Figure 1. Leaf spot caused by Phoma exigua (Ascochyta) on cotton leaf. Figure 2. Pycnidia of the fungus P. exigua in a leaf spot on cotton. Figure 3. Cotton stem canker caused by P. exigua.

Phoma exigua is a fungus that infects numerous species of plants, including common bean, soybean, sunflower, and possibly corn. Several cotton diseases can be attributed to this organism. The leaf spot associated with wet weather blight is characterized by a brown or gray spot usually 1 to 4 mm in diameter (it can be larger in severe cases) surrounded by a red halo (Figure 1). At later stages brown to black specks called pycnidia, which contain the spores of the fungus, may be visible in the lesion (Figure 2). If the leaf spot progresses, cotyledons and young leaves may turn brown and die. The most prominent symptom of the stem canker phase of wet weather blight is often a leaf or leaves that have wilted or died (Figure 3). Below the wilted leaves (sometimes at that node or several nodes lower) is a black canker. The sunken canker at a node is the best diagnostic symptom for cotton stem canker (Figure 3). As the cankers become older, they become brown with a shredded appearance. Often, the lower part of the plant is relatively healthy, although the lower leaves may have dropped off as a result of the leaf spot phase of the disease. The disease may be confused with Fusarium wilt because of the wilting and dark streaks in the vascular system. Typically, with cotton stem canker the streaks in the stem will not extend to the root system as they do with Fusarium or Verticillium wilt. Late in the season, plants infected with cotton stem canker may be brittle and break off in high winds.

Environmental Factors in Disease

Cool, wet weather is required for development of this disease in cotton. Leaf spot caused by P. exigua is common most years in North Carolina and can contribute to damping-off of cotton seedlings. Typically, dryer and warmer weather will prevent further infections and the disease cycle ends. Wet weather in combination with cool temperatures (night temperatures below 60 F) can result in infection of the main stem at a node. If cool wet weather persists, the stem may be girdled causing the upper portion of the plant to wilt and die. Infection and spread of the disease is favored by a combination of factors: the presence of juvenile plant tissue, low temperatures that damage this tissue, and wet weather. Cotton plants may be defoliated and appear dead as a result of the continued progress of the disease. Infected stems on older plants may break off easily from the rest of the plant.

Impact on Yield

Yield loss related to cotton stem canker is highly variable. In fields with more than adequate stands and only 1% to 5% of plants infected, the impact of the disease will be minimal. Many infected plants will survive but may be so late as to be unproductive and thus act like weeds. Cotton fields with a high incidence of stem canker may mature late. The biggest challenge for growers will be in making decisions as to management of an irregular crop. Losses of as much as 1% of cotton lint yield statewide in North Carolina have been reported.

Management

Measures to prevent this disease are limited. Currently, no fungicides are labeled for foliar application to cotton in the southeast. All cotton varieties are probably susceptible to P. exigua. The fungus is very common in North Carolina and affects several crops including soybean as well as numerous weed species. Rotation will thus have little impact on disease. Cultural practices such as tillage do not appear to influence disease incidence.

Other Links

Cotton Pest Management Strategic Plan for the MidSouth
NC Agricultural Chemicals Manual

Recommendations for the use of chemicals are included in this publication as a convenience to the reader. The use of brand names and any mention or listing of commercial products or services in this publication does not imply endorsement by the North Carolina Cooperative Extension Service nor discrimination against similar products or services not mentioned. Individuals who use chemicals are responsible for ensuring that the intended use complies with current regulations and conforms to the product label. Be sure to obtain current information about usage and examine a current product label before applying any chemical. For assistance, contact your county North Carolina Cooperative Extension Service agent.

Distributed in furtherance of the acts of Congress of May 8 and June 30, 1914. North Carolina State University and North Carolina A&T State University commit themselves to positive action to secure equal opportunity regardless of race, color, creed, national origin, religion, sex, age, or disability. In addition, the two Universities welcome all persons without regard to sexual orientation. North Carolina State University, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.

This webpage last updated January 29, 2004 by Tom Creswell.