High populations of Fusarium oxysporum f. sp. vasinfectum, mostly between 100 and 1000 propagules/gram of soil, were found in 10 field soils assayed eight times during 10 yr after susceptible and infected cotton crops had been grown. During some of the years following the cotton crops, cotton and perennial crops such as grapevines and almond trees were grown. Among the isolates of F. oxysporum sp. vasinfectum two culture types were found: low sporing and high sporing types. The high sporing type was more frequently isolated from soil. The two types were further characterized by RAPD analysis and pathogenicity tests.

Preplant soil fumigation with methyl bromide is scheduled to be phased out by 2005. Chemical and nonchemical alternatives are being researched and identified. Soil solarization and/or biofumigation can help fill the gap in certain cases. These alternative methods of soil disinfestation are also of value to organic growers, home gardeners and others who will not or cannot use the soil fumigation chemicals employed by many conventional commercial growers.

ABSTRACT Development of Fusarium wilt in upland cotton (Gossypium hirsutum) usually requires infections of plants by both Meloidogyne incognita and Fusarium oxysporum f. sp. vasinfectum. In this study, the soil densities of M. incognita and F. oxysporum f. sp. vasinfectum and the incidence of Fusarium wilt in three field sites were determined in 1982-1984. Multiple regression analysis of percent incidence of Fusarium wilt symptoms on population densities of M. incognita and F. oxysporum f. sp. vasinfectum yielded a significant fit (R (2) = 0.64) only on F. oxysporum f. sp. vasinfectum. Significant t-values for slope were also obtained for the interaction of M. incognita and F. oxysporum f. sp. vasinfectum, but densities of M. incognita and F. oxysporum f. sp. vasinfectum were also related on a log(10) scale. The physiological time of appearance of first foliar symptoms of Fusarium wilt, based on a degree-days threshold of 11.9 degrees C (53.5 degrees F), was used as a basis for determining disease progress curves and the phenology of cotton plant growth and development. Effects of Fusarium wilt on plant height and boll set were determined in three successive years. Increases in both of these plant characteristics decreased or stopped before foliar symptoms were apparent. Seed cotton yields of plant cohorts that developed foliar wilt symptoms early in the season (before 2,000 F degree-days) were variable but not much different in these years. This contrasted with cohorts of plants that first showed foliar symptoms late in the season (after 2,400 F degree-days) and cohorts of plants that showed no foliar symptoms of wilt. Regression analyses for 1982-1984 indicated moderate to weak correlations (r = 0.16-0.74) of the time of appearance of the first foliar symptoms and seed cotton yields.

The purpose of this study was to assess the biocontrol efficacy of the fungus Trichoderma virens in combination with fungicides against cotton (Gossypium hirsutum) seedling disease pathogens in the field under different soil and ambient environmental conditions. Cotton seed treated with fungicides and/or coated with a latex sticker and air-dried granules of T. virens was planted in field plots with a history of seedling disease in California (CA), Oklahoma (OK), Arkansas (AR), Louisiana (LA), Mississippi (MS) and Indiana (IN), and surviving seedlings counted. Seedling stands in CA from T. virens plus metalaxyl-treated seed plots were greater than the untreated control and most of the plots in which seed was treated only with fungicide. In AR and MS, T. virens plus metalaxyl treatments produced significantly better stands than the untreated controls. In some cases, this combination produced stands equal to those in fungicide-tre ated controls. In LA, OK, and IN, T. virens plus metalaxyltreated seed resulted in stands equal to those in untreated controls, but less than those in the fungicidetreated controls. The addition of triademenol or fludioxonil to the seed treatment combinations did not improve stands above T. virens plus metalaxyl alone. The treatment of cotton seed with T. virens plus metalaxyl generally resulted in greater seedling stands than those in untreated controls and equal to those of the fungicide control, except where disease pressure is very heavy (IN) or light (LA and OK).

Two isolates of Verticillium dahliae, a black microsclerotial isolate and an isolate from potassium deficient cotton plants that forms white colonies on agar media, were examined for their effects on the potassium content of cotton plants. The potassium content of petioles from fully expanded leaves collected at random from branches 6 to 7 nodes below the terminal node were monitored during July and August in 1993 to 1995. Potassium contents of petioles from plants inoculated with V. dahliae did not differ significantly from plants injected with sterile water until the plants were nearing peak boll load. Both isolates caused a gradual development of potassium deficiency symptoms in leaves of inoculated plants and a decrease in petiole potassium, often accompanied by chlorosis and necrosis typical of Verticillium wilt. These results suggest that infection of cotton plants by V. dahliae causes an impairment in the uptake and translocation of potassium that is often associated with the development of potassium deficiency symptoms in leaves of plants with large boll loads.

