Cowpea chlorotic mottle virus local lesion area and infectivity increased by 2-thiouracil

Treatment with 2-thiouracil caused cowpea chlorotic mottle virus (CCMV) lesion area on hypersensitive soybean to be enlarged 8–75 times. Lesion enlargement was detected 48 hr after inoculation, and lesions increased in size rapidly for 6–8 days. Although soybean becomes more resistant (fewer and smaller lesions) with increasing age, thiouracil caused lesions to reach a similar size regardless of host age. Compared to controls, lesion area increased 2, 13, and 4 times at 21, 27, and 32°, respectively. Increased lesion area was noted when thiouracil treatment began at 0, 12, and 24 hr after inoculation, but not at 48 hr. The full effect, however, occurred when treatment began between 0 and 12 hr. When lesion area was increased 10–13 times, the infectivity was increased 33–38 times. The increase in lesion size could be prevented when uracil (10 −3 M) was added to thiouracil (10 −4 M). Thiouracil treatment caused CCMV to move from one leaf half to another, but no systemic movement from one leaf to another was detected. CCMV sap infectivity was enhanced 3 times whereas tobacco mosaic virus sap infectivity was inhibited 7 times by thiouracil in Chenopodium amaranticolor, a local-lesion host for both viruses.