Assay of tobacco mosaic virus in bean leaves expanded under far-red light

Detached, etiolated leaves of Phaseolus vulgaris L. cv. Pinto, expanded at constant temperature under continuous far-red light, formed local lesions in response to infection with tobacco mosaic virus. Numbers of lesions increased when seedlings grown in vermiculite were watered with nutrient rather than with demineralised water, when leaves were inoculated 6, 7, or 8 days after seeds were sown rather than after 5 days, and when leaves received far-red light after inoculation rather than darkness. A preinoculation dark treatment of 24 hr decreased numbers of lesions in leaves inoculated 5 days after seeds were sown but did not significantly affect those in older leaves. Leaves could be used for virus assay, since there was a linear relationship between the number of lesions formed and the concentration of virus inoculated (from 0.25 to 1.00 μg/ml). For assay purposes, satisfactory lesions developed in leaves of 7-day-old seedlings grown under continuous far-red light and incubated after inoculation in darkness for 48 hr. Advantages of the system are noted.