MOVEMENT AND MULTIPLICATION OF SPIROPLASMA-KUNKELII IN CORN

After the introduction of Spiroplasma kunkelli into the leaves of corn (Zea mays) seedlings by inoculative corn leafhoppers (Dalbulus maidis), the pathogen was detected by enzyme-linked immunosorbent assay. Spiroplasmas were detected in leaves 14 days after exposure to inoculative leafhoppers (as much as 2 weeks before symptoms appeared) and in roots after 20 days. The probabilities of detecting spiroplasmas in different plant organs, determined by logistic regression, generally increased over time; the probability was greater in roots than in leaves at most testing dates. Spiroplasmas moved into tassels as they developed. The age and/or size of the plant at the time of infection affected pathogen movement; spiroplasmas were detected earlier in roots of plants inoculated at the one-leaf seedling stage than in those of plants inoculated at the four-leaf stage. Spiroplasma titers remained relatively low in all the plants until the third or fourth week after inoculation. After that time, populations in different plant samples varied within a large range, and the upper limit of the range generally increased with time. The maximum titers were reached earlier in plants inoculated at the four-leaf stage than in plants inoculated at the one-leaf stage. Because S. kunkelii may be present in various plant organs well before symptoms appear, spiroplasmas might be acquired by leafhoppers before disease is obvious in the field.