Many Nebraska growers are mistakenly blaming the deterioration of some alfalfa fields to insects. The real culprit may be fungal leaf diseases, say the University of Nebraska's Bruce Anderson, extension forage specialist, and Stephen Wegulo, extension plant pathologist. Numerous alfalfa fields, especially in south-central and southeastern Nebraska, are severely infected with multiple fungal leaf diseases. The two most prevalent diseases are spring blackstem and leptosphaerulina leaf spot. Common leaf spot also is occurring, but less frequently. In most severely affected fields, plants will likely lose leaves before harvest and little can be done to prevent it.
Many fields look very ragged with uneven regrowth, somewhat yellowish or a dull green, and appear to be deteriorating. Areas that recently received heavy rains look the worst. Spring blackstem and leptosphaerulina leaf spot are initiated by spores released from fruiting structures formed on leaves and stems the previous year. During rainy spring weather, spores are spread by splashing water and wind and can cause rapid and heavy infections.
Fungicides are probably not cost-effective and won't help the current alfalfa crop. They could, however, reduce infection of regrowth following harvest, Anderson and Wegulo say.
Deciding when to harvest diseased fields is difficult. Harvesting early is recommended to salvage leaves, but it also could weaken already weak plants even more. Some may die. The best strategy may be to wait until plants have had seven weeks to recover from the freeze, then harvest no matter how ugly the forage. In many fields, much of the stem bottom may have no attached leaves due to diseases. After harvest, expect regrowth to be slow.
To identify spring black stem and leptosphaerulina leaf spot click on these links: