Bacterial Wilt and Canker in Field-Grown Tomatoes in California

Clavibacter michiganensis subspecies michiganensis (Cmm)

Bacterial canker has periodic outbreaks that can cause significant damage in a single season from primary and secondary spread. It is a vascular disease of tomato that spreads easily during the season by superficial movement of the bacterium via workers and equipment. This disease causes loss of photosynthetic surface, wilting, premature death and unmarketable fruit. Control and management relies primarily on clean seed, healthy transplant practices, sanitation, and crop rotation.

SYMPTOMS

All plant stages are susceptible, though initial infection typically starts at the seedling stage. As seedlings, small, white raised spots may occur on the leaves,   wilting and possibly death. However, disease may not develop in infected seedlings until conditions are more optimal, resulting in asymptomatic infected seedlings that get planted.

Early symptoms in-field include wilting, curling of leaflets and browning of leaves. Due to the vascular location of the organism, symptoms are often limited to one side of the plant.   Early infection can be difficult to diagnose based on symptoms alone, but vigilance is important to minimize spread. Therefore, diagnosis of suspect plant material by a lab or a highly sensitive and effective in-field diagnosis kit is suggested.

Leaf symptoms are often called ‘firing' due to the scorched appearance and rapid development. Initial appearance is yellow to tan patches between veins, ¼ inch diameter on the upper leaf surface of mature leaves. The leaf margin turns brown with a yellow border. As the leaves die, the petioles remain green and firmly attached to the stem.

Fruit symptoms are small (⅛ inch), creamy white spots with tan/brown centers found on the upper, exposed parts of the fruit. These characteristic ‘Bird's-eye spots' may eventually become brown, necrotic and merge with other spots. Fruit symptoms can be observed at any stage, but are usually seen on green fruit ½ to 2” diameter.

Vascular discoloration, initially yellow, then light brown to reddish, can also be seen in the stem and petiole. Pith appears dry and mealy, as well as discolored. Yellow sticky fluid may emerge from cut stem when squeezed. Stems can split open, forming cankers that facilitate secondary spread.

Whole plant wilting begins with the lower leaves but travels up the stem. Infection tends to be more severe in plants infected early, as seedlings, versus late in their growth cycle due to secondary spread.

"Bird's-eye spots" is a classic symptom, but can be hard to find

Sunken stem cankers are often abundant as the disease advances

Unilateral (left) and vascular symptoms (right)

BIOLOGY and EPIDEMIOLOGY

Disease development is favored by warm (75-90°F), moist conditions. Greenhouse operations are highly susceptible to disease development.

Initial inoculum can come from several sources, including infected plant debris in soil, infected weed hosts, infected volunteer tomato plants, diseased transplants, contaminated wood stakes, and infested seed. Clean seed is very important, for disease can develop and spread rapidly under greenhouse conditions, and a 1% seed transmission rate is sufficient to give 100% disease. In-field spread occurs easily by workers' hands, equipment, during vine pruning, particularly when plants are moist with dew or irrigation water. Bacteria present in and on the plant can be transmitted to adjacent plants by pruning or abrasion wounds, or simply contact with infected leaves, entering through pores on the leaf margin (hydathodes) which links directly to the plant xylem vasculature.

Cmm can survive as free-living bacteria in soil for short periods, in non-decomposed tomato plant debris, on weed hosts (nightshade) and volunteer tomato/pepper plants, on contaminated stakes or equipment and in association with seed. Volunteer plants from a previous infection or susceptible weeds can maintain low levels of the pathogen. It survives better in cool, dry conditions than in hot, moist conditions.

CONTROL

Bacterial canker can be challenging to control because of challenges with early detection, the highly infectious nature of the disease, and the number of inoculum sources. Sanitation and preventative measures must be enforced.

• Use certified, disease-free seed from canker-free plants. The standard hydrochloric acid method or fermentation process should produce clean seed. However, these methods will not treat embryonic infection. Centrifuge extraction can lead to high levels of seed contamination.

• Certified disease-free transplants. It is not usually possible to identify infected seedlings at the time of transplanting.

• Remove disease plants as soon as possible. If infection occurs early in the season, infected areas should be plowed down to prevent spread to nearby healthy fields. Several healthy plants adjacent to symptomatic plants should also be removed.
• Hands, tools, shoes and crop support materials should all be disinfected. Hypochlorite (bleach) is not satisfactory as a disinfectant. Quartenary ammonium compounds are recommended.

• Copper-fungicide compounds in foliar sprays are not highly effective for bacterial canker control because they only act at the tissue surface while many of the bacteria are internal to the plant. Sprays should not be applied at high pressure as this may damage the plant (causing many micro-wounds) and lead to more disease. Fixed copper field sprays may help in protecting healthy plants, especially if only superficial symptoms are present.

• When leaves are moist with irrigation water or dew, do not allow movement through the field by workers or equipment

• Remove or plow in infected plants as soon as possible after harvest in order to ensure good decomposition. The bacterium cannot survive in soil for long, but can survive in the field in complex with plant material.

• Discard wooden stakes and strings from infected areas. The porous condition of wood makes sanitation imperfect.

• Rotate out of tomato, peppers, eggplant and related crops for 2-3 years

• Control Solanaceous weeds and volunteer tomatoes and peppers.

• There are currently no resistant tomato varieties for bacterial canker

REFERENCES
Koike, S., P. Gladders, and A. Paulus. (2007) Vegetable Diseases: A Colour Handbook. Boca Raton, FL: CRC Press.

http://vegetablemdonline.ppath.cornell.edu/factsheets/Tomato_Bacterial.htm#Click

https://extension.umn.edu/plant-diseases/bacterial-canker-tomato

Dr. Sally Miller, Dr. Ruguo Huang Bacterial canker of tomato, Asta: A commercial growers guide.

