What's Attacking my Cotton?

Root-knot, reniform, lance and sting nematodes are the four major species of nematodes present throughout the cotton-producing states. Fusarium Wilt and Verticillium wilt are diseases that also can result in significant damage.

Root-knot nematode damage in Louisiana                                Louisiana State University

• Root-knot nematodes are named for the galls, which look like knots in a rope, that they create on plant roots. These nematodes are some of the most dangerous nematodes and are found in cotton fields across the nation. The density of root-knot nematodes is typically scattered within an infested field. They can be found in nearly any field, but are most commonly found in sandy soils. The occurrence and severity of Fusarium wilt and Verticillium wilt is increased significantly in fields with root-knot nematodes.
  
  Potential host weeds: Prickly sida, smallflower morningglory, ivyleaf morningglory, bermudagrass, johnsongrass, cocklebur, goosegrass, red root pigweed, purple nutgrass, yellow nutsedge, purple nutsedge, smartweed, teaweed, lambsquarter, ground cherry, crotolaria, sicklepod

• Reniform nematodes, the most difficult nematodes to detect, are commonly found starting from North Carolina and moving south along the Atlantic coast, across the Gulf Coast and into Texas. Reniform nematodes do not cause galls, as they feed on the outside of the roots. They can prosper and quickly reproduce in a variety of soils, and cause increased occurrence and severity of cotton-seedling diseases. Reniform densities tend to be uniform within an infected field. Typically, infestations are the result of both reniform and root-knot nematodes. Fields with a high density of reniform nematodes could cause greater than a 70 percent loss in yields.
  
  Potential host weeds: Florida beggarweed, purple nutsedge, sicklepod, smallflower morningglory, crotolaria, cocklebur, sow thistle, jimsonweed, Florida pusley, velvetleaf

• Lance nematodes, specifically Columbia lance nematodes, are the least widespread nematode, populating several states in the southeastern United States. Lance nematodes tend to be scattered throughout an infected field and cause a stunted taproot and reduced root system. They may increase occurrence and severity of seedling diseases and Fusarium wilt. Crop rotation is not an effective management tool for lance nematodes because they thrive on a wide range of hosts. Severe infestations of lance nematodes could result in up to a 60 percent yield loss.
  
  Potential host weeds: Nutsedge, pigweed, sicklepod, henbit, crimson clover, showy crotalaria

• Sting nematodes result in reduced root and plant growth, and the root tip often swells, causing the root to appear to be cut off. Sting nematodes prefer sandy soils and are commonly found along the Atlantic and Gulf Coasts. They cause the most damage to young plants with developing root systems, remaining in the soil and feeding on root tips. Sting nematodes generally need at least 80 percent sand content in soils to survive.
  
  Potential host weeds: Beggarweed, cockcrabgrass, cudweed, dogfennel, johnsongrass, morningglory, ragweed, wild carrot

• Fusarium wilt quickly spreads throughout the field after infection. The fungus may attack cotton seedlings, but the disease usually appears when the plants are more mature. Affected plants are at first darker green and stunted, followed by yellowing of the leaves and loss of foliage. Symptoms first appear on lower leaves around the time of first flower. The leaf margins wilt, turn yellow and then brown while moving inward. Infected plants fruit earlier than normal with smaller bolls that open prematurely.
  
  
• Verticillium wilt causes young plants to exhibit yellow leaves and stunting, and often death. Following the seedling stage, older plants display a chlorotic mottling on the leaf margins and between the major veins. Plants attacked during later stages of growth display a mottling on the lower leaves first, progressing toward the top of the plant as the season progresses. A single branch can often show symptoms in the early stages of disease. Yellowing progresses inward, followed by browning and death. Verticillium wilt is widespread among fields growing cotton and causes the most damage during cool and wet growing seasons.