R. M. Davis

Verticillium wilt of cauliflower in California.

Since 1990, commercial cauliflower in coastal California has been severely affected by a vascular wilt disease. Symptoms consist of chlorosis, defoliation, stunting, wilting, and vascular discoloration. Disease has been widespread and has caused significant damage in summer and fall crops. Verticillium dahliae was consistently isolated from xylem tissue in stems and roots of affected plants. Techniques tested for inoculation of cauliflower plants were dipping clipped or nonclipped roots into spore suspensions, injecting spore suspensions into cauliflower stems, and planting seedlings into soil along with an agar block colonized with microsclerotia. Only dipping roots into spore suspensions was consistently successful in causing Verticillium wilt

Effects of chitin and chitosan on the incidence and severity of Fusarium yellows of celery

The effects of chitin and chitosan on disease incidence and severity of Fusarium yellows of celery and on populations of Fusarium oxysporum were investigated between 1994 and 1996. Field experiments were conducted at two locations with a history of severe Fusarium yellows. Disease incidence and severity were significantly reduced by pre-plant chitin amendments to soil. Chitosan applied as a root dip alone did not reduce disease incidence but significantly (P < 0.05) reduced disease severity when used with a tolerant celery cultivar. Standard soil dilution methods were used to enumerate populations of soil microflora. Chitin increased bacterial and actinomycete populations in soil in 2 of the 3 years of study. The effects of potential biocontrol agents recovered from chitin-treated plots in 1995 were studied in 1996; enriching the transplant medium with isolates of bacteria and actinomycetes 4 weeks and 1 week prior to transplanting did not alter the established equilibrium in the field, and no biocontrol effect was observed. Chitin amendments to soil or chitosan treatment of transplants did not reduce soil populations of F. oxysporum. Whether these treatments affected the F. oxysporum f. sp. apii subpopulation within the F. oxysporum population could not be determined.

Origin of Race 3 of Fusarium oxysporum f. sp. lycopersici at a Single Site in California

ABSTRACT Thirty-nine isolates of Fusarium oxysporum were collected from tomato plants displaying wilt symptoms in a field in California 2 years after F. oxysporum f. sp. lycopersici race 3 was first observed at that location. These and other isolates of F. oxysporum f. sp. lycopersici were characterized by pathogenicity, race, and vegetative compatibility group (VCG). Of the 39 California isolates, 22 were in VCG 0030, 11 in VCG 0031, and six in the newly described VCG 0035. Among the isolates in VCG 0030, 13 were race 3, and nine were race 2. Of the isolates in VCG 0031, seven were race 2, one was race 1, and three were nonpathogenic to tomato. All six isolates in VCG 0035 were race 2. Restriction fragment length polymorphisms (RFLPs) and sequencing of the intergenic spacer (IGS) region of rDNA identified five IGS RFLP haplotypes, which coincided with VCGs, among 60 isolates of F. oxysporum from tomato. Five race 3 isolates from California were of the same genomic DNA RFLP haplotype as a race 2 isolate from the same location, and all 13 race 3 isolates clustered together into a subgroup in the neighbor joining tree. Collective evidence suggests that race 3 in California originated from the local race 2 population.

Occurrence and Pathogenicity of Fungi Associated with Melon Root Rot and Vine Decline in California

The occurrence of fungi associated with root rot and vine decline of melon (Cucumis melo) in commercial fields in California was surveyed over 3 years. The fungi most frequently isolated from discolored vascular tissue or root rot were Acremonium cucurbitacearum, Rhizopycnis vagum, Monosporascus cannonballus, Fusarium solani, Macrophomina phaseolina, Pythium spp., and Verticillium dahliae. The frequency of isolation of the various fungi varied with root symptomology. Pythium spp., and M. phaseolina were frequently associated with a wet, brownish root rot, while A. cucurbitacearum, R. vagum, and Rhizoctonia solani were generally associated with a dry, corky root rot. Presence of Monosporascus cannonballus was associated both with a wet, brownish rot as well as with discrete, reddish, corky lesions. The frequency of isolation of a given pathogen varied with geographic location, with M. cannonballus present only in the southern production areas, while A. cucurbitacearum and Rhizopycnis vagum were most common in the northern production areas. In pathogenicity tests in field microplots, M. cannonballus caused vine collapse and severe root rot of cantaloupe, reducing root length density by 93%. California isolates of R. vagum and A. cucurbitacearum, although only weakly pathogenic in field microplots, caused root discoloration and reduced vine growth in greenhouse tests. Reduction in dry weight of greenhouse-grown cantaloupe was 40, 23, and 39% for R. vagum, A. cucurbitacearum, and M. cannonballus, respectively.