Jones, J. B., T. Zitter, M. T. Momol, and S. A. Miller. 2014. Compendium of Tomato Diseases and Pests, Second Edition. St. Paul, Minnesota: The American Phytopathological Society.

Special thanks to input from Dr. Gitta Coaker and Dr. Cassandra Swett.

Flea beetles have been abundant this spring and have been a surprise to many because larger genera of flea beetles and resulting larger holes have been present. The stable, warm spring with few heat waves has been favorable to their development.  The characteristic large hind femurs allow the insect to spring up for movement, but they can also fly. One species recently noted in the valley is the palestriped flea beetle (Systena blanda) which are larger than most flea beetles, about 0.12 inches long, and are dark brown with a white stripe on each wing.

Photo: Palestriped flea beetle on a pepper plant in Yolo County (May 21, 2018)

Flea beetles enjoy a wide host range of crops including peppers, carrots, beans, beets, eggplants, lettuce, melons, peas, pumpkins, tomatoes, potatoes, alfalfa, radishes and most cole crops. These beetles feed on the underside of leaves, usually leaving pits or holes in the leaves. However, in the photo, you can see larger hole damage that one may not expect from a flea beetle. Flea beetles lay eggs in the soil around the plant, on the leaves, or in cavities of hollowed out stems. Larvae living in the soil can feed on roots causing stunting. While this is generally not a problem, it can cause serious damage on occasion.

MANAGEMENT

Mature vegetable crops can tolerate high numbers of flea beetles. They do the most economic damage when seedlings are just emerging or transplants are not yet well established. Once plants have five true leaves, they are usually not highly impacted by flea beetle damage. However, stress from leaf damage can be exacerbated under very hot, dry, or windy conditions.

When plants are in the cotyledon and first leaf stage, monitoring for this pest should be done every two weeks. Thresholds for treatment vary depending on the plant, but as a general rule, beetles should be treated when scouting shows the following levels of infestation:

Protective coverings, such as floating row cover, can be used to exclude insects from a new or emerging planting. If flea beetles have been a problem in the past and/or have been identified in the surrounding weed margin, protective coverings can be very effective to establish a crop.  However, it must be provided before the crop emerges or immediately after transplanting to be most effective.

Larvae overwinter in the soil and on crop residue. Burying crop residue as well as disking weeds in the field margins are usually effective control mechanisms. Cereal grains are non-hosts and will reduce population buildup in the fall and spring.

Sufficient irrigation and fertilization are also key to improving recovery from flea beetle damage.

If an outbreak is particularly bad, one application of an approved insecticide is usually sufficient to control the population. This can be applied as a spot treatment or on the field margins, if populations are identified there. Baits are not effective.

If flea beetles are an on-going challenge, trap cropping with the highly desirable crop Chinese Southern Giant Mustard (Brassica juncea var. crispifolia) can be effective.  One row every 55 yards between or bordering the main crop can be protective.  More on trap cropping is described in the ATTRA guide.   

For more information about the pale-striped flea beetle and control mechanisms, see:

Carrot Palestriped Flea Beetle

Flea beetles

Flea Beetles on Peppers

Alfalfa Pale-striped Flea Beetle

ATTRA Flea Beetle Guidelines (attached)

Working with the Geisseler Lab at UC Davis, we have been evaluating a number of commonly used organic amendments to understand when the nitrogen (N) applied with various amendments actually becomes available for plant use. We have assembled this graph from our incubation studies, showing nitrogen availability. Although the incubation methods do not necessarily replicate field conditions, they are useful to understand how the amendments behave compared to each other under the same conditions. These relationships are likely to play out similarly on-farm.

This graph can be used to understand 1) how quickly N is released from products and 2) how organic fertilizers, composts and liquid fertilizers compare.

Summary

1)   All the products show a similar N release curve, except composts. The trend is a spike of rapid release in the first 10 days, followed by a steady release between 10- 40 days, and leveling out to minimal release thereafter*.

2)   The amount of N released is highest in seabird guano, feather meal, food-based liquid fertilizer and liquid fish emulsion.

3)   Generally, the higher the carbon-to-nitrogen (C/N) ratio, the less readily available plant N.

4)   Yard trimmings composts are unlikely to contribute much N fertility in the year they are applied, though they are useful for building long-term soil fertility. Composts with a high C/N ratio may tie up soil nitrogen.

5)   All amendments with a steep initial release suggest a relatively high proportion of initial ammonium. This means that a good amount of N is immediately available for plant uptake, and that it's important to incorporate these materials to avoid N loss by volatilization as ammonia.

 *Since the amendments were incubated under optimum conditions, the release rates in the field will generally be lower.

Methods

To measure potential nitrogen release rates from different amendments under controlled conditions, we obtained soil from the top 6” of an organically farmed loam soil in the Capay Valley. This soil was mixed with several different amendments at a rate of 300 lbs N/acre, and incubated at optimum moisture at 73°F for 12 weeks.

We are continuing to evaluate products and welcome your suggestions.

Contact: Margaret Lloyd at mglloyd@ucanr.edu

Check out the calendar to see what's happening in March in Yolo/Solano/Sacramento Counties.

Insectary Cover Crop Field Day

Guild Raising

Barn Owls and Bats and their Benefits

Farm Finance Expo

http://ccsmallfarms.ucanr.edu/