Symptoms and Signs
Symptoms include a water-soaked, brownish-gray to brownish-orange, soft wet rot that occurs on the oldest leaves in contact with the soil. These old leaves are often damaged or senescent and are therefore particularly susceptible. From these leaves, the pathogen moves into the healthy parts of the lettuce plant and causes a decay of the crown. A characteristic fuzzy gray to brown growth covers diseased areas, especially basal leaf and crown tissue that is shaded and protected from drying by overlying foliage (which is why the disease is also called gray mold).
Flat, black sclerotia may form on infected tissues. In advanced stages of the disease, lettuce crowns can become completely rotted and entire plants will wilt and collapse. The wilting and collapsing symptoms are somewhat similar to lettuce drop and phoma basal rot. Romaine cultivars appear to be especially susceptible.
Transplanted lettuce can become infected where the stem is in contact with soil; such infections can cause the plant to be delayed in development or to collapse. Botrytis crown rot on young plants can therefore result in significant stand reduction.
Comments on the Disease
Botrytis crown rot is usually a minor disease of lettuce but can cause significant damage if field conditions are favorable for the pathogen. The risk of Botrytis crown rot damage increases with plant injury and cool, wet conditions.
To prevent botrytis crown rot:
- Limit damage caused by farming practices, environmental extremes, or other pathogens and pests.
- Avoid extensive and prolonged wetting of the bed tops to help lower disease severity.
- Use lettuce transplants that are not overgrown; old transplants are subject to additional leaf breakage and damage during planting, and hence are more susceptible to infection by B. cinerea.
Reduce crown rot problems by applying protectant fungicides after thinning.
- Apply a fungicide before plants become too large.
- Direct applications to the base of the young plants.
|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.|
|(Rovral 4F)||1–2 pt||24||14|
|MODE-OF-ACTION GROUP NAME (NUMBER1): Dicarboximide (2)|
|COMMENTS: Do not make more than three applications per crop.|
|(Switch 62.5WG)||11–14 oz||12||0|
|MODE-OF-ACTION GROUP NAME (NUMBER1): Anilinopyrimidine (9)/Phenylpyrrole (12)|
|MODE-OF-ACTION GROUP NAME (NUMBER1): Aromatic hydrocarbon (14)|
|MODE-OF-ACTION GROUP NAME (NUMBER1): Carboxamide (7)|
|COMMENTS: Do not make more than two applications per season.|
|(Cannonball WG)||Label rates||12||0|
|MODE-OF-ACTION GROUP NAME (NUMBER1): Phenylpyrrole (12)|
|‡||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.|