Plants have been evolved to obtain the ability to induce the cascade of defense response upon exposure to wide variety of plant pathogens. Disease incidence, thus, should be considered as an exceptional event for the plants. One of the most important mechanisms leading to this exceptional event has been shown to be due to the suppression of the induction of defense response of host plants by the pathogen. At least in the case of bacterial pathogens, we found that this suppression was shown to be elicited by most of the avirulence (avr) genes. This rather unexpected finding (ie. dual function of avr genes) has opened a new strategy to construct the plant resistant to any plant diseases by escaping from the suppressor activity conferred by avr gene.
A baiting method using Lupinus luteus was applied to the relative estimation of the inoculum density of the white root rot fungus, Rosellinia necatrix in soil. Chips (20 g in dry weight) of dormant twigs of Japanese pear colonized by the fungus were serially diluted with 5 L of an orchard soil and incubated for a week. Thirty seeds of L. luteus were sowed at 5 cm intervals into the infested soil, and the plants were grown at 25 ℃ in a greenhouse during a month. Infection on the plants was then checked. Necrotic and/or rotten legions were observed on the hypocotyls and/or tap roots. There was a significant correlation between the dilution rate of the inoculum and the detection frequency of the white root rot fungus in artificially infested soil. This method is considered to be useful for estimating the inoculum density of the white root rot fungus in field soil.