Symptoms and Signs
Bacterial spot develops on seedlings and mature plants. On seedlings, infections may cause severe defoliation. On older plants, infections occur primarily on older leaves and appear as water-soaked areas. Leaf spots turn from yellow or light green to black or dark brown. Older spots are black, slightly raised, superficial and measure up to 0.3 inch (7.5 mm) in diameter. Larger leaf blotches may also occur, especially on the margins of leaves. Symptoms on immature fruit are at first slightly sunken and surrounded by a water-soaked halo, which soon disappears. Fruit spots enlarge, turn brown, and become scabby.
Comments on the Disease
The bacterial spot bacterium persists from one season to the next in crop debris, on volunteer tomatoes, and on weed hosts such as nightshade and groundcherry. The bacterium is seedborne and can occur within the seed and on the seed surface. The pathogen is spread with the seed or on transplants. Secondary spread within a field occurs by splashing water from sprinkler irrigation or rain. Infection is favored by high relative humidity and free moisture on the plant. Symptoms develop rapidly at temperatures of 68°F (20°C) and above. Night temperatures of 61°F (16°C) or below suppress disease development regardless of day temperatures. Some pathogen strains are virulent on either tomato or pepper and some may be virulent on both.
Management
Cultural practices and preventive sprays of copper help to manage bacterial spot.
Cultural Control
Bacterial spot occurs commonly in tomatoes throughout California. Using pathogen-free seed and disease-free transplants, when possible, is the best way to avoid bacterial spot on tomato. Avoiding sprinkler irrigation and cull piles near greenhouse or field operations, and rotating with a nonhost crop also helps control the disease.
Organically Acceptable Methods
Cultural controls and some copper formulations are acceptable for use on organically certified produce.
Monitoring and Treatment Decisions
Copper-containing bactericides provide partial disease control. Apply at first sign of disease and repeat at 10- to 14-day intervals when warm, moist conditions prevail. Copper is strictly a protectant and must be applied before an infection period occurs.
Resistance to copper has been observed, but can be somewhat overcome by combining copper with mancozeb.
| 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. | COPPER HYDROXIDE# | |||
| (Kocide 3000) | 0.75–1.75 lb | 48 | 0 | |
| MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M1) | ||||
| COMMENTS: Not all copper compounds are approved for use in organic production; be sure to check individual products. | ||||
| B. | COPPER HYDROXIDE# | |||
| (Kocide 3000) | 0.75–1.75 lb | 48 | 0 | |
| MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M1) | ||||
| . . . PLUS . . . | ||||
| MANCOZEB | ||||
| (Dithane M-45) | 2 lb | 24 | 5 | |
| (Dithane F-45 Rainshield) | 1.6 qt | 24 | 5 | |
| (Penncozeb 75DF) | 1–1.5 lb | 24 | 5 | |
| MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M3) | ||||
| COMMENTS: The addition of mancozeb increases the efficacy of copper. Check with your processor concerning allowed materials and rates. Be sure to follow label directions on all products when making a tank mix; the most restrictive label precautions and limitations must be followed. | ||||
| ** | See label for dilution rate. | |
| ‡ | 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 until harvest can take place. In some cases the REI exceeds the PHI. The longer of these two intervals is the minimum time that must elapse before harvest may take place. | |
| # | Acceptable for use on organically grown produce. | |
| 1 | Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of action. Fungicides with a different group number are suitable to alternate in a resistance management program. In California, make no more than one application of fungicides with mode of action Group numbers 1, 4, 9, 11, or 17 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 fungicide with a different mode of action Group number. |