In the present paper the writers described the result of experiments on the pathogenicity of Pellicularia filamentosa to cucumber seedlings, and on some characters on culture media of thirty eight isolates obtained from many different host plants.
The pathogenicity of this fungus to cucumber seedlings is generally intensive in the isolates obtained from seedlings. Generally, the isolates with no pathogenicity showed a poor mycelial growth, while pathogenic ones showed rapid growth of mycelium as much as 8∼11mmper 5hrs. The isolates of sasakii-type were found in the isolates with more rapid growth of mycelium. As carbon sources of this fungus, hexose was more available than pentose. It seems, however, that the isolates of sasakii-type having a weak pathogenicity represent a poor utilization of maltose. The isolates which were able to utilize rhamnose seemed to have a pathogenicity. However, the isolates which were not able to utilize much of phenylalanine did not utilize also much of both histidine and tryptophan, nor one of them in excess. Moreover, the isolates showing a poor utilization of urea and cysteine seemed to belong to the sasakii-type. Although both solani- and sasakii-type have many different characters, they are not divided so distinctly into two groups, solani- and sasakii-types, and intermediate types are found.

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Rice dwarf virus has been transmitted mechanically from viruliferous insects and from diseased leaves of rice plants to its insect vector, Nephotettix cincticeps Uhl. by injection using glass capillaries.
It appears that the concentration of virus in viruliferous insects is somewhat higher than that in the diseased leaves. The concentration of virus in eggs laid by viruliferous female leafhoppers, seems to be nearly same as that is viruliferous leafhoppers. When the extracted juices of viruliferous leafhoppers were stored at 0°∼4°C., the virus survived for 48 hours. When viruliferous insect and affected rice leaves were frozen and stored at -30∼-35°C., the infectivity was maintained up to one year. By heating small glass tubes containing the extract of viruliferous leafhoppers in hot water for 10min., the thermal inactivation point of this virus was shown to lie between 40°and 45°C.

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1. Conidia of Piricularia oryzae Cav. are unicellular, small spheres in their primary stage of development and expand into oval form dividing into two cells. The lower cells of the pairs remain scarcely changed, while the upper ones lengthen and divide, giving forth mature bottle-shaped spores with two septa.
2. Conidia in water drops upon living rice sheath or upon onion-scale epidermal strips, immersed for a while in alcohol and washed with running water, develop germ tubes from the apical cells, forming appressoria. The contents of the central and basal cells are promptly transferred to germ tubes and appressoria. Then the basal, central and apical cells lose their contents, becoming shrivelled successively in this order.
Conidia in water drops upon slide glass generally develop germ tubes first from apical cells, next from basal ones and lastly but rarely from central ones. Translocation of contents of three cells is slow and the central cells seem to be disposed to keep the contents for a longer period.
Conidia in rice-straw decoction produce germ tubes in succession from three cells as in water drops and the germ tubes extend into long hyphae branching profusely. Translocation or utilization of the spore contents is far slow compared with that in the above two cases, apparently because the germ tubes are nourished by the decoction.
3. Among three cells of conidia, the apical cells are generally the shortest-and the central ones the longest-lived, but the difference is not so conspicuous. No particular translocation and accumulation of contents into one of the three cells is observed and no chlamydospores or chlamydospore-like structures are formed.

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The effect of Piricularin (a toxic substance of Piricularia Oryzae Cav.) on the resistant activity of rice plants to the blast disease has been studied by the sheath inoculation method and the following results have been obtained.
1, The treatment of rice plants with a highly diluted Piricularin solution (1∼2μg/ml) gives rise an increase of resistant activity of the plants to the blast disease, being accompanied with the following phenomena.
The callus formation of the host's cell against the penetration hyphae is activated, and checks the invasion, and even when the invasion took place, the invaded hyphae can scarcely grow in the host's cell. The infected cell loses the ability of plasmolysis at the initial stage of infection, as usually observed on the cell of the rice plant variety which is non-susceptible to the blast disease.
2. When the plants were killed dy steam heating for 30 minutes, no difference has been observed, in the resistant ability against the growth of invasion hyphae between the plants pre-treated with highly diluted Piricularin solution and control. Considering the fact that Piricularin is very stable against the heating, these observations lead to the conclusion that the rise of the resistant ability of such a pre-treated plant must be attributable to the activated function of the host cell and not to the antifungal activity of Piricularin itself which has an inhibitory effect to the germination of the blast fungus conidia even in a more highly dilute solution as 0.5μg/ml.
This conclusion has been further supported by the observation that the treatment of the plants with a Piricularin solution of relatively high concentration (10μg/ml) causes a decrease of resistnat activity of the host's cell against the growth of invasion hyphae, being accompanied with the impediment of the cell-function.

