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Host-Pathogen Interactions Preceding the Hypersensitive Reaction of Malus sp. to Venturia inaequalis. R. L. Nicholson, Department of Botany and Plant Pathology, Purdue University, W. Lafayette, IN 47907; S. Van Scoyoc(2), E. B. Williams(3), and J. Kuć(4). (2)(3)(4)Department of Botany and Plant Pathology, Purdue University, W. Lafayette, IN 47907, (4)Present address: Department of Plant Pathology, University of Kentucky, Lexington, KY 40506. Phytopathology 67:108-114. Accepted for publication 19 July 1976. Copyright 1977 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-67-108.

Etiolated apple seedlings, either susceptible or hypersensitively resistant to each of four races of Venturia inaequalis, were inoculated simultaneously with spores from a particular race of the pathogen. Development of the fungus and concurrent host responses were followed in the living host-parasite system by light microscopy of epidermal strips from etiolated hypocotyls. Germination, appressorium formation, and penetration by V. inaequalis occurred at the same rate irrespective of susceptibility or hypersensitivity of the host. Following penetration of the cuticle of susceptible apple hypocotyls, growth of the fungus proceeded without interruption, and subcuticular stromata were formed. Penetration of hypersensitive hosts occurred at the same time as that on susceptible hosts, but the formation of subcuticular primary hyphae (which follows penetration) depended upon the pathogen. In addition, subcuticular stromata were not formed in hypersensitive host-pathogen combinations. Thus, inhibition of fungal growth in hypersensitive combinations occurred close to the time of penetration since primary hyphae either did not form or formed at a rate equal to or less than that in susceptible host-pathogen combinations. Granulation of cytoplasm in hypersensitively reacting cells and subsequent cell-browning occurred significantly later than the inhibition of subcuticular fungal growth. Similarly, changes in phloridzin and phloretin content of etiolated hypocotyls, which were measured by gas-liquid chromatography, did not occur until about 33 hours after inhibition of fungal growth. The data suggest that containment of V. inaequalis in the hypersensitive response is not mediated by changes in phloridzin or phloretin as previously assumed.

Additional keywords: apple scab, physiological contact, Phaseolus vulgaris.