1.4.9
INDUCED RESISTANCE OF ACIBENZOLAR-S-METHYL TO CUCUMBER ANTHRACNOSE AND SCAB

H ISHII1, Y NARUSAKA1, T HORIO1 and Y NAKAZAWA2

1National Institute of Agro-Environmental Sciences, Tsukuba, Ibaraki 305, Japan; 2ZEN-NOH Agricultural R&D Center, Hiratsuka, Kanagawa 254, Japan

Background and objectives
The benzothiadiazole compound acibenzolar-S-methyl (CGA245704, benzo(1,2,3)thiadiazole-7-carbothioic acid S-methyl ester) has been developed and is being used for the control of wheat powdery mildew as a 'plant activator' [1]. This compound also showed control efficacy against anthracnose and scab on cucumber, although it has no antifungal activity (Ishii H et al., unpublished data). The timing of resistance induction was examined and some pathological and physiological observations are made in this study.

Results and conclusions
On cucumber plants, acibenzolar-S-methyl induced not only localized acquired resistance (LAR) but also systemic acquired resistance (SAR) to fungal diseases. Induction of such resistance to both anthracnose and scab was triggered quickly. Control efficacy was observed even when whole cucumber plants were inoculated with conidial suspensions of each pathogen 3 h after dipping the first leaves in suspensions of this compound. Infection behaviour of the scab fungus was examined microscopically. Fungal penetration was not strongly inhibited and infection hyphae were frequently formed under resistance-induction conditions.

It is suggested, therefore, that critical events for the suppression of growth of infection hyphae occurred after the fungus achieved penetration, resulting in a decrease in disease development. A remarkable increase of peroxidase activity was found in the first treated leaf and in untreated upper leaves. This level of activity increased further when the plants were inoculated with either pathogen after treatment with the compound. Lignification of cucumber tissue was examined, but seems unlikely to be involved in disease resistance induced by acibenzolar-S-methyl.

References
1. Ruess W, Mueller K, Knauf-Beiter G et al., 1996. Proceedings Brighton Crop Protection Conference: Pests and Diseases, pp. 53-60.