Forages: Common Leaf Spot and Leptosphaerulina (Lepto) Leaf Spot

Excerpt from Agronomy Guide for Field Crops (Chapter 5)

Order OMAFRA Publication 811: Agronomy Guide for Field Crops

Table of Contents

1. Incidence
2. Appearance
3. Disease Cycle
4. Management Strategies
5. Updates on Forages: Common Leaf Spot and Leptosphaerulina (Lepto) Leaf Spot
6. Related links...

Incidence

Although both these leaf spot diseases occur in Ontario, common leaf spot is the more destructive. Leaf spot infection can cause premature leaf loss and thereby reduce the quality of hay, yield, health and vigour of the crop.

Lepto leaf spot can be confused with common leaf spot since leaf symptoms begin as small, black spots (1-2 mm) that have a light tan or brown centre. A yellow halo usually surrounds the leaf spots. Unlike common leaf spot, these lesions will join together to form larger lesions.

Plate 98. Leptosphaerulina (lepto) leaf spot starts as small dark spots that enlarge until spots join together. Spots will have a tan centre and a yellow halo.

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Appearance

Leaf spot diseases are first seen on the lower leaves and then develop or move up the plant. Common leaf spot produces small, circular (1-2 mm) leaf spots that are brown to black. These lesions rarely join together to form larger lesions. Lesions on the upper leaf surface often have a raised centre. Within these raised centres, the black fruiting bodies (bumps) are easily seen with a hand lens. If you are not sure, put some infected leaves into a plastic bag with some wet paper towels. This will help speed the production of these fruiting bodies. Infected leaves become yellow (chlorotic) and drop prematurely.

Disease Cycle

Cool, wet weather favours leaf spot development, so it is found primarily in the early cuttings (spring and early summer) and regrowth (fall). These fungi survive in infected leaves and on dead leaves found on the soil surface. Spores produced on living and dead leaves are spread through the air where they infect new growth. Young leaves are the most susceptible to leaf spot diseases.

Management Strategies

Timely harvesting of forages is important to reduce leaf loss and minimize disease in the regrowth. Some varieties tolerant of common leaf spot are available, but no resistance or tolerance to lepto leaf spot has been found. There are few practical control strategies available for leaf spot diseases in forages. Leaf spots can reduce the protein level in legume leaves, so it is important to balance the time of harvest between the optimum stage for highest protein (bud stage in alfalfa) and the level of leaf spot disease.

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Updates on Forages: Common Leaf Spot and Leptosphaerulina (Lepto) Leaf Spot

No updates available at this time.

Related links...

• Diseases
• Forages
• Field Crop Protection Guide - Publication 812
  • Forage: Leaf Spot and Stem Spot

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