Distinguishing Among Common Fungal Diseases in Soybeans

The following diseases are caused by fungi that are present in crop residue and in the soil. In each case, wet weather and moist soil conditions favour infection and disease development. When present, foliar symptoms can be very similar and include chlorosis and browning of the tissue between the veins. These foliar disease symptoms can be hard to distinguish from early crop maturity and drought stress. However, as described below, there are many ways to distinguish among fungal soybean diseases.

Septoria Brown Spot (SBS)

Septoria brown spot develops from the fungus Septoria glycines. Symptoms are generally mild during early growth stages. Small, irregular, dark-brown spots occur primarily on leaves. Symptoms occur first on lower leaves during warm, wet conditions and then progress to the upper leaves. Tissue surrounding lesions may be yellow. Late in the growing season leaves become rusty brown or yellow and drop prematurely. A toxin produced by the fungus contributes to yellowing leaves. Fields severely affected by septoria brown spot may have yield loss ranging from 5 to 10 percent.¹

Northern Stem Canker

Northern stem canker is caused by Diaporthe phaseolorum var. caulivora in Eastern Canada and first appears during the early reproductive stages as small, red-brown lesions (Figure 3). Initial lesions are usually found near a lower leaf node and expand lengthwise as the season progresses. Lesions caused by northern stem canker turn dark brown as they age, are 2 to 10 cm long, and eventually girdle the stem, causing wilting and plant death. Foliar symptoms, including interveinal chlorosis and necrosis that appear as a result of a phytotoxin produced by the fungus, are quickly followed by plant death. Top dieback with a characteristic curling or shepherd’s crook of the terminal bud may occur.

Sudden Death Syndrome (SDS)

Sudden death syndrome is caused by Fusarium virguliforme. Sudden death syndrome can be associated with compacted soils and soybean cyst nematode. The fungus infects the roots and colonizes the base of the stem, sending toxins to the leaves. Symptoms of SDS may be seen during the soybean vegetative growth stages. However, they are most commonly seen during the early reproductive growth stages through pod fill. Leaflets on plants with severe foliar symptoms may detach from petioles; however, petioles remain upright and attached to the stem. Splitting the stem of a soybean plant with SDS will reveal a slight tan to light brown discolouration of the cortex, especially at the stem base, and a normal white to cream coloured pith.

Brown Stem Rot (BSR)

Brown stem rot is caused by 2 genotypes of Cadophora (Phialophora) gregata. This disease is found in soybean growing areas of Ontario, but is more common in southwestern counties. Genotype A causes severe foliar symptoms on soybean plants, while genotype B causes little or no foliar symptoms. Most commercial soybean products have resistance to genotype A. The infection can begin early in the growing season with symptoms not appearing until a month before harvest. Cool and wet conditions during pod-fill followed by hot and dry conditions favour the development of BSR.¹ Vascular tissue and pith browning is a characteristic symptom of BSR (Figure 4). Browning starts at the root level and usually at the leaf nodes, and then as the disease progresses, browning becomes more severe up the stem. Foliar symptoms consist of interveinal yellowing and browning of leaves. Affected leaves shrivel up, but remain attached to the stem. These symptoms can easily be confused with those of SDS. Plants can become stunted and may die prematurely. Severely damaged plants will have fewer pods, and limited seed number.

Summary

Fungi that cause SBS, NSC, BSR, and SDS can be present in the soil or crop residue. Symptoms of these diseases are most severe when infection occurs early and with persistent wet weather conditions. Since foliar symptoms of these diseases look similar and initially develop during reproductive stages, an accurate diagnosis is critical for future management options.