Bacterial spot

Bacterial spot is the most destructive leaf disease of capsicum. Black rot of brassica plants often follows infection by bacterial spot.

Cause

The bacterium Xanthomonas campestris.

Symptoms

The first symptoms are usually small, yellow V-shaped areas developing along the leaf margin. These areas soon turn brown and dry out. Vein blackening may extend down the leaves into the petiole and the stem. When the stem is cut across, a black ring will be seen in the water-conducting tissues just below the bark.

Soft, rotting organisms often quickly follow the invasion of fleshy leaves and petioles. Black rot bacterium then cause the plants to rot rapidly.

Tan, circular spots with yellow halos can also develop between the veins. These coalesce and dry out to give the tattered leaf scald appearance.

Infection of young, immature curcubit fruit causes small water-soaked areas of pronounced, light brown ooze. As the fruit enlarge, the ooze dries to form a raised, yellow crust over the spot. The flesh beneath the spots is often water-soaked, extending to the seed cavity and resulting in seed contamination.

How it spreads

The bacterium may be introduced in seed or in surviving undecomposed crop residue or other host plants. Bacteria spread in water splash during wet, windy weather or by overhead irrigation. It can also disperse on insects, or on people or equipment moving through the crop.

Warm, humid weather favours rapid disease development.

Crops affected

A serious disease of many crops such as capsicum, tomato, potato, eggplant, Asian vegetables, cabbage, brussel sprout, pumpkin, melon, lettuce, bean, carrot and stone fruit. It is a minor disease of sweet corn.

The disease can survive between crops on nearby host crops and on many weeds.

Control

Worldwide, there are 11 known races of bacterial spot that affect capsicum. Not all of these have been found in Queensland. No variety has resistance to all races of bacterial spot, but to be effective, the variety used must have resistance to all the races present.

Use as many of the following practices as possible in an integrated program to manage bacterial spot.

Seed management

• Buy seed from a reputable dealer. Ask the seed company for seed that has been tested for bacterial spot. Infected seed can be an important source of the disease in capsicum.
• Seed can be treated with hot water or calcium hypochlorite to kill the pathogen. Hot water treatment is more thorough than calcium hypochlorite because it can kill bacteria inside the seed as well as those on the surface.
• Take care when treating seed with hot water as high temperatures can reduce seed germination if proper precautions are not taken. Plant seed as soon as possible after treatment. If you treat the seed after purchase, the seed company's liability and guarantees are null and void.

Farm hygiene

• Destroy old capsicum plants immediately after final harvest.
• Do not allow volunteer capsicum or tomato plants to grow between seasons or amongst cover crops, because they can harbour the disease.
• Control solanaceous weeds such as nightshades in and around the crop.
• If disease becomes widespread decide quickly about ploughing out the crop.

Crop management

• Do not rotate capsicum crops with tomato, potato or eggplant and do not grow these crops together.
• Physically separate successive plantings. When space is limiting, successive plantings in the same field can be separated by planting several rows of a plant that will form a tall, thick barrier, such as sorghum, sudan grass or sweet corn.
• Provide appropriate amounts of fertiliser and water. Plants that are stressed or those that are growing too luxuriantly can be more susceptible to disease. Low nitrogen and potassium, and extra high magnesium, calcium or nitrogen levels have been associated with increased susceptibility of crops to bacterial spot.
• Use drip rather than overhead irrigation. Water splashed during overhead irrigation spreads the pathogen. When overhead irrigation is the only option, water during the day with enough time to ensure that the plants are dry by nightfall.

Disease management

• Monitor crops thoroughly for bacterial spot and other diseases each week.
• Avoid working fields when plants are wet. As well as movement by splashing water drops, the pathogen can be spread mechanically on workers' hands and on farm machinery when plants are wet. Work in infested areas last if possible.
• Thoroughly spray crops with copper fungicides at recommended rates. In Queensland, populations of bacterial spot have developed tolerance to copper sprays which means that copper is now less effective than it once was in controlling bacterial spot infections.
• Disinfect spray rigs and other machinery with a general disinfectant product, such as a quaternary ammonium compound, after the job is completed or before using the equipment in uninfected crops.

Chemical registrations and permits

Check the Australian Pesticides and Veterinary Medicines Authority database for chemicals registered or approved under permit to treat this pest on the target crop in your location. Always read the label and observe withholding periods.