Abstract

Sandal spike disease was first observed in Reserve 51 of Marayoor forest range of Munnar forest division during June 1980; 50 of the sandal trees in this reserve were affected. Rate of disease spread in two disease monitoring blocks, calculated in terms of apparent infection rate, was 0.080 and 0.066 per unit per month (giving estimated times for 95 incidence of 70 and 87 months respectively). Zizyphus [Ziziphus] oenoplia, Stachytarpheta sp. and Jasminum rigidum showed witches' broom symptoms in several places in the reserve and were suspected to be the collateral hosts of the pathogen. Symptomatological studies are described, including electrophysiological studies using a Shigometer, which indicated a positive correlation between severity of the disease and electrical resistance of inner bark in diseased trees. Histopathological studies using aniline blue and Hoechst 33258 gave evidence on the occurrence of MLO in the diseased phloem tissues; this technique could be used as an easy method of disease detection. Although TEM studies showed large numbers of MLO in the phloem tissues, attempts to culture the organism in vitro were not successful. Redarator bimaculatus was identified as an insect vector of the disease through disease transmission studies and from the presence of MLO in the tissues of the intestine and salivary glands of insects fed on spiked sandal, and in the phloem tissues of plants infected through the vector. Infusion of an aqueous solution of 500 mg of tetracycline in 500 mlof water gave a remission of disease symptoms in spiked trees which lasted for 3-5 months. Repeated infusion of tetracycline-HCl in alternate months for one year did not give complete recovery. Higher does of tetracyclines (2-8 g/tree) prolonged the remission period up to 7-8 months. Infusion of tetracycline at 12 g/tree was phytotoxic. Combinations of different tetracyclines did not improve antibiotic action. Digitonin and 2, 1-guanidinododecane acetate also gave temporary remission of disease symptoms, but cephalaxin was ineffective