Influence of Meloidogyne incognita and Thielaviopsis basicola populations on early-season disease development and cotton growth

Microplot studies were used to examine the effect of various population den sities of Meloidogyne incognita and Thielaviopsis basicola on cotton-plant development and disease severity. Plots were infested with 0, 20, or 100 T. basicola chlamydospores/g and 0, 5, or 10 M, incognita eggs and juveniles/ cm(3) of soil in a factorial arrangement in 1997 and 1998. Combinations of M. incognita and T. basicola reduced plant survival in both years compared to the noninfested control, except in 1998 for the high rate of T. basicola over all nematode rates. Plant height-to-node ratios were reduced by patho gen combinations compared to the noninfested control or to either pathogen alone. Plant dry weight was reduced by M. incognita in 1998 and the high ra te of I basicola in 1997. Root necrosis was increased by increasing rates o f ir: basicola in 1997 and by M, incognita over all rates of T. basicola in both years. Colonization of root tissue by T. basicola was increased by th e low inoculum density of the nematode at 20 CFU/g soil in 1997 and 100 CFU /g in 1998. Nematode reproduction with the high M. incognita treatment rate was reduced in both years of the study by the high T. basicola rate. This study suggests the importance of population level of each pathogen to the s everity of disease and confirms the potential of this disease interaction t o impact cotton production.