Stem Canker

Family: Diaporthaceae

About Stem Canker

Life Cycle

Stem canker can be divided into two varieties: Northern stem canker and southern stem canker. Northern stem canker is caused by the fungus Diaporthe phaseolorum var. caulivora, and southern stem canker is caused by the fungus Diaporthe phaseolorum var. meridionalis. These fungi overwinter in infected plant debris for up to years at a time. Infection can occur in seeds, but fungus overwintering in plant materials is more common. Some weed species can also be hosts of the fungus, but many do not display symptoms of the disease. Spores are produced in rainy weather, which are then splashed onto surrounding plant tissue. Infection typically occurs early on in the vegetative stages of soybean development, but cankers will not be visible until the plant undergoes its reproductive stages. The pathogen enters through leaf tissue, then continues to the petiole, and then it reaches the stem. Although most plants survive initial infection, some young plants are susceptible to rapid death upon infection. Cankers grow as the plant enters the mid pod fill stages of development. As lesions develop at the base of branches or leaf petioles, plant structures above lesions may wilt and die.

Stem Canker Identification and Habitat

Identification

The main characteristic of stem canker is a dead plant with its dead leaves still attached to the petioles late in the season. After flowering, small, reddish-brown lesions appear at the bases of branches or petioles. Eventually these spots will become sunken-in cankers with reddish borders. These cankers can remain on one area of the stem, or they can spread to multiple nodes and girdle the stem, causing death to the plant. The fungi can also cause grey streaking in the taproots and lower stem. Chlorosis and necrosis may occur due to the toxins that the fungus creates.

Varieties

• • Northern stem canker (Diaporthe phaseolorum var. caulivora)
• • Southern stem canker (Diaporthe phaseolorum var. merdionalis)

Habitat

Wet and humid conditions favor the development of stem canker. Early in the season, prolonged wet weather is important for fungus to be able to develop, spread, and germinate effectively. Extended periods of moderate to warm weather (72° to 86°F) are best for disease development. Soils that are not tilled may also promote growth and spread of the disease. Symptoms are often visible from mid-July to harvest.

Stem Canker Management and Control Methods

Cultural Control

Planting resistant hybrids is a good way of reducing the risk of crop infection. In general, planting high quality seed should be a priority. Later planting dates may be appealing if planting in a field with a history of stem canker. Early planting dates (late April to early May) may be more favorable for disease development based on the weather conditions. Tillage can be useful to help infected plant residue break down in the soil, reducing chances of further spread and development of the disease. Crop rotation is important to limit the amount of infective fungi for the next time soybean crops are planted in that location.

Chemical Control

Before the V3 stage where infection commonly occurs, applying foliar fungicide sprays can help to control stem canker. For this disease, timing is fundamental in controlling infection. Before using fungicides, be sure to read labels for cautionary advice and application guidelines.

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