Infection, colonization of Gossypium hirsutum and Gossypium barbadense, and development of black root rot caused by Thielaviopsis basicola

On 28 March and 28 April 1986 and on 12 April and 16 May 1988, Gossypium barbadense was planted in naturally infested cotton fields containing approximately 600 colony-forming units (cfu) of Thielaviopsis basicola per gram of air-dried soil. Soil temperatures ranged from 18 to 20 C and 24 to 26 C at a depth of 15 cm during the first and second planting dates, respectively, during both seasons. During the 1986 and 1988 seasons, 100% of seedlings sampled 1 mo after the first planting were infected, with the severity of cortical root decay ranging from 75 to 100%. Two weeks after the second planting, 89% of the seedlings sampled in 1986 and 92% of the seedlings sampled in 1988 were diseased with 50-75% cortical root decay during both seasons. Cotton stand counts made at 1 and 2 mo after seedling emergence indicated that the stand was reduced by 28% in 1986 and 32% in 1988 in the first planting and 11% in 1986 and 8% in 1988 in the second planting when compared with the original number of seedlings in each planting. In October 1986, 32% of the plants from the March planting and 5% of the plants from the April planting had darkened stelar root tissue near the crown that contained hyphae and aleuriospores of T. basicola in xylem and phloem tissue. Gossypium hirsutum was grown at 20 and 28 C under controlled conditions in soils containing T. basicola at 0.90, and 600 cfu/g of soil. Seedlings were evaluated 10 days after seedling emergence. Seedling stunting increased with inoculum density at both temperatures when compared with the control plants without the fungus. However, levels of disease increased at the low-temperature or high-inoculum-density treatments. Scanning electron microscopy demonstrated that phialoconidia and aleuriospores germinated and germ tubes penetrated within 12 and 48 hr, respectively, after root inoculation at 24 C. Germ tubes of both spores produced appressoria before penetration. Hyphae colonized cortical tissue 72 hr after inoculation and grew intracellularly and centripetally. Five days after inoculation, infected cells were filled with hyphae and aleuriospores.