Lack of Systemic Colonization of Alfalfa Plants After Inoculation of Uninjured Leaves with Conidia of Verticillium albo-atrum. R. M. Jimenez-Diaz, Fulbright Research Scholar, Department of Plant Pathology, Cornell University, Ithaca, NY 14853. R. L. Millar, Professor, Department of Plant Pathology, Cornell University, Ithaca, NY 14853. Plant Dis. 70:509-515. Accepted for publication 19 December 1985. Copyright 1986 The American Phytopathological Society. DOI: 10.1094/PD-70-509.

Healthy uninjured or injured leaves of alfalfa plants were inoculated with a conidial suspension (10⁵ or 1.2 × 10⁶ per milliliter) of the alfalfa strain of Verticillium albo-atrum. Leaflets were pierced with a sterilized needle, rubbed with Carborundum, or touched with a hot glass rod or a piece of dry ice to injure them. Inoculation consisted of spraying plants or individual leaves with the conidial suspension and placing single drops of the suspension either at the bases or the edges of selected leaflets. Ingress and systemic colonization were determined by attempts to isolate the pathogen from leaves and stems. On uninjured healthy leaves, conidia germinated readily, usually producing two germ tubes, which in turn led to profuse branching of hyphae and development of phialidelike conidiophores and conidia. Some hyphae penetrated and grew within the cuticle. No fungal structures were observed either within epidermal cells or in palisade or mesophyll tissues by 16 days after inoculation, and no deleterious effects were detected. The pathogen remained restricted to the inoculated leaves, and neither symptom development nor systemic colonization by the pathogen occurred. Symptoms characteristic of Verticillium wilt and systemic colonization by the pathogen occurred only when leaves had been injured before inoculation.