Cotton seedling diseases
Seedling disease of cotton in Arizona is caused primarily by Rhizoctonia solani and to a lesser extent by Thielaviopsis basicola.
Rhizoctonia is a soil borne fungus found throughout all cotton growing areas. It commonly causes post emergent damping-off or collapse of seedlings when plant growth is slowed by low soil temperatures, residual herbicide phytotoxicity, planting too deep, poor seed vigor or crusting. Infected seedlings may have sunken reddish colored lesions on the upper root (photo 1), or roots may be soft and darkened with a noticeable constriction at the soil line. Fungicide treatment of seed reduces disease, but the best prevention is planting into soils when temperatures are favorable for fast germination, using high quality seed. Fields with a history of severe disease should be rotated out of cotton.
Thielaviopsis also is a soil borne fungus that is easily found in soils throughout the state but causes disease only when soils are relatively cold and wet. It causes a dark colored root rot that also gives the disease the name "Black root rot" (photo 2). Seed treatments are recommended for fields with a history of disease.
