ARS | Publication request: Changes in Susceptibility to Soybean Rust Caused by Phakopsora pachyrhizi Associated with Plant Age and Leaf Node Position

Submitted to: National Soybean Rust Symposium
Publication Type: Abstract Only
Publication Acceptance Date: October 31, 2009
Publication Date: December 9, 2009
Citation: Srivastava, P., Marois, M., Wright, D., Walker, D.R., Leandro, L. 2009. Changes in Susceptibility to Soybean Rust Caused by Phakopsora pachyrhizi Associated with Plant Age and Leaf Node Position [abstract]. In: Proceedings of the National Soybean Rust Symposium, December 9-11, 2009, New Orleans, LA. p. 5.

Technical Abstract: 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 the 2nd, 5th and 7th nodes of plants ranging in age from 39 (V4) to 75 (R8) days old were inoculated with urediniospore suspension (4000 spores/ml). After inoculation, leaves were either detached and placed on water agar in petri plates in a growth chamber (72o C), or left attached to the plants. Two weeks after inoculation, leaves were evaluated for rust severity (%), lesion density (lesions/cm2), sporulation (1-5 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 to younger plants. Leaf node generally did not have a significant effect on rust parameters assessed, but outdoor plants in both attached and detached assays tended to show greater severity and sporulation on the 2nd (lower) node compared to the 7th (upper) node. Overall, the effect of plant age was more evident on outdoor plants than greenhouse plants. Linear regressions showed a significant predictive relationship between detached and attached leaf assays on outdoor plants, with R2=0.34-0.70 for rust severity and R2=0.22-0.42 for lesion density. This study confirms that soybean plants are susceptible to rust at all stages of development and suggests that susceptibility is greatest in older plants due largely to processes associated with plant physiological maturity.