In response to the appearance of Phakospora pachyrhizi, or soybean rust, in the United States, the USDA developed a federal, state, university and industry framework for surveillance, reporting, prediction and management of soybean rust for the 2005 growing season.
The purpose of this coordinated framework is to provide growers, researchers, pesticide manufacturers and suppliers with timely information for managing soybean rust this year.
Local plot. As part of this effort, we have planted a soybean rust sentinel plot in Columbiana County. The plot was planted April 11 (early planting is essential to make sure susceptible plants are available whenever disease spores arrive).
This plot will be monitored weekly until disease risk is high (after rust has been found in a neighboring state) and then twice weekly until rust is discovered in the sentinel plot.
If rust is discovered in the sentinel plot, it will be treated with fungicide and monitoring will continue throughout the growing season.
The plot will also be monitored for bean leaf beetle, soybean aphid, soybean cyst nematode and all other seedling and plant diseases, including rhizoctonia seedling disease, bacterial blight, phytophthora stem canker, sudden death syndrome and sclerotinia stem canker.
Greater risk. In 2005, soybean rust has been confirmed in one continental U.S. county – Pasco County, Fla. The find was made Feb. 23 on overwintering kudzu.
Because soybean rust successfully survived the winter in the U.S., the risk of spread into the U.S. soybean crop in 2005 is greatly increased.
Soybean rust sentinel plots have been planted throughout the South and Corn Belt to monitor the spread of the infection from Florida.
Carried by wind. Soybean rust spores are carried on wind currents just like spores from corn rust, corn leaf blights and the common alfalfa insect pest, potato leafhoppers, are carried to more northern areas from overwintering sites in the South.
Weather patterns and specific weather events have a major impact on the spread of such diseases and insects.
You have probably noticed that diseases like corn rust and corn leaf blights are much more severe in some years than in others. The primary reason for this variability is the date when infestation occurs in the field.
Other factors in disease severity relate to conditions for growth of the disease after the spores start to grow on a host plant. Plant wetness and temperature play a major role in the progress of plant diseases.
Once spores have arrived, the longer plants are wet at the proper temperature for disease growth, the more severe the disease will be.
All beans susceptible. Many corn varieties have some resistance to corn rust and/or corn leaf blights. However, we have very little information about resistance of soybean varieties to soybean rust.
You should assume that all varieties you plant are susceptible.
There are two strategies for controlling soybean rust in soybean fields: 1. protecting the crop through application of protectant fungicides before spores arrive, but after risk is determined to be high, or 2. application of curative fungicides once infection has occurred.
What stage are your beans? One, two, or three applications may be needed, depending upon when the disease comes in and at what stage the first application is made.
Soybeans have two growth phases, vegetative and reproductive. The vegetative (V) stages include all stages from emergence to initial flowering. Reproductive (R) stages include flowering and seed production, through beginning maturity.
The vast majority of reports from Africa and Brazil indicate that soybean rust does not need to be controlled when detected in the vegetative crop stages as long as a curative spray program is initiated as soon as crop flowering begins.
Spraying before crop flowering, however, may be prudent if disease incidence is increasing and the crop is approaching initial flowering (R1). This is especially true for late-planted crops or late maturing varieties that may develop a large canopy before flowering.
Disease incidence is the number of leaves out of 100 with any rust.
Yield loss is very likely once rust can be found in the mid-crop canopy.
Checking for rust. Soybean rust is not easy to diagnose. The disease starts in the lower canopy , so you must go into the crop and actively look for it.
The rust lesions (pustules) show up so small, they look like tiny bumps on the under side of leaves. You need a 16- or 20-power hand lens to see the tiny flesh-colored spore-forming pustules. At this time, the upper surface of the leaf is still completely green.
If the leaves have turned yellowish or started to drop off the plant, it is too late to spray.
All fungicides will work on soybean rust. Timing is the important factor in successful control. One leaf with rust pustules out of 500 carefully examined gives a two-week window for spray applications.
(The author is an agricultural extension educator in Columbiana County. Questions or comments can be sent in care of Farm and Dairy, P.O. Box 38, Salem, OH 44460.)