AUBURN UNIVERSITY, Ala. – Bacterial spot is a common disease of tomatoes and peppers. Most often, damage from this disease is on the leaves of a plant. There are several steps that growers can take to control and prevent this disease. Alabama Extension professionals, Dr. Ed Sikora, a plant pathologist and Dr. Kassie Conner, a specialist focusing on plant disease diagnostics, offer the following tips and information on bacterial spot.
Causes and Symptoms
The bacteria that causes this disease may be found on seeds, transplants or crop debris from previous diseased crops. Bacteria spread from plant to plant by wind, rain and humans. Some suspect that infected transplants often introduce bacterial spot into a field.
Many small, dark spots first appear on older leaves near the base of the plant. For tomatoes, symptoms quickly move up the plant and blighted leaves often stay attached. Pepper plants infected with bacterial spot show similar symptoms, but leaf spots may be larger and infected leaves usually drop prematurely. Green fruit on tomato and pepper plants may appear spotted or scabby. Once the fruit begins to turn red, they can no longer be infected.
Controlling bacterial spot involves using multiple management tactics. Controlling the disease with chemicals alone usually results in failure. The following are several steps that growers can take to control bacterial spot.
Producers should rotate their fields out of tomato and pepper production for 12 months before planting either crop again. This period allows crop debris that can carry the bacteria to decay, which kills the pathogen.
Promptly destroying crops after the last harvest will stop bacteria from multiplying in abandoned fields. Destroy spring crops before transplanting later settings to avoid spread of the pathogen. Do not replant into plastic or organic mulch used for a previous crop. There is enough crop debris on or in the mulch to carry-over the bacteria from crop to crop.
Using wide spacing between rows will allow plants to dry faster after a rain or irrigation event. Wet field conditions favor disease development and widening row spacing may reduce disease severity. Leave extra space between plants within rows, a minimum of 24 inches for tomatoes and 18 inches for peppers. If possible, producers should avoid overhead irrigation.
Workers handling wet, bacterial spot-infected plants easily spread the bacteria. Delay practices such as suckering, staking and tying plants until dew has dried on the plant surface. It is best to work disease-fee fields first and bacterial-spot infested fields last to avoid transferring the disease from a diseases field to a healthy setting.
All tomato varieties are susceptible to bacterial spot. Many pepper varieties have some resistance to the disease.
Bacterial spot populations resistant to copper have been identified in Alabama. Apply copper at the highest labeled rate and tank mix with a mancozeb-type fungicide. The product *Actigard has a unique mode of action that mimics the natural systemic activated resistance (SAR) response found in many plants.
Applied preventatively, Actigard stimulates the plant’s natural defenses against bacterial spot and bacterial speck. Actigard can be used during the preharvest period and is a compliment to a copper-based program. No products have curative activity and it is suggested that all spray programs be started before bacterial spot symptoms are visible on plants. Sprays should begin two weeks after transplanting in the spring and 1 week after transplanting in the fall.
*Trade names are used only to give specific information. The Alabama Cooperative Extension System does not endorse or guarantee any product and does not recommend one product instead of another that might be similar.
Much of this information was originally prepared by Anthony P. Keinath, Extension Plant Pathologist, Clemson University, Coastal Research and Education Center, Charleston, SC.