Nationwide, Fusarium wilt of cotton takes a backseat to the more widely reported and researched Verticillium wilt. This relationship is certainly true in California. Here, hundreds of research studies have involved Verticillium wilt, a disease that can be found in most cotton fields throughout the San Joaquin valley. Fusarium wilt on the otherhand, has attracted relatively few studies; since 1960 it has been found in at least 70 field sites scattered throughout the Valley. In each of these locations, without exception, plants with leaf symptoms of Fusarium wilt have had root nodules or galls caused by the root-knot nematode and severe vascular discoloration caused by Fusarium oxysporum f. sp. vasinfectum.

Nematicides have been commonly used for control of Meloidogyne incognita, the root knot nematode; they also are very effective in controlling Fusarium wilt primarily through their effect on the nematode. Due to increasing environmental considerations, the future use of these nematicides may be increasingly restricted or lost. Thus, we need to find other control measures. In previous studies, we have obtained excellent field tolerance to Fusarium wilt from cotton cultivars with high levels of tolerance to root-knot nematodes. Greenhouse inoculation tests indicated that these cultivars had poor tolerance to F. oxysporum f. sp. vasinfectum. However, cotton cultivars with high levels of tolerance to Verticillium dahlias, the cause of Verticillium wilt, also have shown increased field tolerance to Fusarium wilt.

In 1991, trials were planted at the U. S. Cotton Research Station at Shafter, CA in a field heavily infested with root-knot nematodes and F. oxysporum f. Sp. vasinfectum. The wilt tolerance of the cotton cultivars was compared to a chemical seed coating (CGA 173506-4FS, Ciba-Geigy Corp.) at 0, 0.4, 0.8, and 1.6 oz/cwt acid delinted seed. The seed coating was applied to seed of Acala SJ-2, a cultivar with low field tolerance to Fusarium wilt and root-knot nematodes. Acala SJ-2 also has moderate field tolerance to Verticillium wilt. A second cultivar of cotton used in the comparison was Acala Prema which has excellent field tolerance to Verticillium wilt. This cultivar in four years of field testing has had the lowest levels of root-knot nodule formation of any cultivar currently approved for planting in the San Joaquin Valley.

Disease pressure in 1991 in each of four field tests planted in randomized block design with four replications of each treatment was extremely high. Chemical control of Fusarium wilt by CGA 173506-4FS was ineffective in all field tests. By early July, over 50 percent of the Acala SJ-2 plants had leaf symptoms of the disease or had died. By the end of August most plants were dead and the few survivors were diseased. In contrast, only one percent of the Acala Prema plants had symptoms of Fusarium wilt in early July and by the end of August, no leaf symptoms or dead plants were evident. Differences of the cultivars in field tolerance to Fusarium wilt was also indicated in the yields of cotton harvested from the experimental plots. Acala Prema had 300 to 400 percent higher yields than Acala SJ-2. Following harvest, roots from the plants in experimental plots were evaluated for the extent of root nodules or galling caused by the root-knot nematodes. Because most of the Acala SJ-2 plants had died by late August, their roots were rotted. Thus, by harvest, only the few surviving plants of this cultivar could be evaluated. These plants were severely galled. and were evaluated 4 on a scale of 0 to 4. In comparison, galling on the roots of Acala Prema plants was much less, averaging 2.6 on the same scale. Acala Prema is not in the same class as the most resistant cultivars developed in the California program, but it does represent a significant advance over other cultivars that have been released for planting.

We suggest that the excellent field tolerance of Acala Prema to Fusarium wilt is a result of a combination of genetic traits; it has better than average tolerance to root-knot nematodes and we believe that this trait also works in combination with considerable tolerance to F. oxysporum f. Bp. vasinfectum. This assumption is based on previous studies which showed that cultivars with high levels of tolerance to Verticillium wilt have higher field tolerance to Fusarium wilt. Some of these cultivars, however, have below average resistance to root-knot nematodes. Accordingly, their field tolerance to Fusarium wilt should be lower than that of a cultivar with above average field tolerance to both the nematode and fungal pathogens.

With the present uncertain status of nematicides, the cotton breeding program in California will continue to search for and develop wilt tolerant cultivars as well as more effective chemical or biological seed or soil treatments with potential to manage Fusarium wilt. In the interim, it might prove prudent for California growers, if faced with a serious root-knot nematode problem, to consider planting a currently available cultivar with the highest tolerances to the root-knot nematode and F. oxysporum f. sp. vasinfectum.