Planting resistant varieties is one way to avoid sudden death syndrome (SDS) and stem canker in soybean fields, but researchers are looking for other approaches to preventing these devastating diseases.
Dr. John Rupe, whose research has helped provide the information needed to identify resistant varieties, says, "We would like to provide other control options so the farmer's choice of varieties will not be limited by the requirement for SDS or stem canker resistance."
The goal of stem canker research is to develop a fungicide scheduling program.
Plant pathologists Rupe and Dr. R. T. Robbins and agronomist Dr. H. D. Scott are principal investigators in a University of Arkansas Agricultural Experiment Station project to study the relationship of the environment to the development of SDS and stem canker.
This research is funded in part by the Arkansas Soybean Promotion Board, which also helps support the field screen| ing of soybean varieties for performance under stem canker, phytophthora rot and SDS pressure.
Organisms from diseased plants in Arkansas fields are used to establish disease nurseries for screening varieties. The disease ratings from other states can be misleading in terms of the actual tolerance or susceptibility of a variety under local field conditions.
Disease nurseries include one for stem canker at the Southwest Research and Extension Center, Hope; for SDS at Cotton Branch Experi ment Station, Marianna; and for phytophthora at the Rohwer unit of the Southeast Research and Extension Center.
Variety screening at the disease nurseries, along with other research on the effects of environment on SDS and stem canker development, will help producers select the best varieties for fields with a history of these diseases and help researchers identify other disease control options.
Stem Canker - The goal of the stem canker research is to develop a fungicide scheduling program. "To do that it is essential to understand when infection occurs and the relationship of infection to symptom development," Rupe said.
'Work in other states indicates that fungicides can be effective, but without knowing when infection occurs, it's difficult to apply them in a timely fashion."
Experiments in the field, greenhouse and growth chamber are designed to answer questions such as whether symptoms are triggered by time - measured as growing degree days - or by plant growth stage; which plant parts need to be protected by a fungicide; and how disease development is affected by air temperature, humidity, leaf wetness and other environmental factors.
Preliminary results indicate that infection is most likely during relatively cool and wet; periods early in the season. "There has to be rain because S the fungus spores are splash dispersed," Rupe said.
"And it looks like infection is most likely when plants are wet for two or three days and the temperature is in the range of 70 to 85 degrees Fahrenheit. "Eventually, we hope to be able to define the conditions a farmer can look for as a guide to when to apply a fungicide to: protect a susceptible variety 0 from stem canker, and tell him how long that protection will last."
SDS - There is no evidence that fungicides could be used to prevent SDS, but a model for predicting SDS occurrence could identify cultural practices such as planting date or X irrigation timing to avoid infection or reduce its severity, Rupe said.
Experiments are being conducted to document factors such as the rose of temperature and moisture and the growth stage at which SDS is most damaging.
Developing the predictive models will require many observations under experimental conditions to identify patterns of disease and plant response to environmental conditions. A model would then have to be tested and validated under experimental and field conditions.