The R6 growth stage (pods containing green seed that fill the pods to capacity at one of the four uppermost nodes of the main stem) is an important turning point in the soybean growing season. Most importantly, economic thresholds for soybean aphids and two-spotted spider mites skyrocket. It’s highly unlikely that controlling these pests at the R6 growth stage or later will produce a return on investment. Either our natural predator friends have won the battle or the damage to yield has already been done. Similarly, any disease control measures had to implemented well before R6 to provide an economic return. Seed fill is wrapping up and dry down is beginning.
So are we done scouting our crop……….Not so fast!!
This is the Best Time to Scout for Stem Diseases!
We’re entering that time of year when soybean stem diseases become the most noticeable and easiest to identify. Currently in Manitoba soybean production, stem disease are the cause of our greatest yield losses. It’s vital that we take this annual opportunity to guage how severe are stem disease problems really are.
Let’s take a quick look at the diseases we are most concerned about right now in southern Manitoba.
White Mold (Sclerotinia)
White mould can cause two to five bushels per acre of yield loss for every 10 per cent increase in incidence (per cent of plants affected) of early plant infections. Cool, wet conditions throughout July and August and a dense crop canopy favour disease development. Infection begins low in the canopy at nodes along the stem. Crops grown on high nitrogen land (such as previously manured ground) can be more susceptible to the disease because of excessive canopy growth.
Symptoms may first be noticed from afar as lodged plants or wilted, drying leaves at the top of the canopy. Inside the canopy, symptoms may appear as white mycelium (mouldy growth) along the stem, with black sclerotia bodies inside the stem.
Phytophthora Root Rot (Phytopthora sojae)
Phytophthora root rot (PRR) is considered both a root and stem disease. Late-season infection of PRR appears as a chocolate-brown lesion on the stem that extends upward from the soil line. In some cases, this lesion may develop up one side of the plant only. Dig up roots to confirm. If roots appear rotten (e.g., root mass is reduced and discoloured, nodulation is minimal), it’s likely PRR. If a stem lesion is present and roots appear healthy, it may be another stem disease like northern stem canker or pod/stem blight.
Internal stem will also have brown discolouration and plants will pull easily from the soil. Other symptoms may include include stunting, yellowing plant tissue and wilted leaves remaining attached to the plant. Infected plants may occur in patches or at random among healthy plants. This disease can affect plants at all stages, but it’s easier to identify at reproductive stages. Foliar fungicide will not salvage infected plant tissue.
Northern Stem Canker (Diaporthe caulivora, D. aspalathi)
Initial symptoms typically appear on the lower third of the stem shortly after flowering. The disease begins as small, reddish-brown lesions at the base of a branch or leaf petiole. These lesions then expand to form slightly sunken cankers that are reddish-brown with reddish margins. Cankers may span several nodes on the main stem. Stem canker may at first resemble Phytophthora root and stem rot, where lesions wrap around the stem and extend to the soil line. However, plant roots will appear healthier than those infected by Phytophthora.
Stem canker is found each year in Manitoba, but its incidence is currently low.
Pod and Stem Blight (Diaporthe phaseolorum var. sojae)
Pod and stem blight is present in Manitoba, but at low levels. It’s identified by distinct lines of raised, black dots (pycnidia) on infected stems, pods and petioles.
Seed infection (Phomopsis seed decay) only occurs if pods are affected, which can reduce seed quality. Symptoms may be easier to detect in August through September when soybean plants are maturing.
Anthracnose (Colletotrichum truncatum)
Anthracnose is less common in Manitoba-grown soybeans. It appears as reddish-brown irregularly-shaped blotches on the leaves, stems and petioles. Black fungal bodies develop in these blotches later in the season. Leaf symptoms include reddish veins. Leaf petioles may also become twisted and bent into a Shepherd’s crook, resulting in early defoliation.
Sudden Death Syndrome (Fusarium virguliforme)
Sudden death syndrome (SDS) infects roots early in the growing season, but foliar symptoms usually appear after flowering. Interveinal leaf chlorosis becomes necrotic, then leaves eventually die and prematurely fall from the plant leaving the petiole attached. Fields infested with SCN can increase the severity of SDS symptoms.
For more information and images of all diseases listed here, check out the Crop Protection Network Scouting for Stem Diseases publication.
More information on these diseases can also be found on the Crop Protection Network publication database – Scouting for Soybean Stem Diseases.