Resistance of soybean leaves to the scab, caused by Sphaceloma glycines

It has been known that there are distinct varietal differences of resistance to the soy-bean scab, caused by Sphaceloma glycines Kurata et Kuribayashi. In inoculation tests on 11 varieties, no lesions are found on 4 resistant varieties (Nôrin No. 4, No. 5, Tokachi-nagaha and Kitami-nagaha), but, on 4 moderately resistant (Nôrin No. 2, Nadeshiko No. 1, Akasaya and Iwate No. 1) and susceptible (Nôrin No. 1, Tokachi-hadaka and Ugo-daizu) varieties many lesions are produced. The number of lesions differ at the growth stages of leaves (young, mature and old), the young tissues being more susceptible than the mature or old ones. When the susceptible varieties are inoculated, the causal fungus penetrates into the epidermal cells of leaves, stems and petioles by the infection peg produced from the germ-tube, within 24 hours after inoculation. The penetrated hyphae spreads gradually into the palisade and spongy tissue. Within the infected cells, the decrease of chloroplasts, collapse of nucleus, and the browning and coagulation of cytoplasm are observed. In this case, the incubation period is ordinarily about a week and the sporulation is seen in two weeks after inoculation. On the resistant varieties, when inoculated on young and old leaves, only the epidermal cells are invaded, and the infection is not observed generally in the palisade and spongy tissue, revealing no sign of changes in these tissues. When mature leaves are infected, however, abnormal cell divisions at the palisade tissue under the infected epidermal cells are observed, forming cork cambium layer finally. This layer consists of sclerenchymatous cells of 3-5 layers, and it seems that the mycelium of the causal fungus can not penetrate through the layer. On the moderately resistant varieties, the occurrence of lesions and the resistance reaction of the tissue are influenced by the environmental conditions and nitrogen fertilizer.