Varietal susceptibility of sweetpotatoes to black rot and its mechanism. I.

Factors influencing the infection of roots in the field and their interrelation.

Abstract

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.