Fusarium spp. often initiate growth slowly on high glucose media in the absence of added biotin. Among the isolates of F. oxysporum studied, two types were apparent: low sporulating and high sporulating cultures on agar media. Saprobic isolates of F. oxysporum found in sandy soils usually grew slower than the wilt pathogens from these soils. Within the group of isolates of F. oxysporum formae speciales which cause wilt in host plants, there were two distinct cultural types that were consistently isolated from all infested field soils and from infected plants in each field observed. These two types differed in their degree of sporulation and growth on high glucose agar medium without added biotin; they also could be distinguished based on whether or not they produced a substance easily detected by its odor.

Soil solarization, alone and combined with metam sodium (MS), was evaluated as an alternative to methyl bromide and chloropicrin (MBC) fumigation, the standard soil disinfestation technique in the California strawberry (Fragaria ×ananassa Duch.) industry. Tests were conducted in two consecutive annual production cycles in Irvine, Calif., an environment representative of the coastal strawberry production area. Solarization treatments were applied from late July through September for October plantings. Treatments were equally effective in reducing baited populations of Phytophthora cactorum [(Lebert and Cohn) J. Schröt] (1989-90) and P. citricola Sawada (1990-91) when compared to pathogen survival in nontreated soil. Solarization and MBC reduced Verticillium dahliae Kleb inocnlnm in 1989-90, but MBC gave superior control in 1990-91. Solarization significantly controlled annual weeds, but was less effective than MBC. In 1989-90, solarization alone increased strawberry yield 12 % over the yield of nontreated plots; when combined with MS, yield increase was 29%, equivalent to that achieved with MBC fumigation. Treatments were equally effective in increasing yields in the 1990-91 test. Chemical names used: sodium N -methyldithiocarbamate (metam sodium), chloropicrin nitrotrichloromethane (chloropicrin).

Apricot (Prunus armeniaca) and almond (P. dulcis) trees at the first leaf stage were planted in soil infested with Verticillium dahliae and mulched with transparent or black polyethylene film, or not mulched, in the San Joaquin Valley of California, March‐August 1990. During the 19‐week mulching treatment, summer soil temperatures reached as high as 46, 41, and 33°C at 18 cm depth; and 41, 37, and 32°C at 30 cm depth under clear film, black film, and no film, respectively. Trees mulched from the time of planting with transparent polyethylene (solarization) did not survive or grow as well as those mulched with black film or not mulched. Incidence of foliar symptoms due to Verticillium wilt was reduced by 86–100% in both apricot and almond trees by black, as well as transparent film mulch the following season. Incidence of vascular discoloration symptoms of trunks and primary scaffolds due to Verticillium wilt was similarly reduced by both mulches. Mulching with black polyethylene film gave better overall results than solarization with transparent film. The intermediate soil temperatures produced did not chronically harm trees, as judged by tree survival and annual growth of trunk diameter, yet the prolonged period of soil heating provided control of Verticillium wilt equivalent to that of solarization with transparent polyethylene. These studies provided further evidence that in‐season mulching can be used to conserve water during establishment of new orchards or replant trees in warm, arid climates.

Potassium deficiency symptoms occur on Acala SJ-2 cotton when the soils, by test, are not deficient and when other crops grow well without additions of potassium. Field experiments in 1988, 1989 and 1990 resulted in significant lint yield increases when the soils were fumigated with methyl bromide and chloropicrin. Foliar potassium deficiency symptoms were drastically decreased without altering soil potassium levels.

Treatments of sporangia and zoospores of Phytophthora infestans race 1.2.3.4 with methionine or riboflavin for durations of up to 8 h under fluorescent light did not affect its colonization of rye-seed agar. In contrast, exposure of sporangia and zoospores to methionine-riboflavin mixture for 2 h or more resulted in the failure of race 1.2.3.4 to grow when transferred to rye-seed agar medium. Hyphal growth of races 1.2.3.4 and 0, when incubated in liquid synthetic medium, was inhibited by free riboflavin. Potato plants, Solanum tuberosum cv. Kennebec, when pretreated with a methionine–riboflavin mixture and then spray inoculated with race 1.2.3.4 developed fewer diseased leaves per plant than plants pretreated with water, methionine, or riboflavin. Key words: Solanum tuberosum, hydroxyl radical, superoxide anion.