Fusarium Wilt of Cotton: Population Diversity and Implications for Management

Fusarium wilt of cotton, caused by the fungus Fusarium oxysporum Schlechtend. f. sp. vasinfectum (Atk.) Snyd. & Hans, was first identified in 1892 in cotton growing in sandy acid soils in Alabama (8). Although the disease was soon discovered in other major cotton-producing areas, it did not become global until the end of the next century. After its original discovery, Fusarium wilt of cotton was reported in Egypt (1902) (30), India (1908) (60), Tanzania (1954) (110), California (1959) (33), Sudan (1960) (44), Israel (1970) (27), Brazil (1978) (5), China (1981) (17), and Australia (1993) (56). In addition to a worldwide distribution, Fusarium wilt occurs in all four of the domesticated cottons, Gossypium arboretum L., G. barbadense L., G. herbaceum L., and G. hirsutum L. (4,30). Disease losses in cotton are highly variable within a country or region. In severely infested fields planted with susceptible cultivars, yield losses can be high. In California, complete crop losses in individual fields have been observed (R. M. Davis, unpublished). Disease loss estimates prepared by the National Cotton Disease Council indicate losses of over 109,000 bales (227 kg or 500 lb) in the United States in 2004 (12).

Relationship between kernel pericarp thickness and susceptibility to Fusarium ear rot in field corn

In a 2-yr study, the thickness of kernel pericarp and aleurone layers was compared among 12 dent corn hybrids representing a range of resistance to ear rot caused by Fusorium moniliforme. The pericarp layer on the cap of the eight hybrids with high or intermediate levels of resistance (determined by the percentage of kernels visibly infected with the fungus) was thicker than the pericarp of the four susceptible hybrids. In contrast, the aleurone layer of susceptible hybrids was thicker than the aleurone layer of the intermediate and resistant hybrids. It is suggested that the relatively thin pericarp layer of the susceptible hybrids allows access of the fungus into the kernels, especially through insect wounds

Importance of the Husk Covering on the Susceptibility of Corn Hybrids to Fusarium Ear Rot

The role of the husk covering and the influence of intra-ear thrips infestation on severity of Fusarium ear rot (Fusarium moniliforme) were evaluated in 1990, 1991, and 1992 among corn (Zea mays) hybrids previously rated as susceptible, intermediate, or resistant to Fusarium ear rot. For the three treatments imposed on individual corn ears in 1990, ears were either left untreated, wrapped with a strip of parafilm at the ear tip to maintain a tight seal, or husk layers were split open along one side of the ear I to 2 weeks after pollination to partially expose the developing kernels. In 1991 and 1992, two additional treatments, split husks followed by acephate insecticide application and insecticide application to ears with intact husks, were added. Split husks compromised the natural barrier to insects and fungi and significantly increased Fusarium ear rot severity among all corn hybrids in 1990 and 1991. In 1992, a year with naturally high levels of disease, split husks significantly increased disease severity among the resistant hybrids, but did not affect the susceptible hybrids. Acephate applied to ears with split husks reduced ear rot among all hybrids. Grain yields were significantly less in ears with split husks for all hybrids in 1990 and 1991, and for the resistant hybrids in 1992. Grain yields were higher for all insecticide-treated hybrids each year. These results demonstrate that susceptibility to Fusarium ear rot is influenced by both husk morphology and intra-ear thrips infestation.

Detection and eradication of Alternaria radicina on carrot seed

Alternaria radicina was recovered from carrot seed following a 10-day incubation on a semiselective agar medium, Alternaria radicina Selective Agar (ARSA). A significantly higher proportion of A. radicina-infested seed was detected in individual seed lots assayed on ARSA than was detected by the standard freezer-blotter seed assay. The identity of A. radicina recovered on ARSA was confirmed by inoculation of carrot root tissue. Based on seed assays using ARSA, 24% (seven of 29) of randomly selected commercial carrot seed lots planted in the Cuyama Valley, California, in 1992, contained low levels of A. radicina (≤0.1%)

Susceptibility of selected cotton cultivars to seedling disease pathogens and benefits of chemical seed treatments

The susceptibility of 12 Upland cotton cultivars to three soilborne fungi, Pythium ultimum, Rhizoctonia solani, and Thielaviopsis basicola, was evaluated in greenhouse experiments. Based on symptom development and seedling survival, cultivars highly resistant to P. ultimum included Delta Pine (DP) 6166, Prema, DP 6100, and Maxxa. A relatively low incidence of pre-emergence damping-off caused by R. solani occurred in ChemBred 7, DP 6100, and Royale, although all cultivars subsequently suffered significant post-emergence damping-off. All cultivars were equally susceptible to T. basicola. In both greenhouse and field evaluations, there were no benefits of metalaxyl seed treatment in Pythium-resistant cultivars. Seed treatment with carboxin-pentachloronitrobenzene for the control of Rhizoctonia-induced damping-off resulted in stand increases in all 12 cultivars in greenhouse tests and in 3 of 6 cultivars in field trials.

Factors affecting the development of cavity spot of carrot

Pythium violae and P. ultimum, isolated from cavity spot lesions on carrots produced in California, caused typical cavity spots on carrots grown in artificially infested soil in the greenhouse. In a 2-yr survey of 54 fields in the San Joaquin Valley of California, cavity spot incidence was not correlated with soil pH, electrical conductivity, moisture-holding capacity, organic matter, total and exchangeable calcium, particle size distribution, or planting densities (57, 115, or 230 carrots per meter of row). In a growth chamber maintained at 20 C, six commonly grown carrot cultivars were susceptible to both Pythium spp (.)