The inoculum density (propagules/g soil) of Verticillium dahlias Kleb., causal agent of Verticillium wilt of cotton, was monitored at seven field sites in the San Joaquin Valley of California for a three-year period (1987-1989). The above experimental fields were used to screen twelve variety trials each year for disease resistance and lint yields under field conditions for the San Joaquin Valley Cotton Board. Since Verticillium wilt may be a limiting factor in cotton production, the disease response of these varieties to Verticillium wilt was emphasized.

At each field location the variety trials were replicated four times and four soil samples were taken in a Z pattern in each replication. The soil samples were air-dried for three weeks at room temperature, pulverized and mixed well. Then, 10 grams of soil from each sample were placed into a snap-cap vial and mixed with 15 mg dl-methionine in 2 ml distilled water. After incubation at 33°C for one week, the soil was air-dried again, pulverized with a mortar and pestle and dry plated on sodium polypectate medium (a semi-selective medium for microsclerotia producing species of Verticillium) using the modified Anderson Air Sampler technique (1). The plates were incubated at 22-24°C in the dark for three weeks, then washed with tap water and rated for the presence of V. dahliae colonies. Colonies of V. tricorpus, a ubiquitous, non-pathogenic fungus to cotton, were distinguished from the pathogen by their microsclerotia size and morphology (2). The field data included disease incidence recorded as foliar symptoms, vascular discoloration, and lint yields.

The relationships between population density and disease incidence or cotton lint yields were analyzed using the non-linear regression procedure of SAS (Statistical Analysis System, SAS Institute Inc.) package for a PC microcomputer. For each of the three years, the inoculum density (propagules/g soil) at planting time when plotted against disease incidence (percent foliar symptoms), closely followed the negative exponential curve. The correlation coefficients for 1987, 1988 and 1989 were 0.71, 0.96 and 0.84, respectively. For the three years cumulatively, the correlation between actual data points and predicted values was still very high (R=0.85). The highest correlation between the soilborne populations of V. dahlias and the ratios of cotton lint yields of cv. GC-510 (weak tolerant) over cv. Acala SJ-2 (susceptible to Verticillium wilt) was expressed by the power equation. The correlation coefficients for each of the three years separately or all three years together were 0.88, 0.95, 0.86 and 0.87, respectively. The predicted line suggested that at 2.2 propagules per gram soil both cultivars produced equal cotton lint yields (ratio=100).

The data from the present study suggested that a negative exponential equation can simulate very well the relationship of the inoculum density of V. dahliae to the disease progression at different field locations. Moreover, a more significant correlation between population density and lint yields was revealed. This relationship can be expressed by the power equation and has a very important application value. It shows that at low soil infestation levels (0-3 propagules/g soil) the less tolerant cultivar Acala SJ-2 produces higher yields than the more tolerant variety GC-510. However, above these levels of soil infestation, Acala SJ-2 develops about 40% foliar symptoms and the yield production decreases, while GC-510 responds to the disease with higher yields.

During the three years of this study, the soilborne population of V. dahliae was also recorded at an eighth location at Firebaugh, CA. Interestingly, the disease incidence at this field site was high for all three years, while the inoculum density was at very low levels. The deviation of this experimental field from the negative exponential model could be due to either a new, more virulent strain of the pathogen or different soil or environmental conditions. Research is under way to elucidate the factors involved in this field site.