Symptoms and Signs
The powdery mildew fungus appears as a gray-white, powdery growth on both upper and lower sides of lettuce leaves. Such infections may cause deformity and buckling of the leaves. In advanced stages, leaves may begin to dry out and turn brown. On occasion, the small, brown spherical structures of the pathogen's perfect stage (cleistothecia) may be seen on the leaves.
Comments on the Disease
Powdery mildew on lettuce is favored by warm, dry conditions and occurs primarily on mature lettuce. Spores are dispersed in the air via wind currents. Chains of spores, which are visible with a hand lens, distinguish powdery mildew from downy mildew, which has branched fruiting bodies. Powdery mildew is usually of minor economic importance in coastal areas but is more of a concern in desert growing areas.
Management
Begin fungicide applications when powdery mildew is first observed on 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. | ||||
| A. | QUINOXYFEN | |||
| (Quintec) | 4–6 fl oz | 12 | 1 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Quinoline (13) | ||||
| COMMENTS: Do not apply more than 24 fl oz/acre per crop. | ||||
| B. | SULFUR# | |||
| (Microthiol Disperss) | 5–6 lb | 24 | 0 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M2) | ||||
| COMMENTS: Do not apply sulfur if temperatures will exceed 90°F within 3 days of application. Thorough coverage is required. | ||||
| C. | PENTHIOPYRAD | |||
| (Fontelis) | 14–24 fl oz | 12 | 3 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Succinate dehydrogenase inhibitor (7) | ||||
| D. | MYCLOBUTANIL | |||
| (Rally 40WSP) | 5 oz | 24 | 3 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3) | ||||
| COMMENT: Do not apply more than a total of 1.25 lb of Rally 40WSP per acre per season. | ||||
| E. | AZOXYSTROBIN | |||
| (Quadris) | 12.3–15.4 fl oz | 4 | 0 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11) | ||||
| COMMENTS: Alternate with a fungicide, such as sulfur, that has a different mode-of-action group number. 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. |
| # | Acceptable for use on organically grown produce. |
| 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. |