Diaporthe Phaseolorum Research Articles

EFFECT OF EXPOSURE TO RADIO-FREQUENCY ELECTRIC FIELDS ON SEED-BORNE MICROORGANISMS

Exposure of naturally infected and artificially inoculated seeds to radio-frequency (rf) electric waves of 59 to 64 Mc/sec was of varying effectiveness in eradicating internally borne fungi and bacteria. Diaporthe phaseolorum was eliminated from soybean seeds with little reduction in seed germination. Survival of Ascochyta pisi and A. pinodes (Mycosphaerella pinodes) in field pea seed was greatly reduced by similar treatment, but the fungi were not usually eradicated without appreciable reduction in germination. Germinability of pea seeds was unaffected by increasing the moisture content of the seed to 16% from 10.3%. The A. pinodes content of such seeds, however, was reduced by approximately 50%. Both seed and fungus were more susceptible to damage by rf treatment at the higher moisture level. Bean seed infected with Xanthomonas phaseoli var. fuscans and cabbage seed inoculated with X. campestris were killed by exposures which reduced but did not eradicate the bacteria. Colonies of both xanthomonads on nutrient agar media were destroyed by rf treatment.

SEED INFECTION OF SOYBEAN BY DIAPORTHE PHASEOLORUM AND ITS INFLUENCE ON HOST DEVELOPMENT

Diaporthe phaseolorum (Cke. & Ell.) Sacc., the causal agent of pod and stem blight of soybeans and generally regarded as a facultative parasite, has been found to be pathogenic during the seedling phase. The fungus caused severe losses of germinating seeds and young seedlings. In 1959 more than 30% of the soybean seed crop in southwestern Ontario was infected, and germination was severely reduced. D. phaseolorum was present in seed samples from this area in 11 of 13 years. Studies with a series of seed samples of varying levels of infection revealed that the fungus was still viable and pathogenic in the seed after storage for 2 years. However, during that time the levels of infection in most samples had decreased markedly whereas a concurrent increase in germinability indicated that infected seeds were killed during the germination phase. While several seed treatment fungicides were effective in increasing germination and emergence in heavily infected samples, none completely controlled the disease. At normal planting rates, yield was not affected by levels of seed infection up to 42% but the yield was significantly lower in a sample containing 73% infected seed. It is concluded that the disease may be important in the seed and seedling phases of host development.

OBSERVATIONS ON STEM CANKER AND POD AND STEM BLIGHT OF SOYBEANS IN ONTARIO

Stem canker caused by Diaporthe phaseolorum (Cke. & Ell.) Sacc. var. caulivora Athow & Caldwell is a serious disease of soybeans in Ontario. Artificial inoculation experiments have indicated (a) that soybeans which are highly susceptible to infection at midstage in their development become progressively less so as they approach maturity, (b) that reduction in yield is more or less directly proportionate to earliness of infection, (c) that all varieties presently of commercial importance in Ontario are highly susceptible to the disease, and (d) that six common weeds, and tomato, pepper, and snap and field bean, are not susceptible to infection by the soybean pathogen. The tests also indicated that monoconidial isolates of the stem canker organism are as highly pathogenic as those derived from single ascospores, and that isolates, though morphologically similar, may be differentiated on the basis of their pathogenic capability. The pathogen is definitely seedborne, but infection from such a source is not of primary importance. The likelihood that the pathogen is indigenous to the soil of the soybean-growing area of Ontario is discussed. Pod and stem blight caused by Diaporthe phaseolorum (Cke. & Ell.) Sacc. var. sojae is of negligible economic importance in Ontario. The pathogen, which is observed mostly in the imperfect (Phomopsis) stage, shows little evidence of being able to infect plants until late in the growing season. In the absence of resistant varieties no better methods of control can be recommended than those of completely plowing under soybean debris, of avoiding the planting of the more susceptible varieties, and of practicing crop rotation.