Last week (July 12-July 16, 1999) Sudden Death Syndrome (SDS) was confirmed on three samples of soybeans submitted to the P&PDL. Symptoms of SDS first appear as yellow, interveinal blotches. These blotches rapidly increase in size and interveinal tissues become necrotic. The leaf veins are the last to become necrotic. Petioles and stems of affected plants remain green until considerable leaf tissue has died. Most commonly, these symptoms occur in Indiana soybeans in late July and early August. Dr. Scott Abney, USDA Soybean Pathologist, mentioned that the earlier symptom expression observed this year is most likely related to the earlier planting dates, such that plants were in flowering stage at the time of rainy conditions.
As symptoms progress, leaf blades drop from the petioles, leaving erect, barren, somewhat green petioles attached to stems. Interveinal necrosis progresses faster on the upper than the lower leaves, the upper leaves are usually the first to defoliate. Leaf symptoms alone are not diagnostic for SDS, as similar leaf death may be experienced with brown stem rot. The root and lower stem tissues must be closely examined for diagnosis of SDS.
When initial foliar symptoms appear, the roots of SDS affected plants appear almost normal on the surface. But internally, a reddish-brown discoloration develops near the central core of the tap root and radiates outward and upward. The discoloration of the internal tap root tissues away from the central core become a streaky to uniform, light brown to light gray color. This discoloration spreads into the cortical tissues of the lower stem as the disease progresses. Pith tissues in the stem remain a normal white to green color, which aids in the differentiation of SDS from brown stem rot. Root systems are often badly deteriorated.
Yield reduction due to SDS is dependent upon cultivar, weather conditions, time of disease onset, and severity of disease within a field. Yield losses result from premature plant death, pod abortion, lack of pod fill and low test weight.
SDS is caused by a soil borne fungus, Fusarium solani, form A (the "blue strain"). This strain of the root infecting fungus produces a toxin that is translocated from the infected taproot to the leaves and causes the obvious dying of leaf tissues. SDS has previously occurred primarily in areas where the Soybean Cyst Nematode (SCN) is a major concern. Root damage by the nematode does enhance SDS severity, but the presence of the nematode is not required for SDS to occur.
While SDS controls are limited, the following should help:
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Last updated: 31 October 1999/tlm.