This paper is a follow up to one we presented here last year where we compared the relative field tolerances of Acala SJ-2 and Acala Prema to Fusarium wilt disease. This year we expanded the study comparing a number of cotton cultivars for their relative tolerances to seedling diseases, and to the Verticillium and Fusarium Wilts.

We selected one field location where we knew the soil was infested with the Verticillium wilt fungus, Verticillium dahliae. The soil at another location was known to e heavily infested with rootknot nematodes, Meloidogyne incognita and the Fusarium wilt fungus, Fusarium oxysporum f. sp. vasinfectum. A third site was thought to be free of the wilt organisms, but did have the seedling disease organisms present. At each of these sites we planted seven of the commercial varieties now approved for the San Joaquin Valley. These varieties included Deltapine DP-6166, GC-356, GC-510, and the Acalas Prema, Maxxa and Royale. Acala SJ-2 was planted as a control that in past studies had a lower level of tolerance of rootknot nematodes, the two wilt pathogens and to several of the seedling pathogens. N-8577 was planted in the Fusarium wilt plot location because of its excellent tolerance to rootknot nematodes and exceptional field tolerance to Fusarium wilt disease. In the Fusarium wilt plot we also included three Pima varieties.

In the seedling disease plot we found that all three varieties responded positively when treated with fungicide selected to control Rhizoctonia solani. There were problems with several varieties in the 1992 test due to seed injury that reduced emergence hot with treated and untreated seed, Acala C-306 cultivar that has not been released was outstanding a several location in 1992. Acala SJ-2 also produce excellent stands in these studies. We were not able to isolate Pyyhium ultimum and although Rhizoctonia solani was not very prevailent in these seedlings, Thielaviopsis basicola was by far the most prominent pathogen present.

In the Verticillium wilt plot the only variety that we obviously affected by this disease was Acala SJ-2. Nineteen percent of the Acala SJ-2 plants had the symptoms of Verticillium wilt. Acala C-306 again ha excellent disease tolerance however, all the commercial releases except, Acala SJ-2 had excellent tolerance to this disease in 1992 and produced excellent yields, Acalas Royale and C-306 produced the highest yields and Acalas Maxxa and GC-510,varieties although both with excellent Verticillium wilt tolerance had lower yields than expected.

In the Fusarium wilts study in 1991 we had compared Acala SJ-2, a variety with low tolerance to nematode galling with Acala Prema, which had the lowest rootknot galling rating of any commercial variety we had tested in three years of field trials. Acala SJ-2 is also susceptible to the Fusarium wilt fungus and has a lower level of field tolerance to Verticillium wilt than any of the newly released varieties tested. Acala Prema, on the other hand, has excellent field tolerance to Verticillium wilt disease. In the 1991 test Acala Prema was highly significantly superior to Acala SJ-2 in percentages of plants with disease symptoms, nematode galls and cotton lint yields. From these results we suggested the need to search for varieties with the lowest nematode galling ratings combined with the highest tolerances to Verticillium wilt and/or high field tolerances to Fusarium wilt.

In the 1992 Fusarium wilt plot Acala SJ-2 and all three Pima varieties were devastated by the Fusarium wilt disease. From 42 to 60 percent of these plants were diseased and eventually almost all of these plants appeared to have died. N-8577, which is highly tolerant to rootknot nematodes had the best vegetative growth, the lowest disease rating and the highest yield of all the cultivars tested. Numerically Prema, Royale, DP-6166 and C-306 had the lowest disease ratings. Cotton yields were not necessarily correlated with disease ratings. Acalas Prema and Royale ,however, the varieties with the lowest nematode galling ratings, although not significantly better yielders than GC-510 and DP-6166 were again numerically the highest yielders. Maxxa although it has excellent tolerance to Verticillium wilt reflected its low tolerance to rootknot nematodes because it was obviously adversely effected by the Fusarium wilt disease. We need to recognize that this heavily infested Fusarium wilt plot site together with below ground level drip irrigation seemed to provide a more severe disease situation than occurs in our commercial cotton fields. We again suggest a real need to search for cotton varieties with the highest disease tolerances in combination with the lowest levels of susceptibility to rootknot nematodes.