The recent wet, cloudy weather interspersed with the odd day of warm weather should make alfalfa producers aware of the increased odds for spring black stem, leaf spot of alfalfa and common leaf spot.
In a typical season of sunny, warm, dry conditions in July and August, the potential for disease increase is low. With the onset of cool, wet conditions in the fall, new symptoms can develop. In 2004, spring conditions never ended, and conditions for disease development continued throughout the season.
Diseased leaves, leaf fragments and stems remaining in the field often fail to decompose over the winter, presenting a disease risk in the spring. The level of disease in many fields toward the end of 2004 meant higher than usual levels of infestation in the spring of 2005.
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Rain and dew trigger the disease’s spores. The impact of splashing rain can spread the spores from dead plant material to the current season’s growth. In fields examined so far in 2005, symptoms of common leaf spot, spring black stem and leaf spot have already appeared.
In addition to new leaf and stem infections, an infection of the crown or upper portion of the taproot could occur by Phoma medicaginis. This could result in a reduction in the number of stems and potentially a complete loss of the plant. If disease levels get high enough, late season infection of seed pods by Phoma medicaginis could also result in infected or damaged seed.
To manage this disease, keep the levels of fungus as low as possible. Dithane DG Rainshield NT is registered for control of these diseases on seed crops. For best control, apply it before disease becomes widespread.
The leaf symptoms of these diseases can be difficult to differentiate, and are best examined using a magnifying glass.
Spring black leaf spot: Black tar spots appear first on the lower leaves and petioles. Irregularly shaped dark brown to black spots eventually merge into larger spots. Should the disease become severe, the leaves take on a chlorotic, yellow appearance. Infected leaves die and fall off.
Spring black stem: Individual lesions combine into larger patches, imparting a blackened to brownish appearance to the diseased stem portions. As the infected stem grows, the stem can develop lengthwise cracks. In severe cases of disease, stems can be girdled and break.
Common leaf spot: Disc-shaped lesions of one to three millimetres in diameter appear, with one light brown raised fruiting body. Lesions do not usually coalesce as with spring black leaf spot. Infected leaves turn yellow and drop as the disease progresses.
