Abstract
Root tumors developed on melon plants after either pouring inoculum of pathogenic Streptomyces sp. into soil surrounding growing plants or sowing melon seeds in soil inoculated with the pathogen with a latent period of ca. 7-14 days. In melon sown in infested soil, root tumors were first observed on branches of the primary root at 14 days. These tumors then began to form on other branches. Although numbers of root tumors increased until about 42 days after sowing in infested soil, the number of new tumors decreased about 49 days after sowing. By 49 days after sowing, length of infected melon plants were about 60% of that of healthy melon plants and the infected plants had about 85% of the number of leaves on healthy plants. When melon plants were inoculated at different growth stages, the most severe symptoms were observed on plants inoculated when 14 to 21 days old. Severity of the disease was mitigated with aging in plants inoculated after 21 days of growth. Development of symptoms is therefore closely related to the growth stage at the time of inoculation, with formation of root tumors more active when the root system is actively developing. On the other hand, injecting the pathogen into aerial melon parts causes morphological abnormalities on inoculated tissues. Although this pathogenicity was most severe on hypocotyl tissue, abnormalities on stem and petiole were milder than those on hypocotyl and were more conspicuous when plants were inoculated with mycelia than with spores.
