Maize plants cv Kawanda Composite A growing in soil in a screenhouse and ranging in age from 1 - 9 weeks were inoculated with maize streak geminivirus (MSV) using viruliferous Cicadulina mbila adults caged either in groups of two or five per plant on top, middle or bottom leaves in order to determine whether a consistent relationship is present between symptom development and plant age. Symptoms generally developed first on the leaf emerging at the time of inoculation or, for older plants, on the leaf directly below. Leaves which had already emerged did not subsequently develop symptoms, so the lowermost leaf with symptoms indicated the growth stage at which a plant was infected. This relationship allows the stages at which different plants in maize crops were infected with MSV to be determined from just a few surveys, so facilitating epidemiological studies at sites which can be monitored only infrequently, such as distant ones or farmers' fields.
Gibson, R.W.; Page, W.W. The determination of when maize plants were infected with maize streak virus from the position of the lowest diseased leaf. African Crop Science Journal (1997) 5 (2) 189-195.