Symptoms and Signs
Also known as shot hole or ring spot, anthracnose initially causes small [less than 0.125 inch (3 mm long)], water-soaked spots on outer leaves. Spot development is particularly severe on the underside of the lower leaves. Spots enlarge, turn yellow, and are usually irregular and angular in shape. Under cool, moist conditions, white to pink spore masses of the fungus may be visible in the center of the lesions. If the disease is severe, the lesions will coalesce and cause significant dieback of the leaf and in some cases result in stunting of the plant. As spots age, the affected tissue will dry up and become papery in texture. Eventually the centers of these spots will fall out, resulting in the shot hole symptom.
Anthracnose lesions are often clustered along the midribs of the underside of lower leaves. Romaine cultivars, in particular, exhibit severe disease along leaf midribs. If infected early and severely, young lettuce seedlings can be killed by anthracnose.
Comments on the Disease
This disease requires cool, wet conditions for infection and symptom development. Anthracnose is always associated with rainy springs. Splashing water moves microsclerotia onto lettuce leaves, resulting in infection. The fungus, Microdochium panattonianum, is host-specific to lettuce and can survive for up to 4 years as microscopic resting structures (microsclerotia) in the soil. Romaine cultivars are particularly susceptible.
Management
Cultural Control
To prevent disease development:
- avoid planting early spring lettuce in fields having a history of the disease.
- rotate with any crop other than lettuce to help reduce soil inoculum levels, though such rotations will not eliminate the pathogen unless lettuce is not planted for over four years.
- use irrigation systems (furrow or drip irrigation) that eliminate leaf wetting.
Resistant cultivars are not widely available.
Organically Acceptable Methods
Use cultural controls in an organically certified crop.
Treatment Decisions
In fields with a history of this disease, begin fungicide applications on early season (spring) lettuce crops if rainfall is imminent or at the first indication of disease symptoms.
| Common name | Amount per acre** | REI‡ | PHI‡ | |
|---|---|---|---|---|
| (Example trade name) | (hours) | (days) | ||
| Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least likely to cause resistance are at the top of the table. When choosing a pesticide, consider information relating to the pesticide's properties and application timing, honey bees, and environmental impact. Always read the label of the product being used. | ||||
| A. | AZOXYSTROBIN | |||
| (Quadris) | 6.2–15.4 fl oz | 4 | 0 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11) | ||||
| COMMENTS: See label for special phytotoxicity precautions. | ||||
| ‡ | Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest. |
| 1 | Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions. Fungicides with a different group number are suitable to alternate in a resistance management program. For fungicides with mode-of-action group numbers 1, 4, 9, 11, or 17, make no more than one application before rotating to a fungicide with a different mode-of-action group number; for fungicides with other group numbers, make no more than two consecutive applications before rotating to a fungicide with a different mode-of-action group number. |