Reproductive Stage Diseases & Soybean Cyst Nematode Awareness
BY Dairyland Seed Agronomy Team
Phytophthora Root and Stem Rot
Phytophthora sojae is an oomycete pathogen that is fungal-like in nature. It can infect throughout the entire growing season. Commonly referred to as a water mold, PRR produces spores that germinate and then swim in the soil toward soybean roots during water events. Later season infection symptoms of this pathogen are very much like other early season seed infecting “damping off” types of disorders. Diseases typically experienced early in the season, such as fusarium, pythium, and rhizoctonia.
The primary driver of late season phytophthora infection is plentiful moisture and appropriate soil temperatures. PRR likes it moist and warm (77-85⁰F soil temperatures), so do summer thunderstorms that bring PRR’s travel mechanism into play. This makes for scenarios where a water mold such as PRR to move and congregate where that water is allowing increased infection rates. Poorly drained and/or saturated areas are certainly most notable for infection.
Symptoms can also occur in well-drained fields when the pathogen is present, and the soils are saturated for 7 to 14 days due to heavy rain or over irrigation. The stem rot phase is easily recognizable by the presence of a distinct chocolate-brown lesion moving up the stem from the soil line. Initially, the upper stem of the plant may remain green, but eventually the plant will wilt, turn yellow and then die with leaves remaining attached.
Management for this malady begins with planting LumiGEN® treated soybean products along with resistant varieties with single or multiple gene resistance Rps genes (“Resistant to Phytophthora sojae”). The most common Rps genes that are available include Rps1a, Rps1c, Rps1k, Rps3a, and Rps6, or a combination of one or more Rps genes stacked together. Observe and attempt to select cultivars based on which specific resistance genes are known to be effective in your region. Also, strongly consider planting the better field or horizontal tolerance scoring product, as this metric carries a bit more weight in later season infections most years. Improvements of soil drainage, aeriation, and structure will also pay returns on your investment. Rotation of crops is critical to combat recurrence of this disease in soybean.
White Mold
White Mold, Sclerotinia sclerotiorum, primarily infects the soybean plant through the flowers. Humid conditions coupled with rain and moderate temperatures during soybean flowering are conducive for white mold infection. This fungus can survive in the soil as sclerotia for several years so management is important. Fungicides are effective against white mold if applied around the R1 to R3 growth stage. Other management practices include lower planting populations, wide row spacing, crop rotation and tillage.
Foliar symptoms are interveinal coloring of grey-green and can resemble other soybean diseases such as brown stem rot or stem canker. Symptoms usually appear in soybeans between the R3 and R6 growth stage. Fluffy white mycelium growth can be found on the stems near the nodes. Leaves of infected plants eventually die but remain attached to the stem.
Sudden Death Syndrome
Sudden Death Syndrome (SDS) is caused by the fungus Fusarium virguliforme and infection occurs in early spring. Disease symptoms don’t usually appear until around August. This is because rain during the reproductive stage can cause the disease to start producing toxins which can kill the leaves. SDS tends to favor early planted soybeans, cool damp soils and compacted soils. The other factor common with SDS is Soybean Cyst Nematode (SCN). The SDS organism overwinters in the soil, in crop residue and in the soybean cyst which is why there is a close association between SCN and SDS. Losses from SDS range from minimal to, in some cases, severe.
The first noticeable SDS symptoms are chlorotic blotches that form between the veins of the soybean leaflets, which expands and turns necrotic. Those leaves will die and can drop off, leaving only the leaf petiole. The final diagnosis includes splitting the stem to look for discolored stem cortex. The cortex will have a light brown discoloration while the pith will remain white. Brown stem rot (BSR) can have similar leaf symptomology but will have a brown discolored pith when the stem is split.
Controlling SDS after infection is not possible so prevention is key. Keep accurate records of problem fields and varieties. Planting problem fields later in the spring and eliminating soil compaction can also help in reducing the incidence of SDS. Selecting soybean varieties tolerant to SDS and containing SCN, along with using seed treatments like ILeVO® will help control SDS and SCN.
