Bacterial canker ravages processing tomatoes

Learn how to recognize bacterial canker now to manage this disease in the future.

Bacterial canker has devastated many fields of processing tomatoes in Michigan. Losses to growers from drastic yield reductions is expected to be substantial.

Symptoms of bacterial canker on tomato depend on the age of the plant when infected and weather conditions. When infected at a young age, plants can wilt and die. Less severe infections on transplants include blistering on the petiole and browning on the midvein. Infected transplants can also appear healthy and not show symptoms. On older infected tomatoes, sometimes the leaflets wilt on one half of the petiole, while the other leaflets remain healthy (Photo 1).

Cankers can occur on stems and often occur after wilting is seen. There can also be browning of the leaves, especially around the margins; this is commonly referred to as the “firing stage” of the disease. When the stem of an infected plant is cut open, a slight browning or discoloration of the internal tissue is often seen; internal tissues of advanced cases are greatly deteriorated and cavities are formed. Infected fruits have “birds-eye” spotting that starts as small, white dots (Photo 2).

As the spots get larger, the centers die and turn dark, giving a “birds-eye” effect. Plants infected with bacterial canker do not always show these fruit lesions. While it may be difficult to diagnose bacterial canker based on any one symptom (except birds-eye lesions), when two or more of these symptoms appear in a plant, they are likely the result of bacterial canker infection and identification should be made through isolation of the bacterium.

Unilateral wilting
Photo 1. Unilateral wilting of a tomato leaf due to bacterial canker infection.

Birds-eye spotting
Photo 2. Birds-eye spotting of a tomato fruit infected with bacterial canker.

Plants showing symptoms of bacterial disease and those immediately adjacent to the infected plants should be immediately removed from the greenhouse and destroyed. In some situations, all of the plants will have to be destroyed. Unfortunately, if the disease begins in a flat that is too far from the walkway to be seen easily, the disease may go undetected until several flats are severely infected. Although epidemics may seem to appear overnight, chances are it had rather humble beginnings in just a few plants and simply progressed unnoticed for a couple of weeks.

Plug sheets containing infected transplants should not be reused. Removing infected transplants from the greenhouse is the most critical component of managing bacterial diseases once they’ve been introduced. Planting diseased transplants into the field ensures that the disease is established early with the greatest potential for yield and quality reduction if the environment favors the disease. Planting infected plants in low tunnels can result in very early defoliation.

Tomato plantings should be rotated. Do not include any other solanaceous crop such as pepper in your rotation program. Although it is widely thought that the bacteria cannot live in the soil, they can survive on tomato debris, especially debris on the soil surface. Two to three years is a rule of thumb to allow such tomato debris to break down. Weeds, especially solanaceous weeds such as nightshade and horsenettle, should be controlled around the fields. Although it is suspected that the bacteria that cause disease on tomatoes survive on weeds, the importance of weeds as a source of inoculum is not known.

Once a greenhouse or field is contaminated with bacteria, steps must be taken to assure that future crops remain disease free. If a greenhouse is contaminated, remove all plant material from the greenhouse (including weeds and dead plant tissue on the floor), wash and disinfect floor surfaces, hoses, equipment, etc. with a 10% solution of bleach or a commercial disinfestant (GreenShield is an example).

Wooden structures such as benches or trays should be soaked in a disinfestant such as bleach (10%) or GreenShield for a minimum of an hour and preferably overnight. A simple washing of wooden surfaces is inadequate because of the cracks and crevices that may allow the bacteria to escape a surface wash. Bacteria that overwinter on a wooden surface may be carried to the plants in water droplets next season during the splashing of overhead irrigation.

A contaminated field should be rotated out of tomatoes for at least three years. At one time it was believed that a rotation of at least five years was necessary, however, it is now known that the level of bacteria in a contaminated field drops dramatically after the first year of rotation. Any equipment used in the problem field should be washed and disinfected prior to entering a clean field. Equipment and workers should begin work in the cleanest field and finish with the contaminated field.

Copper sprays every five to seven days may help reduce the spread of bacterial canker. However, if the environment is favorable for bacteria (75-90ºF with rain) coppers may be limited because the bacteria have a decided advantage in a wet environment.

Avoid working in a diseased field when it is wet to avoid spreading the disease. Bacteria may enter the plant through natural openings, or wounds created by wind, pesticide spraying or insects. A film of water on the leaf surface allows the bacteria to remain viable and move. If workers are moving within a wet field and creating new wounds on the plants, new infections are likely. If plants have been staked, all stakes should be treated as discussed previously for wooden trays and benches.

Copper or copper/mancozeb mixes have been used in an attempt to control bacterial diseases. To maximize the benefit, copper sprays should be applied soon after planting in the field and frequently (every 5 days) during wet weather. Although growers commonly express concern about applying copper during bloom, it is thought that infection of the bloom may result in subsequent fruit symptoms. Studies from research plots suggest that copper applications during bloom do not decrease yields.

Also see: MSU’s Research results for bacterial canker in tomatoes

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

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