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The present paper reports the results of the experiments on the production of phytoalexin (PA) by the host-parasite interaction using rice plant as the host and Xanthomonas oryzae, the causal organism of rice leaf-blight, as the parasite. The experimental methods and the results were as follows.
1) Leaf-blade pieces of rice plant were sticked with capillary glass tube and mounted with drops of distilled water suspension of X. oryzae on the injured parts, and kept in moist chamber for 24∼48 hours. For the control, distilled water were mounted on the pricks of the leaf pieces (control 1) and the bacterial suspension were kept in test tube (control 2). Drops were collected after above noted hour and centrifuged for 10 minutes by 6000rpm. The supernatants were then added with the new suspension of X. oryzae, and after 24 hours, bacterial population of these solutions were compared by the method for the comparison of bacterial populations by phage technique¹⁴⁾.
The supernatant of X. oryzae supension on leaf inhibited the multiplication of X. oryzae (Table 1). The inhibitory action seems to be due to PA, but can not be said exactly, because X. oryzae showed no multiplication in the control 2, the supernatant of the suspension in test tube.
It is presumed that the inhibition of the multiplication shown in the control 2 may be due to the deficiency of some nutrients in the solution. The second experiment, therefore, was carried out with the synthetic medium instead of distilled water: the bacterial suspension made with the synthetic medium was mounted on the leaf pricks. For the control, the same suspension was kept in test tube (control 2) and the medium only was mounted on the leaf pricks (cnotrol 1). In the present experiment, X. oryzae showed a good multiplication in the controls, and therefore the inhibition of the multiplication of X. oryzae showen by the supernatant of the bacterial suspension on the needle pricks was considered to be caused by PA (Table 2).
The results mentioned above show that PA is produced by the results of the interaction between host plant and the bacterial pathogen as well as in the case of host and fungus⁸⁾.
2) The antibiotic action of the PA-solution mentioned above was compared with the one produced by the interaction of rice plant and Piricularia oryzae, the causal fungus of rice-blast. The technique adopted in the case of P. oryzae was similar to the previous paper⁸⁾. The results given in Table 3 and 4 show that each of these PA-solutions inhibited both the multiplication of X. oryzae and the germination of conidia of P. oryzae. It seems therefore that each of these two PA-solutions have no specificity on its antibiotic action.
3) Experiments were carried out to compare the multiplications of X. oryzae in the drops placed on the needle pricks of healthy leaves with that of ether-narcotized ones. The bacterial suspensions were mounted on the pricks. The suspension was collected after 24 hours and the bacterial populations with in unit volumes of these drops were compared by means of the bacteriophage technique. The results show that the multiplication of X. oryzae was inhibited in the case of the healthy leaves (Table 5). From the results, it is presumed that the main inhibiting factor may be due to PA, because the factor seems to exsist in the drops from healthy leaves and no PA action seems to be present in the case of the narcotized leaves.

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The writers have previously reported a decrease of β-amylase activity in rice leaf tissues surrounding the spots which occured after the inoculation with conidia of Cochliobolus miyabeanus. To clarify the cause of this phenomenon, we have investigated the formation of artificial spots by placing a drop of each solution of such chemicals as respiratory inhibitors, metal salts and phenols, on leaves of rice plants placed in the dark, and the influence of such chemicals on the decomposition of starch present in artificial spots and their surrounding tissues. In view of our exprimental results, these chemicals were divided into three groups as follows: (1) chemicals which form artificial spots and inhibit the starch translocation in rice leaves (Group I), (2) those which form artificial spots, but not inhibit the starch translocation (Group II) and (3) those which have ability neither to form artificial spots nor to inhibit the starch translocation (Group III). We have also investigated the direct or indirect inhibition, by the above-mentioned chemicals, of the activity of β-amylase prepared from rice leaves. All the chemicals belonging to Group I strongly inhibited the activity of this enzyme, whereas most of the chemicals belonging to Group II did not inhidit it. From the results of these experiments, it seems that a substance or substances which inhibit the activity of β-amylase may be produced by the tissues surrounding the spots on rice leaves or by the causal fungus itself.

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