Bacterial spot damages Michigan peppers

Spring and summer rains favor disease; coppers can help but not cure bacterial spot on peppers.

Bacterial spot is an important foliar disease affecting pepper foliage and fruit in Michigan and is a problem this year. It is recognized that the bacterium can be introduced to a pepper field by seed contamination, infected transplants, or overwintering field inoculum. Pepper varieties resistant to bacterial spot are available. Using only certified, treated seed and disease-free transplants will ensure that bacterial spot will not be introduced to the field.

Pepper transplants with disease symptoms should not be planted in the field. Under wet field conditions (including overhead irrigation), the bacteria on the transplants will continue to increase. This results in more spotting (Photos 1, 2), severe blight, and defoliation (Photo 3). The wet spring and summer storms likely have contributed to the problem that we are now seeing. The combination of wind and rain with sandblasting causes wounds on the plants and the rain potentially drives bacteria from the surface of the plant into the wounds. High numbers of bacteria at the time of flowering, and during early fruit development will likely result in spotting of the fruit. Healthy-appearing transplants that were growing near infected transplants, while in the greenhouse, may develop bacterial spot symptoms in the field.

Bacterial spot symptoms
Photo 1. Bacterial spot symptoms on the foliage of pepper transplants.

Infected banana pepper
Photo 2. Infected banana pepper fruit with raised, necrotic spots.

Photo 3. Defoliation of infected pepper plant in the field.

Field management of bacterial spot

Managing bacterial spot in the field includes using copper mixed with an EBDC fungicide on a 5 to 7 day basis. There are some limited studies that suggest that a spray interval of 10 to 14 days is not effective if bacterial problems are developing. (NOTE: Some processors have restrictions regarding the use of EBDC fungicides.) Copper is not a “curative” or systemic product, but it may be effective in suppressing the bacteria responsible for this disease and thereby averting a disease epidemic. However, if sprays are not applied until after the occurrence of disease symptoms, it is unlikely that the disease will be slowed significantly. Although a copper application following a storm could be helpful in reducing the populations of bacteria on the plant surface, it will not touch the bacteria that might have washed into wounds during the storm, and copper sprays will not eliminate the spots that were present at the time of applications. Agri-Strep or other antibiotics not registered for use in the field in Michigan.

It is especially important to keep the bacterial population low during flowering and early fruit formation to avoid the brown corky spots developing on the fruit, as a result of this disease. In my field plots, I have not had problems with copper applications causing blossom drop or foliar burning. The occurrence of phytotoxicity is often associated with poor drying conditions such as periods of very cool wet weather, in which the copper stays in solution on the leaf. This problem can be complicated by label recommendations for shorter spray intervals and higher copper rates during such weather conditions.

When using copper applications, the pH of the spray solution is an important consideration. Coppers should not be applied in solutions having a pH of less than 6.5. Solutions containing any more acid than this can result in a loss of efficacy because too many of the copper ions that are responsible for bacterial protection are disintegrated and lost. High pH is not as important or damaging, but to be on the safe side, a pH range of 6.5 to 9.0 is optimum.

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Dr. Hausbeck’s work is funded in part by MSU‘s AgBioResearch.

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