Brown stem rot (BSR) of soybeans has the ability to cause yield losses greater that 30%. A yield loss of 17% was attributed to BSR at U of I's Northwestern Illinois Agricultural Research and Demonstration Center (NWRC) near Monmouth in 2002, even though very few foliar symptoms of the disease were visible. Greater yield loss to BSR has been observed when foliar symptoms are present. The disease is more prevalent in the northern soybean-growing region, partially because the cooler temperatures are more favorable for the development of disease symptoms and survival of the fungus. Though it can be found in all of Illinois, the likelihood of yield loss to BSR increases progressively north through the state.
Brown stem rot symptoms on a leaf.
Pith browning caused by brown stem rot.
The fungus that causes BSR, Phialophora gregata, produces toxins that cause the browning of pith tissue in the stems and the interveinal chlorosis and necrosis in the leaves characteristic of BSR. The stem and foliar symptoms generally become evident after the
pods begin to form (growth stage R4). The stem-browning symptoms are first evident as faint browning at the lowest nodes of the plant, working progressively up the stem and causing browning between the nodes. The fo-liar symptoms usually are most visible around stage R6 (soybeans have filled pod) or later, which can begin in mid- to late August. From the road, a field infested with BSR can look like it is just maturing a couple of weeks early. The cooler nights during pod fill that favor yield also favor the development of foliar symptoms of BSR. The foliar symptoms of BSR have not been very visible the past several years at NWRC and other locations in Illinois because of above-normal temperatures in August and September, though yield loss has still occurred. Dean Malvick, plant pathologist at the U of I, is investigating the effects of BSR on yield when foliar symptoms are not expressed.
The foliar symptoms of BSR are almost identical to those caused by sudden death syndrome (SDS) of soybeans. They cause both yellowing and death of leaf tissue between the veins. Splitting the stems can help distinguish between BSR and SDS, though a lab test may be necessary to confirm which pathogen caused the symptoms. Soybean plants have been collected from NWRC containing both of the fungi that cause BSR and SDS.
Phialophora gregata survives between soybean crops in soybean residue. Management practices that reduce the amount of infested residue remaining in a field when the next soybean crop is planted are very effective at reducing the severity of BSR. Longer crop rotations out of soybeans and incorporating the residue through tillage are methods of reducing the amount of infested residue. Research conducted in Wisconsin has shown that P. gregata could still be isolated from soybean residue that had been buried for 1.5 years and over 2.5 years later from residue left on the soil surface. The ability of the fungus to survive in soybean residue can explain why the corn/soybean rotation is not long enough to eliminate the yield loss to BSR. Planting a soybean variety that is resistant to BSR will reduce the yield loss to BSR and will also reduce the amount of infested residue that could infest the following soybean crop.
Correct identification of BSR is important for managing this disease. Many areas of the state that have BSR also have SDS. As previously mentioned, there are management practices that are very effective at reducing yield loss to BSR; however, these same practices have not been shown to be effective against SDS. Managing to reduce infested residue and choosing from varieties with good resistance to BSR are effective tools. With SDS, however, crop rotation and residue incorporation have not been shown to be effective, nor is the level of resistance as high for SDS as it is for BSR in varieties currently available.--Eric Adee