Several soilborne soybean diseases occur in Arkansas that may look somewhat similar upon casual observation. It is important, however, to accurately distinguish which is which because yield loss potential, as well as disease management and prevention may be radically different. The following is a comparison of symptoms associated with southern soybean stem canker (SC), sudden death syndrome (SDS), aerial blight (AB), Neocosmospora stem rot (NEO), and Phytophthora root rot (PHY). Please remember that while this article may help in initial disease identification, submission of appropriate specimens to the Arkansas Plant Disease Diagnostic Laboratory is the best way to be sure. For more information, see the Soybean Production Handbook.
Although SC infects soybean plants early in the season, plants generally remain symptomless until about R-1. Leaf symptoms include interveinal chlorosis and necrosis (Figure 1), with plant death following within 2-3 weeks initial symptoms with many dead leaves remaining “frozen” to the plant (Figure 2). Stem cankers that run along one side of the stem can usually be seen on infected plants once the foliar symptoms are visible (Figure 3). Cankers may occur anywhere on the stem but usually appear to begin at a leaf node on the lower third of the plant. Cankers do not completely girdle the stem externally, but the pith and vascular tissue inside the stem at the site of the canker will be discolored. Field distribution of SC can vary from a few localized sites to fieldwide depending on the susceptibility of the cultivar, the length of time the pathogen has been present in the field, and the environment during the first 6-8 weeks after planting.
Sudden Death Syndrome:
Leaf symptoms of SDS are similar to those of SC and include interveinal chlorosis and necrosis. Although symptoms vary by cultivar, in general the interveinal spots are less uniform and more spotty on the leaf with SDS than with SC (Figure 4). Also with SDS, many cultivars tend to drop leaflets as the disease progresses, leaving a skeleton appearance (Figure 5). As with SC, SDS symptoms generally become visible about the time the plants achieves mid-reproductive stages of maturity. A characteristic of SDS is the absence of a canker or lesion on the stem. However, vascular tissue of the lower stem will show a brown discoloration while the pith remains white (Figure 6). Field distribution of SDS can be isolated or general, but many times in furrow-irrigated fields severe disease is most prominent near the water outlet where soil remains cooler and wetter.
Phytophthora Root Rot:
PHY is a cool-season, wet soil disease and is usually most severe in seedlings and young plants (Figure 7). However, in some cultivars, under certain conditions, adult plants may be affected. In this case, dark, sunken lesions on the lower stem may occur (Figure 8). In contrast to SC, stem lesions due to PHY start at or near the soil line and girdle the stem. Internal vascular tissue associated with the canker is chocolate brown. In some cultivars leaf symptoms that are similar to SC symptoms may occur.
AB is a mid-season disease that can cause severe defoliation in susceptible cultivars under the right conditions. Infected leaves rapidly wilt and die remaining frozen on the plants (Figure 9), but stems remain alive and green. Reddish-brown lesions may develop on pods (Figure 10) and stems at leaf scars, but in contrast to SC, stem lesions are localized. Pod abortion is common. Under very humid conditions, weblike fungal mycelium may grow on the surface of leaves and stems, and sclerotia of the fungus may form on infected tissue (Figure 11). Field distribution patterns may be localized or general. Narrow rows or broadcast plantings are most conducive to severe disease, and fields with a cropping history that includes rice with severe sheath blight are prone to severe AB.
Plant affected by Neocosmospora stem rot are usually first noticed when the plants reach R-5. Affected plants will begin to have chlorotic leaves that become necrotic and drop off. The plants die prematurely (Fig. 12). This may be noticed in scattered plants or large patches in the field that may resemble low spots in the field where waterlogged soil caused plant wilting. Pods on the affected plants do not completely fill. At harvest, the affected plants, which are noticeably more weathered due to the premature death, will appear as grayish patches in an otherwise brown field. Reddish perithecia may be visible on crowns, roots, or nodules (Fig. 13). These perithecia look very similar to red crown rot caused by Cylindrocladium crotalariae which has not been reported in Arkansas, and microscopic identification of the fungus may be necessary to differentiate the two. There may also be white mycelium and/or perithecia within the pith. Neocosmospora stem rot is usually found in fields where soybean monoculture has been practiced.
Other potential look-alikes:
Dectes Stem Borer: Larvae of this insect hatch from eggs near the base of the plant, feed in petioles for a few days, then bore into stems where they continue to feed internally until they reach the soil line. Petioles that have been fed upon dry up and drop off the plant, but leaves in the upper canopy may show interveinal necrosis. In some situations stem borer symptoms may resemble aerial blight (Fig 14). A reddish scar may develop around the entrance hole. Internally, the pith is brown and discolored where the larvae have fed. Larvae or frass may be present in the pith cavity of infected plants (Fig 15).
Occasionally pesticide application may create foliar symptoms that can be confused with diseases. However, with pesticide injury, symptoms are more uniform throughout the field than would be the case with these diseases (Figure 16 – Affected plants generally do not lose their leaves unless the problem is an obvious mis-application, and no stem symptoms (cankers) will be present. Triazole fungicide phytotoxicity can be relatively common 14 to 21 d after application if the fungicides were applied to stressed plants (hot conditions at mid-day) or when non-recommended products are used with the application.)
Compare & Contrast
|Disease||Leaflet interveinal necrosis||Stem lesions present||Defoliation||Sclerotia present||Seedling disease||Internal stem discoloration|
|SC||yes||yes||no||no||no||associated with canker|
|SDS||yes||no||yes||no||no||lower stem & crown onlya|
|NEO||Yes||No||Yes||No||No||No (sometimes a white pith)|
aThe vascular tissue in the crown and lower stem may be brown while the pith remains white.
bThere will be an entry hole and possibly a reddish scar around it.
Authors and contributors: Terry Kirkpatrick, Travis Faske, Terry Spurlock, Amanda Greer, Michael Emerson