Bacterial Canker: a disease to watch for

Published: November 1, 2014

Bacterial canker of tomato, caused by the gram positive bacterium Clavibacter michiganensis ssp. michiganensis (Cmm), is a sporadic but destructive disease of field and greenhouse grown tomatoes in the Midwest US and other parts of the world. Yield losses, resulting from wilting and eventual death of plants as well as culled fruits due to the “birds-eye-spot” symptom, during outbreak seasons could reach as high as 70-80%.

Historically, bacterial canker of tomato was first discovered in 1909 from a greenhouse in Grand Rapids, Michigan and is at times referred as the Grand Rapids disease. The disease was very well described by the pioneer plant pathologist Erwin Smith over one hundred years ago. There were devastating outbreaks of the disease in North America including the US Midwest in 1930s and 1980s. During the years 1983-1985, the disease was of high concern to tomato growers in Canada, Michigan, Ohio, Indiana, Iowa and Kansas.

Undocumented seasonal reports of the disease have been trickling from different counties of Missouri in the past years. There was, however, a noticeable disease outbreak in Missouri and other Midwestern states as recent as 2011. In response to this urgent call, Lincoln University’s IPM program has organized a webinar on diagnosis, identification and management of bacterial canker. Power point presentations by Drs. Sally Miller (The Ohio State University) and Dan S. Egel (Purdue University) as well as a recorded video of the discussions are available at (http://www.lincolnu.edu/web/programs-and-projects/ipm).

The disease slowed down for two years (at least not officially reported) until it showed some resurgence in 2014 with three confirmed cases of the disease from Daviess, Vernon and Lawrence counties. All these three cases have been persisting for the past few years, according to the farm owners and conversations with extension educators. Records from the University of Missouri’s plant diagnostic clinic indicated 7 confirmed cases of bacterial canker in years 2009 and 2011, and 2 confirmed cases in 2014. On the other hand, a farm in central Missouri which was hit hard by the disease during the years 2011-2013 seemed to show a significant improvement. It may most likely be that other unreported cases of the disease could exist elsewhere. But one thing is certain, the disease unfortunately continued to prevail in Missouri although great strides have been made in containing the epidemic. As growers are getting ready for next year and the cold winter is just on the door, we felt that they should be proactive about this disease and put a disease prevention strategy in place.

Marginal necrosis or "firing" symptom on tomato leaves.

Yellow to brown discoloration of vascular tissues on infected tomato stem.

Bacterial canker is a seedborne disease capable of spreading fast in nurseries or greenhouses and hence difficult to manage once it is established in a farm. A very wise decision in managing this disease is to put together a holistic prevention plan for each component of the production system.

A useful hint would be to start by drawing the map of production flow. The foremost strategy will be to target pathogen avoidance. Make sure that your starting plant material (seeds or transplants) is clean and obtained from a reliable source. Keep a good record of the seed lots used and location of the fields. This will help to trace back the source of the problem and to succeed in a smart rotation plan. Next, identify and train individuals responsible for each activity. These activities include maintaining aseptic environment in the nursery, getting rid of any crop residue or alternate hosts, sanitizing all equipment used in tomato production, keeping the field clean, monitoring seedlings and transplants frequently and detecting the disease as early as possible.

Once detected, a prompt action shall be taken before the bacterial inoculum spreads with splash or windy rain. For further information on this disease and any aspects of its management you may contact Zelalem Mersha (Assistant Professor and State Extension Specialist, Lincoln University Cooperative Extension, 900 Chestnut St., 214 Allen Hall, Jefferson City, MO 65102; e-mail mershaz@lincolnu.edu, Tel. 573-681-5634.

Symptomatically, the disease can be differentiated on the basis of whether it arises from systemic or localized infections. A systemic infection, initially manifested by wilting symptom, occurs from the bacterial inoculum that is carried with the seed or penetrates through wounds (created during pruning or cultivation) and reaches vascular tissues. Very often only half of the plant wilts or leaflets on one-half of the petiole wilt while the other half remains healthy looking. Localized infections may result in localized symptoms such as marginal necrosis (“firing”) when infection occurs through broken trichomes or natural openings such as hydathodes. Vascular tissue of infected stems will show a yellowish discoloration which may change to brown. These yellow to brown streaks gradually darken and sometimes open resulting in dark brown cankers. One way to easily distinguish bacterial canker from other wilt diseases caused by fungal pathogens like Fusarium and Verticillium is by slicing a symptomatic plant and placing them in water. If bacterial masses ooze out of the vascular tissues, the wilting is most likely caused by bacteria. An accurate diagnosis is a key to a successful and effective control of this disease. If you see any suspect transplant, get it diagnosed at MU’s diagnostic lab or contact staff of Lincoln University’s Plant Pathology Program (Dr. Zelalem Mersha mershaz@lincolnu.edu, Tel.: 573 681 5634 or Ms. Martha O’Connor OConnorM@lincolnu.edu, Tel.: 573 681 5633) right away before the disease spreads.