NPIPM:Stem canker on soybean
Compiled by: Buyung Hadi, from the materials by: Loren Giesler, Dean Malvick and Lawrence Osborne
Causal Organism
Recently stem canker has been recognized as a complex composed of Northern stem canker, caused by Diaphorte phaseolorum var. caulivora, and Southern stem canker, caused by D. phaseolorum var. merdionalis. In the 1950s and early 1960s, Northern stem canker was recognized as one of the major disease of soybean in the Midwest. Usage of less susceptible cultivars helped curbing the disease for a period of time. Throughout 2000s, an increase of occurrence of Northern stem canker in the Northern plains was noted. Southern stem canker arose in the southern regions of the US in 1970s and remain a problem there.
Symptoms and Signs
Early symptoms appear in the later half of the growing season. Reddish brown lesions appear at the stem nodes, usually during the early reproductive stages of the plant. These lesions develop into elongated, sunken, dark brown canker that spread on the stem. Tiny black dots may appear on the stem. These dots are stroma, a mass of D. phaseolorum spore producing structures. The pathogen produce toxin that are transported to the leaves, causing interveinal yellowing and subsequently tissue death of the leaves. Dead leaves usually remain attached to the leaf stems (petioles). The canker may eventually girdle the stem, preventing water transport to the leaves and causing plant wilt.
The symptoms of stem canker may be confused with those of Phytophthora root and stem rot. Stem canker usually forms higher on the plant than Phytophthora root and stem rot although canker lesions may occur at or elongate towards the soil line. A diagnostic symptom of stem canker is green stem tissue above and below the individual cankers on the stem.
Disease Impacts
Northern stem canker reduces yield by girdling the stem, negatively affecting pod fill. Early occurrence of plant girdling may severely reduce the yield. Late girdling may still affect the yield by reducing the seed size and weight. In severe cases, yield loss as high as 50-80% has been reported.
Life Cycle and Epidemiology
The fungus Diaphorte phaseolorumoverwinters in infested plant debris and infected seeds. The rate of seed infection byD. phaseolorum var. caulivora, the fungal pathogen causing Northern stem canker, was estimated to be between 10-20%. In the spring, the fungus left on soybean residue of the previous season begins to produce spores. These spores are carried by wind or rain splash onto plant surface. The growth stage of the plant when infection occurs influence the final severity. Infection at V3 growth stage correlate with the highest final disease severity. Infection that occurs after V3 may lead to lower level of final disease severity. As the pathogen develop within the hosts, more spores may be produced and spread to adjacent host plants, forming a secondary infection.
Infection is favored by high cumulative rainfall during vegetative growth period. As the disease develops, dry weather in the reproductive stages may exacerbate the negative impact of stem canker on host plants. Although infection occurs during the first half of the soybean season, symptoms do not appear until the infected plant is in reproductive stages.
Management Approaches
Cultural Methods
Deep plowing in fields with stem canker history will help to manage stem canker by reducing potentially infested soybean residue on soil surface. Since the pathogen is seed borne, seeds produced from fields with stem canker should not be used for planting. Seed treatment with fungicide helps to reduce stem canker but will not eliminate the pathogen completely.
Host Plant Resistance
Commercial varieties with resistance or tolerance against stem canker are available.
Chemical Control
Foliar fungicides can be effective in managing stem canker when applied during the vegetative stages. However, this option may not always be economically feasible.
