Effect of soybean leaf and plant age on susceptibility to initiation of infection by Phakopsora pachyrhizi

Authors: Bonde, Morris; Nester, Susan; Berner, Dana

Submitted to: Plant Health Progress
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/23/2011
Publication Date: 2/27/2012
Citation: Bonde, M.R., Nester, S.E., Berner, D.K. 2012. Effect of soybean leaf and plant age on susceptibility to initiation of infection by Phakopsora pachyrhizi. Plant Health Progress. PHP-2012-0227-01-R.

Interpretive Summary: Soybean rust is an important disease of soybean caused by a fungus first discovered in the United States in 2004. The incident of disease in the field often has been observed to increase in fall months at a time when plants are approaching full maturity. This increase in disease has caused some researchers to speculate that as plants begin to flower they become more susceptible to infection by the pathogen. To determine if the speculation was correct, we undertook a study of plant and leaf age to susceptibility to soybean rust. Our results under controlled conditions in growth chambers and greenhouse indicate that the increase is not due to changes in the plant but more likely to changes in the environment from one that is less favorable to one that is more favorable for disease development. This information will help in the development of models with which to predict soybean rust epidemics and ultimately in better control of the disease by the farmer.

Technical Abstract: Although previous studies have been conducted to determine the relationship of plant and leaf age to susceptibility to soybean rust, this relationship still is unresolved. Some studies suggest that as plants reach the flowering stage they become more susceptible to initiation of infection. However, others have indicated that younger plants and/or leaves are more susceptible. To help resolve the question, we undertook a study in which 50-day-old plants were inoculated with a urediniospore suspension, placed in growth chambers under a range of temperature conditions, and later examined for number of lesions at the different leaf positions. In addition, plants ranging in age from 20 days (seedling stage) to 80 days (reproductive growth stage R4) were inoculated, placed in a greenhouse, and examined by leaf for number of lesions that developed. Results showed that the cultivar tested, Williams 82, a late group III indeterminate variety, did not vary in susceptibility among leaves at different positions when inoculated 50 days after planting, regardless of temperature. Furthermore, on a per plant basis, as plant age increased from 20 days to 80 days at the time of inoculation, there was a progressive decrease in susceptibility based on numbers of lesions which developed. Although the study does not rule out the possibility that there might be cultivar differences in age response, it does strengthen the possibility that increases in amounts of soybean rust observed in the fall are due to changes in environment from less favorable for soybean rust to more favorable.