Studies on Resistance of Wheat Varieties to Scab (Gibberella zeae (Schw.) Petch.) : X. The improved method of inoculation by either conidiospores or hyphae of the scab cultured on potato agar medium

Abstract

In the field, the primary infections of scab to the heads of wheat occurred by the natural inoculation with ascospores or conidiospores flying in the air; whereas the artificial inoculation of scab is usually carried out by spraying a mixed suspension of spores and hyphae of the fungus. In this experiment of the improved method of scad inoculation, the possibility was tested that the hyphae alone might be infected to leaf blade or ears, as spores do. For this purpose, the hyphae as well as the spores were taken from the scad cultured on the potato agar medium. It was found that the infection could be also obtained by inoculating the hyphae alone, though the conidiospores were successfully used in the infection, of course. The materials (wheat varieties; Nobeoka-bouzu and Wheadling) used in this experiment were much more resistant to scab than the variety Shin-chunaga, which is recognized one of the most resistant varieties in Japan. Nevertheless, no difference was seen in the susceptibility to scab between these resistant varieties and the less resistant ones reported in our previous papers. This fact supports our previous assumption that both the resistant varieties and the susceptible ones show the nearly same degree of infection when the scab was inoculated into leaf blade. From the results of our previous studies concerning the scab infections to leaf blade, head and rachis of wheat by the improved method of inoculation, the conclusions with regard to the mechanisms of infection, resistance or susceptibility to scab were drawn as follows: (1) The absence of natural infection of scab on the leaf blade of wheat in the field may well be due to the fact that it has no morphological and histological factors (steps) to promote the germination of scab spores and the subsequent intrusion of hyphae into the tissue of leaf blade. But by the improved method of inoclation, scab fungus is able to intrude easily into leaf blade, and to show a symptom of scab infection on it, owing to the supply of moisture and nutrient. (2) The fact that the natural infection of scab to wheat is restricted to heads suggests the existence of various morphological and histological factors (steps), which are needed for the intrusion of scab into heads of wheat. Such factors as number of anthers caught by glumes, pollens, number and size of stomata on the inner parts of outer glumes were recognized as the important factors influencing the entrance of scab into heads. Furthermore, the resistance or susceptibility of wheat varieties to scab might be attributable mainly to the characteristic differences of these factors on the head of each variety. (3) The foliage lesion of scab inoculated by the improved method was similar in both the resistant and susceptible varieties. This similarity was explained by the fact that any of the wheat varieties is never immune or even highly resistant to scab constitutionally, and that all the varieties are equally susceptible rather than resistant to scab. (4) The degrees of lesion in various varieties infected by the improved method of inoculation on heads or rachises did not always coincide with those of the varieties whose resistances were judged by the degrees of damages received in the field. The possible explanation of this fact may be that when the tissues of heads or rachises were directly inoculated by the improved method, the tissues permit the entrance and spreading of fungus without the aid of any of the factors above-mentioned. In spite of this fact, however, the existence of slight but recognizable differences of varietal resistance in the predisposition cann't be completely excluded. The available data do not permit the definite conclusion, and these phenomena are open for further research.