As the soybean season winds down there are were a couple of noteworthy diseases observed that are worth reviewing and looking out for in the future.
Sudden Death Syndrome
This is the third year in a row that sudden death syndrome (SDS) of soybeans has been observed in Oklahoma. The disease was observed this year in eastern and western parts of the state. The disease is essentially a Fusarium root rot caused by an unusual species of Fusarium, F. virguliforme.
Symptoms first appear during August during reproductive stages of crop development when leaves show scattered pale green spots between major leaf veins. The spots enlarge and turn brown, leaving only the areas surrounding major leaf veins green.
Severely affected leaves fall to the ground, but the petioles remain attached to the plant. Affected plants have a decayed and discolored root system and affected plants are easily pulled from the ground. The lower stems and crowns show an internal grey to light brown discoloration that is visible when the surface is scraped back with a knife.
Pod and seed development usually ceases when plants begin to defoliate so yield loss is most severe when plants are affected early in reproductive development.
SDS is favored when infested fields are saturated with water from heavy rains during periods of cool temperatures. The optimum temperatures for SDS development are from 70 to 75° F. These conditions were widespread in Oklahoma during the first two weeks of August.
Hardpans in the soil that restrict drainage and root injury from the soybean cyst nematode also favor disease development. SDS often appears in patches where drainage is poor or where the pathogen is present. The fungus persists in the soil for extended periods of time as resistant spores (chlamydospores) that are formed in roots and soil.
Therefore, the disease is likely to appear again when conditions are favorable for SDS development.
Management of SDS is best achieved by planting soybean varieties with the best SDS resistance rating. On a scale from 1 to 9 with 9 being the most resistant, there are varieties with scores of 7 to 8, which provide adequate resistance.
Other management strategies that may help control SDS include managing levels of soybean cyst nematode through crop rotation and planting cyst resistant varieties, and using practices that reduce soil compaction and improve drainage.
A crop consultant reported the possible presence of soybean rust in north central Oklahoma in late summer. Upon examination, the disease turned out to be bacterial pustule, a look-alike of soybean rust, caused by the bacterium Xanthomonas axonopodis pv. glycines. Soybean rust and bacterial pustule can be difficult to tell apart because they both produce raised bumps, or pustules in the spots that they cause.
Bacterial pustule first appears as tiny, pale green spots with raised centers. The spots turn brown and a prominent raised pustule develops. Spots are usually surrounded by a yellow border or ‘halo’. The spots may coalesce and produce large areas leaf browning which may split and fall out giving the leaves a ragged appearance.
Profuse bacterial streaming is observed when lesion sections are observed under a microscope in a drop of water. Without the aid of a microscope, the main distinguishing characteristic between soybean rust and pustule is that the raised pustules of soybean rust usually have profuse sporulation consisting of tan colored spores compared to the lack of fungal spores produced by bacterial pustule.
Spore production can be observed with the aid of a hand lens.
Bacterial pustule can cause severe disease development that results in defoliation and yield loss through a reduction in seed size. However, bacterial pustule is typically of minor importance in temperate regions of the world. The disease has been is more important in tropical areas where it becomes aggressive when temperatures and humidity are high.
Planting varieties with the Rxp gene for resistance is the most effective management strategy for bacterial pustule. However, seed companies do not score soybean varieties for resistance to bacterial pusture. This is likely due to the limited importance of the disease.
The disease also is seed borne and survives in old crop residue. Crop rotation is effective for bacterial pustule.