1978 Volume 44 Issue 5 Pages 578-584
“Kohan” disease of lettuce, resembling lettuce big vein disease and hitherto unknown to Japan, has occurred in Wakayama Prefecture. The main symptom on lettuce was a chlorosis along the leaf vein, and a characteristic boundary line between chlorotic and green areas. The causal agent was not transmitted by mechanical inoculation from a diseased leaf of lettuce to several test plants, by aphid, Myzus persicae, from diseased lettuce to healthy lettuce, and also, through seed of lettuce. Virus-like particles were not detected from diseased leaf tissues of lettuce in electron microscopic studies using direct negative staining technique and ultrathin section method. When healthy lettuce seedlings were transplanted into the soil collected from beneath the infected plants from several farms, they showed typical symptoms at high percentage 20-45 days later. The infested soil remained infectious after air-drying for 3 weeks. Lettuce seedlings that had been transplanted to soil containing the roots of naturally infected plants developed symptoms, whereas those that were transplanted to soil containing the leaves of naturally infected plants, did not. Resting spores and zoosporangia of Olpidium sp. were found in roots of all naturally infected plants. The causal agent was transmitted to lettuce by zoospores collected from roots of naturally infected plants. And also, it was transmitted by grafting scions from diseased plants to the stem of healthy plants, but not through soil containing the roots of diseased plants that had been infected by grafting.