Leaf lesions of blast, caused by Piricularia oryzae, on a susceptible variety of rice were grouped into five classes according to the degree of brown coloration and size of the lesions (see Fig. 2)
Sporulation process on a single lesion of class 4 was observed and microphotographed at definite intervals. After a lesion was placed in a moist chamber, specially designed for the present purposs as shown in Fig. 1, a conidiophore began to emerge in about six hours, and produced the first conidium at its apex one hour later which reached its full size in 40 minutes. Soon after, a branch developed from the conidiophore just below the point of attachment of the first conidium, and the second conidium was formed at its apex.
In the same manner, process of conidial formation continued to form 7 to 9 conidia on one conidiophore, each being formed at about an hour's interval. The mode of branching of the conidiophore is sympodial in accord with the observations of Henry et al. (1948).
Sporulation intensities of lesions of different classes were compared. Sporulation was most rapid and abundant on 4, less on 5 and 3, and very scarce in 2, whereas entirely absent on 1. It seems likely that the browning of the lesion could retard the spread and normal growth of the mycelia in the host tissue, and, consequently, the sporulation on the lesion.
(National Institute of Agricultural Sciences)

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Recently it has been shown that some plant pathogenic fungi require growth factors for their normal development. The present paper describes experimental studies about the growth factors and nitrogen sources of Piricularia oryzae Cav. (rice blast disease fungus).
From the cultural experiment on synthesized media, it is shown that biotin is the essential and indispensable growth factor.
While vitamin B₁ is a complemental factor for this fungus growth.
Optimum concentration of biotin for this fungus lies in the range of 2.3mγ/ml-3.0mγ/ml.
While, as above mentioned, biotin and vitamin B₁ are essential growth factors of the present fungus, the biotin is almost always detectable in slight amounts in the organic compounds, which are usually given as nutrition materials in the culture media of fungi, for example, sucrose. Therefore, the cultural studies of this fungus which were made without paying attention to the biotin should be investigated again. According to the present experiments, nitrate is good nitrogen source for Piricalaria oryzae Cav. and ammonium salts is not good, probably because the dropping of pH value which may be caused by the unequivalent absorption of ammonium ion brings about a bad condition for this fungus growth. Nitrite cannot be assimilated by this fungus. The various amino-acids and amides differ in value from each other as nitrogen source for this fungus. Glycocoll l-alanine, aspartic-acid, d-glutamic-acid and asparagine are used to better advantage than nitrate. The worth of urea, dl-valine, l-leucin, l-cystin dl-norleucine, dl-methionine, tyrosine, arginine hydrochloride, dl-ornithine hydrochloride as nitrogen source is inferior to that of nitrate and creatine, taurine and dl-α aminobutyric acid are sure to be inadequate nitrogen sources for the present fungus.
In conclusion it is sure that for this fungus some aminoacids and amides are better nitrogen source, though some inorganic nitrogenous compounds such as nitrate are also assimilated well.

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A study was made on the factors determining the severeness of the black rot of sweetpotatoes in the field, and the mechanism of infection was discussed, based upon the experiments on the varietal resistance with 53 varieties. The experiments were succssively carried out from 1951 at Shiraimura, Chiba Pref.
1. The roots in the field are infected mostly by the pathogen liberated from the diseased part at the base of seedlings, and the 'soil infection' is rather of minor importance in this respect.
2. When diseased seedlings are planted in the field, there reveal varietal differences with high statistical significance in respect to the enlargement of the stem (underground) lesion.
3. The differences actually influence upon the ratio of the infection of roots at digging time. As to the cause of this it is supposed that the density of the pathogen around the diseased plants is reduced on the side of resistant varieties. This effect is accentuated by the prompt healing of stem lesions of resistant varieties during hot summer.
4. There also are varietal differences with respect to the grade of insury by the wire-worms (Melanotus caudex). These insects transmit the disease, and consequently the chance of the infection of roots varies greatly with varieties of sweetpotatoes.
5. The percentage of diseased roots in the field is more strongly influenced by the varietal difference of wire-worm injury as compared with the resistance of stem for the enlargement of the diseased part.
6. Severeness of the disease in the field seems to be explained in reference to these factors, except for climatic factors may modify it annually.
7. The fields with similar conditions distribute widely in Kanto and Tokai regions and presumably also in other regions of southern and western. Japan.

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The present paper deals with the results of experiments on the turbidity, being measured with the electric photometer, of the juices expressed-out from virus-infected tubers of potatoes of sweet potatoes and tuberous roots of radish or turnip. The results obtained are summarized as follows.
During storage the turbidity of the healthy potato juice is generally slightly lower than that of the diseased. The turbidity was shown to be low according to the severity of the disease and also to the lapse of time in storage. The correlation between the turbidity and the tuber size was not recognized. As for the average turbidity tested with many tubers, the healthy juice is geneaally higher than diseased, though individual tuber shows irregular in turbidity. The largest difference between the healthy juice and the diseased was shown at 1/8 dilution.
The turbidity of radish juice gave the similar results to the case of potatoes, and the difference between the healthy and the diseased is more larger. The turbidity of the diseased younger tuberous roots seems to decrease owing to the accumulation of certain substances, though that of the diseased older one, during the later growing period, increases owing to the stagnation of formation and transportation of the substances. The upper part of the root is higher in turbidity than other parts.
The turnip juice gave similar results to the case of radish, and the sweet potatoes showed slight difference in turbidity between the healthy and the diseased.
The turbidity degree of juice from the living materials shows reversely to the protein content, but the older juice or the diseased seldom shows contrary to such a rule because of the occurrence of coaglation or precipitation by the auto-denaturalization. From these experiments it is clear that the turbidity test is not to be utilizable for the purpose of quantitative analysis, though it may be utilizable for diagnosis.

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[in Japanese]

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