Polygalacturonic acid (PGA) is an important constituent of Sorensen's NP-10 medium (NP-10) for estimating the population density of Verticillium dahliae in soil. Different types of PGA are available, but not all of them favor the growth of V dahliae. Unavailability of PGA sodium salt from orange (P-1879) has created an unprecedented problem for the quantification of microsclerotia (MS) of V dahliae in soil. The PGA from orange (P-3889) that is now available does not support the growth of V dahliae. Therefore, experiments were conducted to optimize the use of NP-10 prepared with P-3889 and various concentrations of NaOH. NP-10 with P-3889 amended with eight concentrations of NaOH were compared with NP-10 prepared from PGA sodium salt from orange (P-1879, now discontinued) and citrus (P-3850) along with cellophane and Na-pectate media for recovery of MS from soil and growth of V dahliae on the media. Seven soils were assayed for MS, and eight isolates of V dahliae were evaluated for growth and production of MS. Concentrations of NaOH >0.035N and <0.02N in NP-10 with P-3889 reduced mycelial growth, microsclerotial production, and recovery of MS from soils. Similarly, NP-10 with P-3850 alone, cellophane, and Na-pectate media had significantly reduced growth on media and recovery of V dahliae from soils. The NP-10 with P-3889 and 0.025N NaOH consistently yielded numbers of V dahliae MS from soil samples and supported the growth and production of MS similar to the NP-10 with P-1879. The medium developed in this study can serve as a direct replacement for the original NP-10 that was developed nearly three decades ago, an important component of which is no longer available.

The seed transmission of Verticillium dahliae was evaluated in lettuce (Lactuca sativa). Seed collected from lettuce plants infected with V. dahliae were plated with or without surface sterilization on Sorenson's modified NP10 medium. Of the seed plated with or without surface sterilization, 90 and 66 %, respectively, yielded colonies of V. dahliae. The incidence of Verticillium wilt ranged from 55 to 80 % among lettuce plants grown from seed harvested from infected plants. All evaluated isolates of V. dahliae were capable of seed transmission in lettuce. A V. tricorpus isolate failed to cause significant disease in lettuce or to become seedborne. Storage of contaminated seed at seven temperatures ranging from -20 to 15degreesC for up to 72 weeks did not reduce the incidence of V. dahliae in seed, whereas storage at room temperature (23 +/- 2degreesC) for 20 to 52 weeks reduced the incidence of V. dahliae without affecting seed viability. Of the 11 weed species collected from fields with a known history of Verticillium wilt of lettuce, four yielded V. dahliae. Pathogenicity tests demonstrated that isolates of V.dahliae from Sonchus oleraceus, Capsella bursa-pastoris, and Solanum sarrachoides were as virulent as or more virulent than an isolate of V. dahliae from lettuce. These results demonstrate the potential of seedborne and weedbome inoculum to disseminate V. dahliae.

Epidemics of Verticillium wilt in pepper fields of the central coast of California and isolates of Verticillium dahliae associated with these epidemics were characterized. The mean incidence of wilted plants per field ranged from 6.3 to 97.8% in fields with Anaheim, jalapeno, paprika, or bell peppers. In general, incidence of wilt in jalapeno and bell pepper crops was lower than in crops of other types of pepper. Inoculum density of V. dahliae in the surveyed pepper fields ranged from 2.7 to 66.6 microsclerotia g(-1) dry soil, and the correlation between disease incidence and density of microsclerotia was high (r = 0.81, P < 0.01). Distribution of Verticillium wilt was aggregated in a majority of the pepper fields surveyed, but the degree of aggregation varied. Vegetative compatibility group (VCG) characterization of 67 isolates of V dahliae indicated that 67% belonged to VCG 2, 22% to VCG 4, and 11% to a new group, designated VCG 6. The pathogenicity of isolates of V dahliae from bell pepper and tomato plants was tested by inoculating 1-month-old bell pepper (cv. Cal Wonder) and tomato (cv. EP 7) seedlings and incubating the inoculated plants in the greenhouse. Seedlings of bell pepper were susceptible only to the isolates of V dahliae from pepper, whereas seedlings of tomato were susceptible to both pepper and tomato isolates. Pepper isolates belonging to VCG 2, VCG 4, and VCG 6 were highly pathogenic to bell pepper and chili pepper. Temperatures between 15 and 25degreesC were optimal for mycelial growth of a majority of isolates of V dahliae. Molecular characterization of pepper isolates of V dahliae using a polymerase chain reaction (PCR)-based random amplified polymorphic DNA (RAPD) technique revealed minor variation among these isolates, but unique polymorphic banding patterns were observed for isolates belonging to VCG 6. Verticillium wilt of pepper is a major production constraint in the central coast of California. More aggressive isolates of V dahliae may have been selected in this region as a result of intensive cropping practices.