Nathan Kleczewski, Extension Specialist – Plant Pathology; nkleczew@udel.edu
Sudden Death Syndrome in Soybeans
There have been a couple of reports of SDS in Delaware and Maryland, and the UD Plant Diagnostic Clinic confirmed one sample earlier this week. This disease is rare in Delaware, but does pop up from time to time. SDS is caused by the soilborne fungus Fusarium virguliforme. Plants infected with F. virguliforme typically do not present symptoms until after flowering (R1). Early symptoms of the disease include mottling and crinkling of the leaves. Tiny yellow to white flecks appear between leaf veins. As the disease progresses the leaf tissue between the veins turns yellow/brown, while the veins remain green (Figure 1). Soon thereafter the leaves shrivel and fall from the plant, but the petioles remain intact. If the plant is removed from moist soil tiny blue structures may be visible at the base of the stem. These are spore masses produced by the fungus. In most cases diagnosis is completed by sectioning the lower portion of the stem lengthwise. The cortex of a stem infected with SDS will be streaked with tan/light brown lesions, whereas healthy plant stems remain white. The pith of infected plants also retains a white coloration. Symptoms of SDS can be confused with other pathogens of soybean including charcoal rot. Stems infected with charcoal rot fungus contain round black/gray structures that look like tiny bits of coal (hence the creative name) or zones delineated by sharp black margins. The roots of SDS infected plants will also be rotted. You will not see the combination of symptoms with plants with nutrient issues, chemical burn, or insect damage.
F. virguliforme overwinters in the residue or free soil as recalcitrant spores, which are resistant to a wide range of temperatures and stresses. The pathogen typically infects plants while they are in the seedling stage, and infect the cortex between V1 and V6. When plants begin flowering the pathogen colonizes the cortex more thoroughly, and toxins are produced that are translocated to foliage. These toxins are responsible for the characteristic interveinal necrosis. Symptoms often appear during heavy rains during the reproductive stages and disease is favored by high soil moisture. Consequently, the disease typically is present in areas of the field that are poorly drained (low lying or compacted areas). The disease is spread short distances on mechanical equipment, workers boots, etc., and spores can be disseminated very short distances in rain.
An interesting twist to the story is that SDS is often found in association with Soybean Cyst Nematode and the pathogen can be isolated from within cysts of SCN. Thus the nematode may play a role in pathogen spread or infection. Consequently, if you detect SDS in your field you should consider sending a soil sample to the UD Diagnostic clinic to determine SCN levels and drive future planting considerations.
The options for management of SDS are limited. Some soybean cultivars are more tolerant to SDS than others and resistance ratings can be obtained from seed dealers or directly from seed companies. A variety should be used that has both SDS and SCN resistance. Avoid planting soybean into cool, wet soils. If you have a field with a history of SDS consider planting it last to minimize the chance of infesting additional soybean fields. If you have a chronic issue with poor drainage in areas of your field, consider improving drainage in the area. A combination of rotation to corn and minimal tillage has been shown to reduce SDS levels in some cases. Overall SDS is not a major issue in Delaware. Plants can recover from this disease and I have only heard about isolated parts of the field having disease.
Figure 1. A leaf with symptoms of SDS. Foliar symptoms are not diagnostic of the disease and can be caused by other agents.
More Reports of Frogeye Leafspot on Soybean
Frogeye leafspot has been detected throughout the region, and may be more prevalent than in normal years. I detected the disease at low levels in full season soybeans in mid-July (http://extension.udel.edu/weeklycropupdate/?p=6038). Symptoms of the disease include tan to brown spots with a purple/dark red halo (Figure 2). When inspecting lesions with a hand lens, it may be possible to see black dots with grey/silver-colored spores at the center. The pathogen infects young leaves more readily than older leaves, but it can take up to two weeks for symptoms to develop. As a result, plants may have a layered appearance. Warm wet conditions, especially warm nights with heavy dews, favor disease development.
Typically this disease moves in late in the season and within season management is not required. However, this year we have many fields of late-planted double-crop beans. These fields should be scouted for frogeye. In general, if the disease is detected in the mid canopy at early reproductive stages (R1-R3) and your risk level is high, a fungicide may be warranted. Factors that increase disease risk in double crop beans include: 1) the use of a Frogeye susceptible soybean variety; 2) presence of a significant level of disease in the field; and 3) favorable weather. Research indicates that applications made at R3 tend to be most effective in states where this disease is common. Economic factors should also be weighed carefully when making any decision to spray fungicides.
Frogeye can be managed between seasons by selecting varieties with resistance to Frogeye leaf spot. Ratings can be obtained from seed companies or dealers. Most varieties with resistance to Frogeye leafspot are group V or higher, so your selection may be somewhat limited. Rotation with non-hosts, such as corn and small grains will help reduce inoculum levels and disease severity. Residue management, when practical, can also help reduce the impacts of this disease.
Figure 2. A leaf with symptoms of Frogeye leaf spot
Update on Soybean Rust
Several people have asked about the status of soybean rust this year, particularly because planting of double crop soybeans was delayed due to inclement weather. Earlier in the season there was some concern from the South that soybean rust may be a greater concern than in years past, mainly because the environment was conducive to the development of the disease. Consequently, the pathogen was detected in sentinel plots and commercial fields 2-3 weeks ahead of schedule. The disease has developed more slowly than anticipated, although disease progress is still ahead of pace for many Southeast soybean growing regions and has spread more rapidly in August. As of August 19, 80 counties in 8 southern states have reported soybean rust (Figure 3). At this time last year 50 counties reported soybean rust, and during the dry season of 2011, only 5 counties reported the disease. Currently the disease has not been detected in our neighboring states.
Soybean rust moves via spores blown from the south. Recent forecasts from the SBR ipmPIPE website (http://sbr.ipmpipe.org/cgi-bin/sbr/public.cgi) indicate that Delaware is not likely to receive any spores in the near future. It is not likely that rust will make it here in time to cause any damage to our soybeans, even late-planted double-crop beans, but I’m not quite ready to throw in the hat quite yet. The take home message: we are not at risk for soybean rust at the current time. We are closely monitoring the situation and will keep you informed of the status throughout the remainder of the soybean growing season.
Figure 3. Soybean rust confirmations as of August 19, 2013
Field Crops Disease Blog
Check out my field crops disease management blog article for the week: Faux Cercospora leaf blight (AKA sunscald): http://extension.udel.edu/fieldcropdisease/. You can now subscribe to my blog and receive updates via email by clicking the “contact me” button under my picture and entering your information into the pop up that appears. Make sure that you check off the box at the bottom to subscribe to blog updates.