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Ontogenic Resistance of Leaves and Fruit, and How Leaf Folding Influences the Distribution of Powdery Mildew on Strawberry Plants Colonized by Podosphaera aphanis

September 2014 , Volume 104 , Number  9
Pages  954 - 963

Belachew Asalf, David M. Gadoury, Anne Marte Tronsmo, Robert C. Seem, Andrew Dobson, Natalia A. Peres, and Arne Stensvand

First and third authors: Department of Plant and Environmental Sciences, Norwegian University of Life Sciences, 1432 Ås, Norway; first, fifth, and seventh: Norwegian Institute for Agricultural and Environmental Research, Plant Health and Plant Protection Division, 1432 Ås, Norway; second and fourth authors: Department of Plant Pathology and Plant-Microbe Biology, Cornell University, New York State Agricultural Experiment Station, Geneva 14456; and sixth author: Department of Plant Pathology, University of Florida, Gulf Coast Research and Extension Center, Wimauma 33598.

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Accepted for publication 20 February 2014.

Ontogenic or age-related resistance has been noted in many pathosystems but is less often quantified or expressed in a manner that allows the concept to be applied in disease management programs. Preliminary studies indicated that leaves and fruit of three strawberry cultivars rapidly acquired ontogenic resistance to the powdery mildew pathogen, Podosphaera aphanis. In the present study, we quantify the development of ontogenic resistance in controlled inoculations of 10 strawberry cultivars using diverse isolates of P. aphanis in New York and Florida, USA, and in Norway. We report the differential and organ-specific development of ontogenic resistance in the receptacle and externally borne strawberry achenes. We further report that rapid development of ontogenic resistance prior to unfolding of emergent leaves, rather than differential susceptibility of adaxial versus abaxial leaf surfaces, may explain the commonly observed predominance of powdery mildew on the lower leaf surfaces. Susceptibility of leaves and fruit declined exponentially with age. Receptacle tissue of berries inoculated at four phenological stages from bloom to ripe fruit became nearly immune to infection approximately 10 to 15 days after bloom, as fruit transitioned from the early green to the late green or early white stage of berry development, although the achenes remained susceptible for a longer period. Leaves also acquired ontogenic resistance early in their development, and they were highly resistant shortly after unfolding and before the upper surface was fully exposed. No significant difference was found in the susceptibility of the adaxial versus abaxial surfaces. The rapid acquisition of ontogenic resistance by leaves and fruit revealed a narrow window of susceptibility to which management programs might be advantageously adapted.

Additional keywords: Fragaria × annanasa, Sphaerotheca macularis.

© 2014 The American Phytopathological Society