Isolates Of Phytophthora Cactorum Research Articles

The ginseng root pathogens Cylindrocarpon destructans and Phytophthora cactorum are not pathogenic to the medicinal herbs Hydrastis canadensis and Actaea racemosa

Goldenseal (Hydrastis canadensis) and black cohosh (Actaea racemosa, syn. Cimicifuga racemosa) are medicinal herbs that are indigenous to North America. Both are potential companion or rotation crops in shadecultivated ginseng (Panax quinquefolius) production systems. The susceptibility of H. canadensis and A. racemosa to three serious root diseases of P. quinquefolius was determined in a series of inoculation trials. Plants of the three species were grown in a shaded greenhouse and inoculated with isolates of Phytophthora cactorum, Cylindrocarpon destructans, or Rhizoctonia solani known to be virulent to ginseng. Panax quinquefolius plants were severely diseased or killed by the pathogens. However, P. cactorum and C. destructans were not pathogenic on H. canadensis or A. racemosa. Rhizoctonia solani caused damping-off of emerging seedlings of all three species.

Antagonism of isolates of Bacillus subtilis to Phytophthora cactorum

Twenty-one isolates of the bacterium Bacillus subtilis isolated from local and exotic sclerotia of Sclerotium cepivorum were tested on cornmeal agar (CMA) for antagonism to six isolates of Phytophthora cactorum, causal agent of apple crown rot. All bacterial isolates produced diffusible antibiotics antagonistic to growth of all P. cactorum isolates on CMA. Antagonists were further evaluated in a replicated greenhouse trial for their ability to control crown rot infection on McIntosh apple seedlings in sterile soil. Six isolates of B. subtilis provided statistically significant reductions of infection on McIntosh seedlings. No correlation was observed between width of inhibition zone in vitro and protection from P. cactorum infections in McIntosh apple seedlings in a greenhouse trial. Significant negative correlations were observed between percent seedling infection and root weight.

Inhibition of Phytophthora cactorum by a bacterial antagonist

Bacterial iolate B8 inhibited the growth of six isolates of Phytophthora cactorum on cornmeal agar. Under greenhouse conditions, B8 reduced infections by three isolates of P. cactorum in sterile field soil. Growth of P. cactorum on cornmeal agar was completely inhibited with 40% autoclaved B8 extract. The growth was significantly reduced by low pH alone (pH 4.5 or less) but even when the pH of B8 extract was raised to 6, P. cactorum growth was completely inhibited. Cornmeal agar buffered with 15 mM citrate, pH 5, was suitable for assaying extracts for antagonism to P. cactorum. Acetic acid and other fatty acids (possible metabolites of B8) were also inhibitory, with degree of antibiosis increasing with the size of the fatty acid molecule. The antibiotic produced by B8 was characterized as neutral (not absorbed by ion exchange resins) and of low molecular weight.

Pythiaceous fungi and plant-parasitic nematodes in California pear orchards: I. Occurrence and pathogenicity of pythiaceous fungi in orchard soils

Pythiaceous Fungi and Plant-Parasitic Nematodes in California Pear Orchards I. During a year’s survey of 126 California pear orchards with varying degrees of pear-decline damage, a total of 3,586 isolates of soil-borne fungi of the genera Phytophthora and Pythium were recovered, and, where possible, identified as to species. A direct relationship was found between the severity of pear-decline damage in an orchard and the percentage of isolates from the orchard that were Pythium spp.; and a corresponding inverse relationship between pear-decline damage severity and the percentage of isolates that were Phytophthora spp. Isolates of Phytophthora cactorum and P. cryptogea were found to be pathogenic on Pyrus communis L. hort. var. Winter Nelis and Pyrus serotina Rehd. seedlings. These pathogens were found infrequently, and it was concluded that pythiaceous fungi were not the primary cause of pear decline. Pear fruit and pear and apple seedlings trapped more Pythium spp. than the fruit of apple, lemon, and avocado, while few Phytophthora spp. were trapped in seedlings. Phytophthora spp. isolates were trapped most often in pear and apple fruit. Lemon and avocado fruit yielded a fair number of both genera.