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Yellow blotch in alfalfa can be a significant problem to forage yield and forage seed quality under wet, humid conditions.
Philip Northover, a plant pathologist with Manitoba Agriculture, said symptoms first appear as small chlorotic spots on upper leaf surfaces about 10 days after infection occurs.
The fungus Leptotrochila medicaginis also causes blotches on stems, petioles and leaves, but blotches are more common and noticeable on leaf surfaces.
As the lesions increase in size, they expand from the mid-vein to the margins of a leaflet. The smaller veins in the leaf initially restrict the spread of the lesion, giving it an angular appearance.
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Cultivar susceptibility affects the appearance of the symptoms, with resistant plants restricting infection to small yellow spots that later turn brown.
In susceptible plants, the small yellow dots expand into chlorotic streaks and later become yellow to orange fan-shaped blotches. Over time under moist conditions, the blotches take on a dark brown to black colour. The leaflets also tend to curl up as they desiccate.
Under humid conditions, with relative humidity greater than 97 percent and temperatures below
25 C, windborne spores are ejected from cup-like structures called apothecia, embedded on overwintered leaves.
Under typical Manitoba conditions – cool, moist spring weather followed by dry, hot summers – apothecia capable of releasing spores can be formed in early autumn, but generally do not release spores until the following spring. Apothecia that form in the fall may continue to release spores well into July.
In a season where it remains wet and humid into the summer, apothecia can be produced much more rapidly. New infections can occur within a season, which means the crop can be under multiple attacks in the same year.
To deal with this condition, forage producers should cut their alfalfa early, before leaf drop, to reduce the amount of overwintering infected leaves and minimize leaf loss.
Alfalfa seed producers should rotate alfalfa with nonlegume crops if possible. Spring burning reduces overwintering inoculum, but may injure the stand if growth has already begun.
Mancozeb-based fungicides can be applied to manage leaf spot diseases. Lance, with the active ingredient boscalid, is registered for blossom blight, common leaf spot, spring black stem and lepto leaf spot, but not for yellow blotch. It is uncertain whether applying Lance for these other diseases would suppress yellow blotch development.
