The determination of when maize plants were infected with maize streak virus from the position of the lowest diseased leaf

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. Les plantes de mais du cultivar Kawanda Composite A plantes en sol dans une serre, variant de 1 a 9 semaines d'age ont ete inocules avec la "streak" geminivirose du mais en utilisant des adultes virulents de Cicadulina mbila en cage en groupe de deux ou de cinq par plante, au sommet, au centre ou bien a la base foliaire. Generalement, les symptomes se sont developpes en premier sur la feuille qui a emerge au moment de l'inoculation ou sur la feuille directement en dessous de la precedente pour les plantes plus agees. Les feuilles qui avaient deja emerge n'ont pas developpe de symptmes, ainsi la feuille plus a la base avec symptmes a indique le stade de croissance a laquelle la plante a ete infectee. Ceci permet ainsi la determination, avec quelque peu d'enquetes, des stades auxquelles les plantes de culture de mais auraient ete infectes avec la virose du mais, devenant ainsi une methode appropriee pour l'etude de l'epidemiologie de la maladie en champs des fermiers ou pour des sites lointains.