Identifying Early-Season Cotton Diseases

KEY POINTS

  • Several species of soil-borne and seed-borne fungi can cause cotton seedling diseases.
  • The fungi can attack the seed prior to or during germination, or attack seedlings after emergence.
 
 

Figure 1. Seedling root damage from Pythium. Source: Howard F. Schwartz, Colorado State University, Bugwood.org.

Pythium spp.

  • These fungi are classified as water molds, and produce spores that move in the soil water.
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  • Pythium fungi are most commonly problematic in soils that have remained saturated for several days.
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  • Cotton seedlings infected with Pythium usually have a water-soaked, almost translucent lesion at the soil line and the outer root layer can be peeled back, creating a ‘wire root’ appearance (Figure 1).
 

Figure 3. Cotton seedlings infected by Ascochyta gossypii. Source: Clemson University - USDA Cooperative Extension Slide Series, Bugwood.org.

Rhizoctonia solani.

  • Damage to seedlings is often described as “sore-shin”.
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  • Plants injured by sand blasting are particularly susceptible to Rhizoctonia.
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  • Less dependent on wet, cool conditions than other seedling diseases, and can be found in either wet or dry soils under warmer soil temperatures.
  • Reddish brown lesions girdle the stem at the soil line. If the seedling survives, the stem will be weakened at the lesion site and plant growth may be stunted.
 

Figure 3. Cotton seedlings infected by Ascochyta gossypii. Source: Clemson University - USDA Cooperative Extension Slide Series, Bugwood.org.

Phoma exigua (Ascochyta gossypii).

  • Damage often occurs post-emergence; cotyledons may turn brown and die. This fungus may occur throughout the growing season, but can cause more damage to young cotton plants.
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  • Disease may be more common in foggy conditions and when night temperatures are in the 50s.1
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  • A. gossypii may also be referred to as Wet Weather Blight or Cotton Stem Canker.
  • In the most serious cases, the fungus attacks the hypocotyl, killing affected plants and possibly destroying many plants within a field.
  • The disease is usually sporadic and plants may recover once warm, dry weather returns.3
 

Figure 4. Figure 4. Damaged vascular tissue from Fusarium infection. Source: Clemson University - USDA Cooperative Extension Slide Series, Bugwood.org.

Fusarium spp.

  • There are several species of Fusarium that infect cotton seedlings.
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  • Fusarium can sometimes be found in conjunction with root-knot nematode infestations, as nematodes can injure young roots and increase disease severity.
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  • Plants may be stunted with interveinal chlorosis in the leaves. The most obvious symptom of fusarium is a group of wilted plants where adequate soil moisture exists.
  • Damage to the vascular system limits soil moisture and nutrient uptake, eventually causing plant death.
  • Infected plant stems may be carefully sliced open to reveal damaged, brown vascular tissue.2
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