Pest of the Month: Bacterial Spot

Identification

Symptoms of bacterial spot appear as small, water-soaked, greasy spots on infected leaflets. On tomatoes, distinct spots occur. Individual leaf spots may coalesce with each other, resulting in the browning of entire leaflets. Fruit spots often begin as dark specks. As spots enlarge, they become raised and scab-like. In pepper, spots may be lighter in color, and fruit lesions may appear blistered. In mature plants, leaflet infection is most concentrated on older leaves and defoliation may occur in severe infections. Positive diagnosis requires lab tests, as other diseases may cause symptoms that appear similar to those of bacterial spot.

Survival And Spread

Entry into the plant occurs through stomata or wounds made by wind-driven soil, insects, or cultural operations. Bacterial spot can be seed transmitted, but most inocula come from volunteer plants or infected plant debris in the soil. Temperatures of 75°F to 87°F are ideal for bacterial spot, but infections can occur at higher or lower temperatures. Since water movement spreads the bacteria from diseased to healthy plants, workers and farm equipment should be kept out of wet fields.

Management

An integrated approach is needed to manage this disease. Sanitation is important. Pepper and tomato volunteers and solanaceous weeds should be destroyed between crops. Transplant houses should be located away from tomato or pepper fields. Purchase only certified disease-free transplants.
Since no variety incorporates resistance to all known races of the disease, growers should use varieties with resistance to races that occur in their area. Commercial pepper varieties resistant to races 1, 2, and 3 have been on the market for several years. Over the past year or so, a number of newer varieties that incorporate additional resistance to races 4 and 5 have come on the market. Seminis has introduced several varieties of sweet pepper that are resistant to Races 1–5, including PS 5776 and PS 8302. Harris Moran has introduced Patriot and Revolution, which includes Race 1, 2, 3, and 5 resistance. Harris Moran 2641 has resistance to races 1–4. All of these have performed well in trials, demonstrating dramatically reduced infection rates.

Apply sprays before and during rainy periods. If conditions are favorable, frequent spraying may not be sufficient to maintain bacterial spot below damaging levels. The traditional recommendation for bacterial spot control consists of copper and maneb or mancozeb. There is some evidence that the use of organosilicate adjuvants and applications of magnesium might increase the incidence and severity of bacterial spot infections.

In the past few years, several new products have come on the market that have given good results in research trials when used in rotation or together with traditional controls such as copper. These include Tanos (cymoxanil-famoxadone, Dupont Crop Protection), Actigard (acibenzolar-S-methyl, Syngenta Crop Protection), and Serenade (Bacillus subtilis, AgraQuest). Some growers and researchers have experienced success with the bacteriophage (bacterial virus) AgriPhage (Omnilytics) for the control of bacterial spot. A number of growers have also reported good results using Oxidate (Biosafe Systems) as a sanitizing agent following cultural operations or weather events favoring the development and spread of the disease.