Three isolates of Verticillium tricorpus were selected, based on greenhouse tests, for biocontrol of seedling diseases and Verticillium wilt of cotton under field conditions. Experimental field plots at Shafter, Dos Palos, and Davis, CA which were naturally infested with seed and seedling pathogens and Verticillium dahlias, were preplant treated with a sand:corn meal mixture (95:5) which had been colonized by V. tricorpus. The inoculum, approximately 6.6 lbs/25 feet of row, was rototilled or worked into the seed row approximately two weeks before planting. Approximately two weeks after seedling emergence, plants were randomly sampled and the roots were plated on a selective agar medium to determine the rhizosphere competence of the isolates of V. tricorpus. In other treatments for control of Verticillium wilt and seedling diseases, a soil treatment was made approximately one month before planting using Basamidgranules (a.i. isothiocyanate) by applying 100 lbs./acre in the seed row. Additional treatments included foliar sprays with Rally ( -butyl- (1)-(4-chlorophenyl)-1H-1,2,4-triazole-l-propanenitrile) at 0.1 and 0.2 lbs a.i./acre. Plants were sprayed to runoff at the cotyledon and the 4th to 5th true leaf stages of development.

The results of these tests at the Dos Palos and Davis field locations showed that V. tricorpus reduced the incidence of Verticillium wilt; however, the results showed only a trend which was not significant at the 5% level. Less than 1% incidence of Verticillium wilt occurred in control plants at the Shafter location. No differences were found in yields of seed cotton or in the incidence of vascular discoloration of plants in the control and different biocontrol treatments at Dos Palos and Davis. The emergence and survival of cotton seedlings were increased but not significantly at the 5% level-by all isolates of V. tricorpus at the Dos Palos site; whereas, at the Shafter location, treatments with V. tricorpus reduced seedling stands. No differences in seedling disease development were observed between the control plots and other treatments at the Davis site. Random collections of diseased seedlings from the Dos Palos and Shafter experimental plots showed that at Dos Palos, P. ultimum and T. basicola were most frequently isolated; whereas, at Shafter, T. basicola was the main seedling pathogen. At Shafter and Dos Palos, the recovery of V. tricorpus from seedling roots and hypocotyls ranged from 50 to 2594 colony forming units/gram (wt. weight) of tissue in soil infested with the various isolates. In contrast, the isolation of V. tricorpus from roots of control plants ranged from 2 to 20 cfu/gram of tissue. Soil treatments with Basamid-granular or foliar treatments with Rally were ineffective in reducing Verticillium wilt or in increasing seed cotton yields in comparison with the control treatment. However, percentage survival of cotton seedlings at the Dos Palos location was significantly higher in plots sprayed with Rally at the cotyledonary stage than in control plots.