiana State University, Winnsboro, LA; N. Dufault, University of Florida, Gainesville, FL; H. L. Mehl, Virginia Tech University, Suffolk, VA; H. M. Young-Kelley, University of Tennessee, Jackson, TN; T. R. Faske, University of Arkansas, Lonoke, AR; T. W. Allen, Mississippi State University, Greenville, MS; and R. L. Nichols, Cotton Inc., Raleigh, NC.
INTRODUCTION
Historically, foliar diseases were not considered a significant threat to U.S. cotton production. Target spot, caused by the fungus Corynespora cassiicola (Berk. & M. A. Curtis) C. T. Wei, was first reported by Jones (9) in Mississippi in 1961 but did not reemerge until 2008 in irrigated cotton in southwest Georgia (10). Subsequently, this disease has spread to all Deep South cotton-producing states (1,2,5,11). Currently, southwest Georgia, Florida Panhandle, and south Alabama comprise the epicenter for target spot in the U.S. Estimated losses of $70 million in 2013 in this area were lower in 2014 and Recent reports of target spot from Brazil (6) and China (14) suggests that this disease may be a global threat to cotton.
SYMPTOMS
Individual leaf spots, which are range from 0.6 to 2.5 cm in diameter, have a distinct ‘zonate’ or ‘target spot’ pattern with alternating light and dark brown bands (Fig. 1A-B). Once multiple lesions develop, symptomatic leaves senesce (Fig. 1C). Typically, lesions first appear on leaves in the lower to mid-canopy shortly after canopy closure, which usually occurs one to three weeks after blooming begins in early to mid-July. Given favorable weather and/or frequent irrigation, rapid disease intensification occurs throughout August until defoliation levels peak around 80% in early September (Fig. 1D-E). While target spot has minimal impact on lint quality, losses on susceptible varieties may exceed 30% of anticipated yields in cotton with a yield potential of 1.7 to 2.2 tonne ha-1 (7).
ETIOLOGY
Corynespora cassiicola, which is a pathogen, endophyte, and saprophyte on 590 plant species in 390 genera, is segregated into 6 phylogenetic lineages based host specialization but not location (3). Sumabat et al. (12) reported that C. cassiicola isolates from cotton across the South are genetically identical but distinct from cucumber, soybean, and tomato isolates. Cotton isolates are much more aggressive on cotton than on other hosts (12).
Intensively managed cotton is most vulnerable to target spot. Frequent mid- to late-summer showers along with irrigation, beginning at canopy closure, intensifies disease. Disease development is suppressed in low-yield cotton that fails to lap the row middles. Cardinal temperatures and moisture patterns favoring disease are not defined nor is the impact of rotation, tillage, and seeding rate on target spot. However, the risk of damaging disease outbreaks declines with increasing distance from the Gulf of Mexico.
CONTROL
Since target spot tolerance among cotton varieties differs (Fig. 2), variety selection is a likely strategy for minimizing yield loss and avoiding costly fungicide inputs (8). However, sizable yield losses have been recorded under ideal conditions for disease development on target spot tolerant cotton varieties, so fungicides may sometimes be needed. In southwest Alabama, producers have largely shifted from target spot susceptible to tolerant cotton varieties (Hagan, personal observation).
Fungicides are widely used in high risk areas in Alabama, Florida, and Georgia to protect intensively managed cotton from target spot but are often unnecessary in other southern states where serious disease outbreaks are sporadic. Despite the modest efficacy of ground-applied fungicides against target spot (4,8), lint yield protection from properly timed single or two applications may reach 280 kg ha-1 on susceptible varieties with lesser yield protection seen on tolerant varieties (7).
With the current market price below $0.27 per kg of lint, the economics of fungicide use for target spot control on cotton is questionable across much of the South. Increased emphasis needs to be placed on the establishing target spot tolerant varieties and limiting fungicide inputs to intensively managed cotton as needed based on scouting reports.
Figure 1. Target spot in cotton is A) characterized by ring or ‘target’ pattern, B) gray to brown lesions, and C) chlorotic leaves with multiple lesions that are prematurely shed. Lesions first appear on D) leaves in lower and mid canopy after canopy closure and E) 80% defoliation on a susceptible variety 6 weeks after disease onset.
Figure 2. Defoliation on A) non-fungicide treated control and B) fungicide-treated target spot susceptible varieties Phytogen 499 and Fibermax 1944 as compared with the target spot tolerant varieties Deltapine 1252 and Deltapine 1538.
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