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Tomato transplant infected with bacterial spot.

Bacterial spot is the most common disease of peppers in Alabama and is a recurring problem on tomatoes grown in the state. The disease can significantly affect marketable yield. Bacterial spot is caused by four species of the bacterium Xanthomonas.

Disease Source and Development

The main sources of the pathogen include:

  • infected seed
  • infected transplants
  • infested crop debris
  • contaminated equipment, such as stakes and tools

Fortunately, the bacterium cannot survive on its own in the soil. The pathogen can also survive on volunteer tomato and pepper plants in the field, as well as in enclosed structures—such as greenhouses and high tunnels—during the winter.

Disease development is more likely in moist, prolonged periods of high relative humidity and under temperatures between 68°F and 95°F. Excess nitrogen levels favor the development of bacterial spot.

Infection is often initiated in the greenhouse during transplant production, with inoculum originating from infested seed. Warm, wet greenhouse conditions favor the spread of the pathogen to seedlings where it infects plants through wounds. Bacteria can colonize the transplants without any symptom development. However, once planted in the field, bacterial spot symptoms start appearing. Thus, asymptomatic transplants harboring pathogen inoculum are often responsible for disease outbreaks in the field.

Symptoms of Bacterial Spot

Symptoms of bacterial spot on tomato can appear on the leaves, fruit, and stems. Leaf lesions are dark and water soaked initially, eventually turning brownish black. Spots are somewhat rounded but may be angular if restricted by leaf veins. The center of the spots frequently dries out and tears giving the leaf a ragged appearance.

Spots on green tomato fruit first appear as small, black, raised pimples surrounded by a narrow water-soaked or greenish-white halo. Older spots are black, slightly raised, and up to 1/3-inch in diameter. In time, the raised centers develop into scabby, tan to brownish black sunken craters. Ripe fruit appears to be resistant to infection by bacterial spot.

Symptoms of bacterial spot on peppers begin as small, yellowish-green, water-soaked spots on the lower leaf surface. Spots first appear on older leaves in the lower canopy and spread upward with the aid of splashing water, high-pressure sprays, or the handling of wet plants by workers. These spots develop into small, circular, brown-to-black spots often surrounded by a narrow yellow halo. This leaf damage often causes significant defoliation of plants, especially in peppers, resulting in yield losses due to blossom drop and increased incidence of sunscald on fruit. Spots on pepper fruit begin as dark, circular, water-soaked lesions up to 1⁄4-inch in diameter. Eventually, the spots become raised and cracked and have a warty appearance.

Management of the Disease

Management of bacterial spot requires an integrated pest management (IPM) approach from seed selection through harvest. Host resistance has been a critical component of an IPM strategy to control bacterial spot on pepper. There are a few bacterial spot-resistant cultivars on the market. Currently, there are no commercial tomato cultivars available with resistance to bacterial leaf spot.

The first step in managing bacterial spot is selecting and purchasing high quality, disease-free certified seed from a reputable dealer.

Sanitation during transplant production is critical to managing this disease. Transplant producers need to remove all plant material from the greenhouse before starting a new crop and between crops. Weeds and volunteer plants also need to be eliminated in and around the production house.

Establish and maintain a balanced fertility program. Tomatoes and pepper crops growing under low fertility conditions are more susceptible to severe outbreaks of bacterial spot. Also avoid excessive fertility levels especially before fruit set as this can result in excessive foliage, low fruit set, and increased disease severity.

Application of fixed-copper products is still the primary method of managing bacterial spot in the field. Tank-mix the copper product with the fungicide mancozeb to increase the bactericidal effect of the copper. Actigard 50 WG (a.i. acibenzolar-S-methyl) is an SAR (systemic acquired resistance) product that can stimulate the plant’s own disease defense system. Actigard has been shown to reduce bacterial spot incidence and severity on tomatoes and should be used in combination with copper-based products in production fields. Always follow the manufacturer’s directions and restrictions when using any pesticide.

Plow crop debris soon after harvest is complete to help facilitate decomposition of the residue, eliminating the substrate on which the pathogen survives in the field.

 

Trade and brand names used in this publication are given for information purposes only. No guarantee, endorsement, or discrimination among comparable products is intended or implied by the Alabama Cooperative Extension System.

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