A Fluorescent Technique for Studying Growth ofPeronospora tabacinaon Leaf Surfaces

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Abstract
Calcofluor was used as a fluorescent nontoxic stain to study germination, appressorial formation, vesicle production, and sporulation of Peronospora tabacina on leaf surfaces. Fungal germination proceeds equally well on leaves of tobacco, pepper, tomato, and potato but not on Nicotiana debneyi in which germ tubes were abnormal and appressorial formation was inhibited. In tobacco, appressorial formation decreased with increasing plant age and gradually decreased in young leaves of older plants. In tobacco, appressorial formation was not affected as long as 21 days after stem inoculation with P. tabacina but was inhibited at 30 days. Germination and appressorial formation in tobacco were not affected by metalaxyl but were strongly inhibited by .beta.-ionone. The technique simple, rapid, and reliable and can be used to study effects on fungal ingress in situ of plant genotypes, toxicants, and environmental factors.