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5

Changes in susceptibility to soybean rust caused by Phakopsora pachyrhizi associated with plant age and leaf node position

Presenter: P. Srivastava

All authors and affiliations: P. SRIVASTAVA (1), J. Marois (1), D. Wright (1), D. Walker (2), L. Leandro (3). (1) NFREC, University of Florida, Quincy, FL; (2) USDA-ARS, National Soybean Research Center, Urbana, IL; (3) Department of Plant Pathology, Iowa State University, Ames, IA

The effect of plant age and leaf node position on susceptibility to soybean rust was evaluated on plants (cv. Williams 82) grown in greenhouse and outdoor conditions. Leaves on node 2, 5, and 7 of plants ranging in age from 39 (V4) to 75 (R8) days old were inoculated with a urediniospore suspension (4000 spores per ml). After inoculation, leaves were detached and placed on water agar in petri plates in a growth chamber (72C) or left attached to the plants. Two weeks after inoculation, leaves were evaluated for rust severity (%), lesion density (lesions per cm2), sporulation (15 scale), and pustule number per lesion. Soybean leaves were susceptible to rust on plants of all ages and on all nodes. However, rust severity, lesion density, sporulation, and pustule number per lesion were greater (P < 0.05) on older plants compared with those on younger plants. Leaf node generally did not have a significant effect on rust parameters, but outdoor plants in both attached and detached assays tended to show greater severity and sporulation on node 2 (lower) compared with node 7 (upper). Overall, the effect of plant age was more evident on outdoor plants than on greenhouse plants. Linear regressions showed a significant predictive relationship between detached and attached leaf assays on outdoor plants, with R ≤ 0.70 for rust severity and R ≤ 0.42 for lesion density. This study confirms that soybean plants are susceptible to rust at all stages of development and suggests that susceptibility is greatly determined by processes associated with plant physiological maturity.

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