Bacterial Diseases of Tomato
Infections on tomato fruits by bacterial pathogens often occur before fruit matures and symptoms appear. In Utah, two bacterial diseases of tomato have been found in the last two years: bacterial speck and bacterial canker.
Bacterial speck is the most common bacterial tomato disease in Utah. The disease is introduced into the garden or field on contaminated seed or infected transplants. It is caused by Pseudomonas syringae pathovar tomato. Infected ripe tomatoes have characteristic black spots with yellow halos. Fruit infections occur early in the season, but symptoms are not visible until fruit matures. Foliar symptoms are evident right after infection, consisting of brown spots that may also be surrounded by a yellow halo (as shown above). If transplants show brown spots on leaves they should not be purchased or planted.
Transmission of bacterial speck occurs through contaminated seed, splashing water, and pruning tools. Plants grown from infected seed will develop brown spots on leaves soon after transplanting. Rain or irrigation water spreads bacteria, causing new infections on nearby plants. The bacteria can also spread from last year's crop residue, where it can survive for up to six months, or from asymptomatic weeds in the Solanaceae family. A final mode of spread is from pruning tools. After cutting infected shoots, bacteria can be left behind on the blades, and may be transferred to healthy plants in subsequent cuts.
Management of bacterial speck is primarily through cultural practices. Use only certified disease-free seed, or save seeds from healthy plants. Plants with suspect symptoms should be submitted to the Utah Plant Pest Diagnostic Lab, and plants with positive diagnoses should be removed and destroyed immediately. Rotate tomato every year, and remove plant debris such as old leaves and vines.
Control weeds to reduce the potential for inoculum to build up. If the disease had been a problem in the previous year, copper products can be applied as a preventive. However, once symptoms are visible, copper will be ineffective and plants need to be removed. Infected plants should not be composted because the bacteria can survive if the compost does not get hot enough.
Another bacterial disease found in Utah this year is bacterial canker. This disease is caused by Clavibacter michiganensis pv michiganensis. Just like the pathogen that causes bacterial speck, C. michiganensis pv michiganensis is also seedborne. Symptoms can occur at any plant stage and on any above ground plant parts.
There are two types of infections: systemic and superficial. Systemic infections occur when plants are grown from contaminated seed, and the bacteria spread through the entire plant. Superficial infections occur when bacteria splash onto the plant surface, causing lesions. Seedlings with a systemic infection will suddenly die when conditions are right for the bacteria to spread within the plant (moisture or humidity, with temperatures between 75-80°F). However, if conditions for the bacteria are unfavorable, infected plants will continue to grow and appear healthy and vigorous until conditions change in favor of the bacteria. Older plants with a systemic infection will show upward-curling, yellow to brown leaves, and overall wilting. A discoloration of the vascular tissue can be seen on cut tissue.
Superficial stem and foliar infections result in brown spots, while fruit infections appear as white, raised spots with a dark brown center known as a “bird’s eye.”
Besides being seedborne, Clavibacter can survive in plant debris left in the yard or field from the previous tomato crop.
Management strategies for bacterial canker are very similar to bacterial speck. It is important to only use certified disease-free seed and not save seed from symptomatic plants. The bacteria can survive on plant debris, but once it has decomposed, it cannot survive in the soil. Therefore, clean up all plant residue from the garden and apply only fully composted organic matter. Crop rotations for one year will also reduce spread. Old wooden stakes, pruning tools, and any equipment that comes into contact with infected plant material should be disinfected with a 10% bleach solution. Copper products can be used as preventive to protect fruit from superficial infections, but will not control systemic infections.
-Claudia Nischwitz, Plant Pathologist
Featured Picture of the Quarter
Colorado potato beetle is the single most important defoliator of potatoes. A single adult female lays up to 800 eggs and consumes close to 1.5 square inches of leaf material per day. If the beetles manage to consume all the foliage of the host plant, they move to the stem and exposed tubers, but these are not their preferred sources of food. Unfortunately, potatoes have not evolved any resistance mechanisms, and after more than 100 years of breeding efforts, no resistant plants have been identified.
-Image by Claudia Nischwitz,