Managing Mid- to Late-Season Diseases in Cotton

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KEY POINTS

  • From fruiting to defoliation, boll rots, foliar diseases, cotton root rot, and wilt diseases are the main diseases to watch for in cotton.
  • Few management options are available for mid- to late-season cotton diseases.
  • Minimizing cotton stress early in the season can help reduce the severity of mid- to late-season foliar diseases and help maintain cotton yield potential.

Boll Rots

Rank growth from periods of warm, humid, and rainy weather conditions as well as excessive nitrogen and/or irrigation applications can lead to boll rots in cotton. Applying a plant growth regulator (PGR) to help control plant growth and increase air circulation in the canopy can reduce boll rots. In areas that are more susceptible, wider row spacing and defoliation can help reduce incidence of boll rots. Insect damage can provide an entryway for boll rots into the cotton plant; therefore, controlling mid- to late-season insects may help reduce boll rot damage.1 Boll rots generally appear first as water-soaked brown or reddish-brown lesions on the boll capsule or bracts. The infection can then spread, turning the boll black (Figure 1). Severe infection may cause bolls to fall off the plant.2

Foliar Diseases

Late-season foliar diseases like Alternaria (Figure 2) are commonly associated with moisture stress and fertility problems in cotton, especially potassium deficiency. The crop should be supplied with adequate nutrients and water throughout the growing season. Crop residue should be chopped and plowed to accelerate decomposition. Rotate to non-host crops and plant varieties resistant to local foliar diseases.2

Cotton Root Rot

Cotton root rot is a fungal disease that is of major economical concern in the southwestern United States. Symptoms include leaf yellowing or bronzing followed by wilting at the top of the plant, then at the bottom, and finally complete wilt by the third day. Plants die with leaves secured to the plant. This fungus can survive in the soil for many years; therefore, heavily infected soils may benefit from a three to four year crop rotation with a sorghum or corn crop.3

Boll Rot

Figure 1. Boll rot.

Wilt Diseases

Fusarium and Verticillium wilts are difficult to manage soil borne pathogens. The severity of Fusarium wilt can be increased by the presence of root-knot nematodes. Verticillium wilt is widespread across the Cotton Belt with a higher incidence during cool, wet growing conditions.1

Differentiation between Fusarium or Verticillium wilt can be difficult as symptomology of the two diseases are similar. Stem discoloration is typically more evenly distributed with Verticillium wilt (Figure 3). For confirmation of a wilt disease, the bottom stems of cotton plants may be sent to a diagnostic lab. Refer to your local university extension service for sampling procedures.2

Alternaria

Figure 2. Alternaria leaf spot.

Verticillium

Figure 3. Stem discoloration from Verticillium wilt.

Proper fertility management, crop rotation to corn or soybean, nematicides, and cotton variety selection can help reduce yield losses from wilt diseases. Cotton varieties with improved levels of resistance to Fusarium and Verticillium wilt and nematode resistance are available to help reduce disease incidence.4

Cotton Diseases