Spring black stem

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Spring black stem is a fungal disease that affects various species of alfalfa (Medicago sativa) and other legumes. The disease is caused by the fungus Phoma medicaginis, which can lead to significant yield losses in affected crops.

Symptoms

The primary symptoms of spring black stem include the appearance of small, dark brown to black lesions on the stems, leaves, and petioles of the plant. These lesions can coalesce, leading to larger areas of necrosis. In severe cases, the disease can cause defoliation and stem breakage, which can significantly reduce the forage quality and yield.

Disease Cycle

The fungus Phoma medicaginis survives in infected plant debris and soil. It produces spores that are spread by rain splash, wind, and insects. The disease is favored by cool, wet weather conditions, which promote spore germination and infection. The fungus can infect plants at any stage of growth, but symptoms are most commonly observed in the spring.

Management

Effective management of spring black stem involves a combination of cultural practices and, if necessary, chemical control. Key strategies include:

• **Crop rotation**: Rotating alfalfa with non-host crops can help reduce the inoculum levels in the soil.
• **Resistant varieties**: Planting alfalfa varieties that are resistant to Phoma medicaginis can help minimize disease impact.
• **Field sanitation**: Removing and destroying infected plant debris can reduce the source of inoculum.
• **Fungicides**: In severe cases, the application of fungicides may be warranted, although this is generally not a common practice for alfalfa.

Impact

Spring black stem can lead to significant economic losses for alfalfa producers due to reduced yield and forage quality. The disease can also weaken plants, making them more susceptible to other stresses and diseases.

See also

• Alfalfa
• Fungal diseases of plants
• Crop rotation
• Plant pathology