RELATIONSHIPS BETWEEN VERTICILLIUM-DAHLIAE INOCULUM DENSITY AND WILT INCIDENCE, SEVERITY, AND GROWTH OF CAULIFLOWER

Microplot and field experiments were conducted to evaluate the effects of inoculum density on Verticillium wilt and, cauliflower growth. Soi l containing Verticillium dahliae microsclerotia was mixed with variou s proportions of fumigated soil to establish different inoculum densit ies (fumigated soil was used as the noninfested control). Seven inocul um density treatments replicated four times were established, and the treatments were arranged in a randomized complete block design. Soil w as collected from each microplot immediately after soil infestation fo r V. dahliae assay by plating onto sodium polypectate agar (NP-10) sel ective medium using the Anderson sampler technique. Five-week-old caul iflower was transplanted into two beds within each 1.2- by 1.2-m micro plot. At the same time, several extra plants were also transplanted at the edge of each bed for destructive sampling to examine the disease onset (vascular discoloration) after planting. Cauliflower plants were monitored for Verticillium wilt development. Stomatal resistance in t wo visually:healthy upper and two lower, diseased leaves in each micro plot was measured three times at weekly intervals after initial wilt s ymptoms occurred. At maturity, all plants were uprooted, washed free o f soil, and wilt incidence and severity, plant height, number of leave s, and dry weights of leaves and roots were determined. The higher the inoculum density, the earlier was disease onset. A density of 4 micro sclerotia per g of dry soil caused 16% wilt incidence, but about 10 mi crosclerotia per g of soil caused 50% wilt incidence. Both wilt incide nce and severity increased with increasing inoculum density up to abou t 20 microsclerotia per g of soil, and additional inoculum did not res ult in significantly higher disease incidence and severity. A negative exponential model described the disease relationships to inoculum lev els under both microplot and field conditions. Stomatal resistance of diseased leaves was significantly higher at higher inoculum densities; in healthy leaves, however, no treatment differences occurred. The he ight, number of leaves, and dry weights of leaves and roots of plants in the fumigated control were significantly higher than in infested tr eatments, but the effects of inoculum density treatments were variable between years. Timing of cauliflower infection, crop physiological pr ocesses related to hydraulic conductance, and wilt intensity (incidenc e and severity) were thus affected by the inoculum density. Verticilli um wilt management methods used in cauliflower should reduce inoculum density to less than four microsclerotia per g of soil to produce crop s with the fewest number of infected plants.