Bacterial canker (Clavibacter michiganensis pv michiganensis) is one of the most destructive and puzzling tomato diseases in Massachusetts. One grower sets out a vigorous crop of transplants that grows into healthy looking plants full of green fruit, only to see "marginal scorch" appear throughout the field and his yields decline despite regular copper sprays. Another has never had it on the farm before, and finds an outbreak that starts in one cultivar and spreads throughout the field. Another has had it for years, and can't seem to escape it no matter how many different strategies he tries.
Everyone would like a simple answer to the canker problem. There isn't one. However, in recent years we have seen some growers having more success in bringing this disease under control. Research at several universities is revealing more about how the pathogen may be arriving on the farm and what the critical practices are to keep it under control. This article will discuss new research results, as well as what we have learned from the UMass Tomato IPM program over the past four years.
Three key principles for preventing losses to bacterial canker can be summed up as follows:
Each of these protects your plants at a different stage of growth - and all are important.
Seeds as a source. The bacterial canker pathogen can be seed-borne, both on the surface of the seed and under the seed coat. Although seed companies take many precautions to produce disease-free seed and test seed for the presence of disease, it is possible for some commercial seed to be infected. The bacterial canker pathogen is more difficult to detect reliably than many other disease organisms. According to recent research at the University of Iowa, the limit of detection using standard testing procedures was 1:3,000 to 1:10,000 (one infected seed per 10,000).
One precaution that growers can take to improve the assurance that their seed is disease free is to use hot-water treated seed. Some companies offer hot water treatment as an option (this includes Johnny's Selected Seed and Stokes Seed). It is also possible to hot-water treat one's own seed, but this must be done carefully with the proper equipment. See the fact sheet entitled, Preventing Bacterial Diseases of Vegetables with Hot-water Seed Treatment for further details.
If you are concerned about bacterial canker, do your best to ensure that all of the seed you start in your greenhouse, is clean including the plants that are destined to be sold as bedding plants.
Greenhouses: what you can't see is what you get. Research at the University of Michigan has shown that: 1) the pathogen can move readily from infected plants onto clean plants in the course of regular greenhouse activities, and 2) carryover in the greenhouse from one season to the next may not be as important as was once thought. In one experiment, seedlings that carried systemic infections with bacterial canker were placed in known locations in a grid of transplants in the greenhouse. Watering was from hand-held sprinklers; no special precautions were taken to prevent disease spread. Plants nearest the infected plants developed bacterial canker symptoms (wilting) and died in the greenhouse. Many other plants showed no symptoms in the greenhouse, but the bacteria could be found residing on the surface of the leaves. When these healthy-looking plants were set out in the field, they developed symptoms during the season and the yield losses were serious. This type of infection - bacteria that enters from the surface of the leaf through natural leaf openings or wounds - causes the "marginal scorch" symptoms that are so common in Massachusetts. Once the bacteria enter the plant - which could occur in the greenhouse or in the field - it can take anywhere from seven to 84 days for symptoms to appear.
The second part of this experiment was to leave the weeds and residue in the greenhouse over the winter, and put new, clean transplants into the house the following year. The second crop of plants did not develop symptoms either in the house or the field, and the bacteria were not found on the plants. Does this mean you should stop your greenhouse sanitation practices? No! It is important to use standard disinfectants to clean benches, trays, hoses, etc. But this may not be the most important source of the problem.
If infected plants are present, the movement of bacteria from one plant to another during normal watering, handling, and ventilating activities occurs readily. Controlling the bacteria at this stage can prevent yield losses. Bacteria on the surface of transplants can be effectively controlled by sprays of copper hydroxide or streptomycin in the greenhouse. Kocide DF® Agri-Strep® and Agri-mycin 17® and Dithane F-45® are labeled for greenhouse use on tomato.
In another experiment at the University of Michigan, Kocide 40DF®, alone or mixed with Dithane F-45®, or Agri-Mycin 17® was applied on a five-day schedule from the first true leaf stage until transplanting. Bacteria on leaf surfaces were effectively suppressed. Plants were not sprayed after being set out in the field. Yields of sprayed, exposed transplants were equal to clean plants that were grown in a separate greenhouse and had never been exposed to the bacteria. Unsprayed, infected plants had lower yields.
It is possible that a less intensive spray schedule would have similar results, but this was not tested. In an experiment conducted at UMass in 1994, weekly sprays of streptomycin on greenhouse-inoculated plants eliminated all bacteria on sprayed plants until mid July.
Using bactericide in the greenhouse means a lower volume of chemical is used compared to multiple copper sprays in the field. Check the label carefully for required REI and personal protective equipment.
Rotate your tomatoes to a different field. Setting clean transplants into a field where canker-infected tomato was grown the previous year will result in early infection with canker and reduced yields. This has been documented by research and by grower experience in Massachusetts.
Bacterial canker survives in the field as long as there is any infected crop debris. It lasts longer in debris on the surface than it does in buried debris, plowing after harvest will help to speed up the decomposition. Keep each field out of tomato (and related crops such as potato and eggplant) for two to three years.
We have seen outbreaks of canker where tomatoes are planted in rotated ground that is adjacent to last year's tomato ground. New fields should, as much as is possible, be located at a distance from last year's fields -- as far as possible given the choices available on your farm.
Prevention is cost-effective. All of these tactics focus on prevention -- ensuring that disease-free plants go out into a "clean" environment. Bacterial canker outbreaks in the field require regular sprays with a copper or copper/maneb mix, with limited success. Prevention strategies are both the least expensive and the most effective way to beat bacterial canker.
Ruth V. Hazzard, Robert
University of Massachusetts Extension, Agroecology Program, Vegetable and Small Fruit Team
Reprinted from UMass Extension
Image from Ohio State University Extension Website
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