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Cytology and Histology

Negative Geotropism in Venturia inaequalis. David M. Gadoury, Research associate, Department of Botany and Plant Pathology, University of New Hampshire, Durham 03824; William E. MacHardy, professor, Department of Botany and Plant Pathology, University of New Hampshire, Durham 03824. Phytopathology 75:856-859. Accepted for publication 7 March 1985. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/Phyto-75-856.

Pseudothecia of Venturia inaequalis developed on both the adaxial and abaxial surfaces of scabbed apple leaves overwintered on the floor of a New Hampshire orchard in 1982. However, the ascocarps were usually not found on both surfaces simultaneously. In a controlled study, scabbed leaves were overwintered in an orchard with either the adaxial or abaxial surface exposed. In mid-February, prior to ascus formation, leaves from each group were collected and incubated in the dark for 30 days at 10 C and 90% RH. The leaves were either inverted upon incubation or were incubated with the original upward surface facing up. Leaves were then embedded, sectioned, and examined microscopically. The neck and ostiole of over 90% of the ascocarps were directed towards the surface that had faced upward during winter, except in the leaves that were inverted upon incubation. In the inverted leaves, ascocarps formed on both the adaxial and abaxial surfaces. Some ascocarps in the inverted leaves developed both adaxial and abaxial ostioles. A central hymenium gave rise to asci that were directed towards both leaf surfaces. The surface colonized by V. inaequalis during the parasitic phase did not affect the orientation of the ascocarps or the number of ascocarps that formed. Orientation of pseudothecia was negatively geotropic. This negative geotropism ensures that ascospores will be ejected toward the atmosphere and may provide a mechanism for conservation of inoculum in disturbed leaf litter.