Brown Stem Rot
Similar to SDS, the soilborne fungi that causes Brown Stem Rot (BSR) infects early in the spring and symptoms don’t usually show up until the soybean is in reproductive stages. BSR is caused by the fungus Phialophora gregata which infects the plant through the roots and then grows inside the vascular tissue. There are two different genotypes of the fungus that causes BSR. Type A causes leaf and stem symptoms and is more aggressive. Type B typically only causes stem symptoms and is less aggressive.
Symptoms usually appear around the R3-R4 growth stages and are favored by cooler temperatures and adequate moisture. Foliar symptoms can appear similar to SDS with interveinal chlorosis that turns necrotic. BSR will have a brown discolored pith that extends from the lower stem upward, while SDS will have a white pith.
BSR cannot be controlled once the plants have been infected. Soybean cyst nematode is also known to contribute to BSR infection so selecting a variety that has SCN resistance along with BSR tolerance will be key in helping to prevent BSR in future years. Other management practices include rotating away from susceptible crops for two or more years and using tillage to incorporate plant residue.
Frogeye Leaf Spot
Frogeye leaf spot is caused by the fungus Cercospora sojina, and typically infects after flowering. Warm, humid conditions and/or frequent rains favor the development of this disease. Frogeye leaf spot lesions are more commonly seen on soybean leaves, with the younger leaves being more susceptible. In severe cases, lesions can develop on stems and pods.
Lesions of frogeye leaf spot are small, circular gray spots with reddish-brown margins. As these lesions age the center becomes light brown to gray with a dark border. The lesions can coalesce to create larger, irregular areas. Frogeye leaf spot lesions on leaves are commonly mistaken for herbicide damage. The best time to scout for frogeye leaf spot is from R3 (beginning pod) through R6 (full seed).
When scouting for frogeye leaf spot, focus on fields that are continuous soybeans and collects moisture for extended periods of time. The best way to manage frogeye leaf spot is to bury infected residue and rotate to a non-bean crop. When planting soybeans back in the infected field be sure to use a variety with high tolerance to frogeye leaf spot. The use of seed treatments and foliar fungicides may also provide some control.
Soybean Cyst Nematode
Soybean cyst nematode (SCN) is a microscopic roundworm that feeds on soybean roots. SCN may suppress nodulation, cause significant root damage, plant stunting and yellowing. The feeding from SCN can cause an entry point for diseases like SDS (sudden death syndrome) to attack the plant. Above-ground symptoms of SCN include oval shaped patterns of stunted, yellow plants with low vigor. Below-ground symptoms are stunted roots that may be discolored, have fewer nodules, and have SCN females present. SCN females are tiny structures attached to the roots, much smaller than nodules and initially white in color. Over time they turn from white to yellow and finally to brown once they are mature.
When scouting for SCN, focus on consistently low-yielding areas of the field. Dig up plants in those areas and carefully crumble the soil off of the roots but not hard enough to knock off the cysts. Closely examine the roots, especially the fine roots for SCN females. The best time for scouting is about six weeks after planting through about three weeks before harvest. If you suspect SCN then collect about 15-25 soil cores from about 6-8 inches deep and send to a diagnostic lab. Cysts can remain viable in the soil for more than 10 years so management is very important. Management strategies for SCN include rotating to a non-host crop, planting soybeans with SCN resistance, rotating between PI88788 and Peking resistance sources, and incorporating ILeVO in your seed treatment.
Brian Weller
Western Region
507.456.3034
Rod Moran
Western Region
507.456.3034
Dan Ritter
Central Region
219.863.0583
Branden Furseth
Northern Region
608.513.4265
Mark Gibson
Eastern Region
260.330.8968
Amanda Goffnett
Eastern Region
989.400.3793
Ryan Mueller
Eastern Region
989.400.3793