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Effect of Planting Date and Host Genotype on the Root-Knot Nematode-Fusarium Wilt Disease Complex of Cotton. D. P. Jeffers, Former graduate student, Department of Plant Pathology, University of California, Davis 95616, Present address: CIMMYT, Lisboa 27, Apdo Postal 6-641, 06600 Mexico, D. F., Mexico; P. A. Roberts, professor, Department of Nematology, University of California, Riverside 92521. Phytopathology 83:645-654. Accepted for publication 9 February 1993. Copyright 1993 The American Phytopathological Society. DOI: 10.1094/Phyto-83-645.

Four cotton (Gossypium hirsutum) genotypes and three biweekly planting dates, beginning during late March, were examined for 3 yr in a split-plot field design to determine effects on Fusarium wilt development. Cotton genotypes included Acala SJ-2, susceptible to Fusarium oxysporum f. sp. vasinfectum and Meloidogyne incognita; Acala SJC-1, tolerant of F. o. vasinfectum and susceptible to M. incognita; N6072, susceptible to F. o. vasinfectum and resistant to M. incognita; and N8577, tolerant of F. o. vasinfectum and resistant to M. incognita. Fusarium wilt disease was low, moderate, and severe, for 3 yr, respectively. A planting-date effect of more Fusarium wilt in earlier plantings was found and was most evident in Acala SJ-2. This effect was indicated by a higher percentage of plant death, total foliar and plant-death symptoms, root discoloration, and yield suppression in earlier compared to later plantings. Analyses of planting-date main effects indicated significant (P = 0.050.001) linear trends in these variables. In the moderate- and severe-wilt years, Acala SJ-2 final plant-death values were 6275 in the first and 2328% in the third planting, and yield was suppressed by 5575% in the first compared to the third planting date. Adult female and total M. incognita numbers in Acala SJ-2 roots early in the season were higher in the first compared to the later plantings during the severe-wilt year, and during 2 yr, significant (P = 0.050.001) linear trends of higher post-harvest nematode root-infection levels in earlier plantings were found in Acala SJ-2 and Acala SJC-1. Acala SJ-2 was colonized most extensively by F. o. vasinfectum, followed by N6072; Acala SJC-1 and N8577 had limited stem colonization. Compared to Acala SJC-1, plant death in N6072 caused by Fusarium wilt was similar at low but greater at high M. incognita initial densities. N6072 had healthier root systems and higher yield than had Acala SJC-1, indicating nematode resistance was more effective than wilt tolerance in protecting plants from the disease complex, whereas combined nematode and wilt resistance in N8577 was most effective. Delayed planting of cotton has important potential as a Fusarium wilt disease-management tactic.