Virginia soybeans: Factors to consider in rust management
Now that Asian soybean rust (SBR) has been confirmed in Suffolk, Va., growers in the state need to make some management decisions in regard to whether or not to spray fungicides for control of the disease. There are two approaches that can be taken at this point: 1) avoid risk and spray or 2) wait and see.
Unfortunately, it is not clear which is the correct approach.
Several factors need to be taken into consideration when deciding how to react to the presence of SBR in the Virginia. These factors are best summed up by what plant pathologists refer to as the “plant disease triangle” which is a visual representation (in the shape of a triangle) of the interaction between the pathogen, crop host, and environment. In the case of SBR, we must consider:
- how widespread the soybean rust fungus is in Virginia (pathogen),
- the maturity of the soybean crop (host), and
- the current weather conditions (environment).
On Sept. 17, 2013, the soybean rust fungus (Phakopsora pachyrhizi) was observed and confirmed on soybean leaves collected from one of the sentinel plots at the Tidewater AREC in Suffolk, Va. The soybeans were not showing symptoms of infection and it was only after incubating leaflets in a moist chamber for over a week and looking at hundreds of soybean leaflets under a microscope that I observed a few soybean rust pustules/spores.
Extension agents have been scouting fields and sending us soybean leaves for analysis over the past several weeks, and thus far Brunswick, Chesapeake, Lunenburg, Mecklenburg Westmoreland, Middlesex, Prince George, Amelia, and Powhatan counties have been negative for SBR (though this does not exclude the presence of the fungus at very low levels). Today my field technician is scouting several counties in the area, and he will bring back soybean leaves to the lab so we can check for the presence of the fungus. This, along with additional samples from Extension Agents, should give us a better idea of how widespread the fungus is in the state.
However, just because the fungus is at a low frequency and symptoms are not visible does not necessarily mean fungicide sprays are unwarranted.
A critical factor to take into consideration for SBR management is the maturity of the soybean crop. Once soybean reache s R6, risk of yield loss due to SBR is minimal and fungicide sprays are no longer needed. However, growers with soybeans that have not yet reached the R6 stage should consider spraying fungicides for control of SBR (triazole or pre-mix fungicide). David Holshouser, the soybean specialist at the Tidewater AREC, has recommended that soybean within 100 miles of Suffolk that has not yet reached R6 be sprayed. This is particularly relevant for late-maturing/double-cropped soybean.
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