How to Prevent Tomato Seedling Diseases

Bacterial diseases can ramp up on greenhouse tomato seedlings long before symptoms become obvious. Bacterial speck (Pseudomonas syringae pv. tomato), bacterial spot (Xanthomonas campestris pv. vesicatoria) and bacterial canker (Clavibacter michiganensis subsp. michiganensis) are three diseases that can wreak havoc in the greenhouse.
 
Symptoms of both bacterial speck and bacterial spot appear as dark blotches and/or spots on the leaves. Sometimes these blotches/spots are nearly black, but they can also be light brown. A yellow “halo” can surround these blotches/spots but may not always be present.
 
Bacterial canker looks different than other bacterial diseases. Canker causes a light browning along the leaf mid-veins and extends down the petiole. It is easy to overlook canker infection because it can mimic other disorders, including stress from drying. Bacterial canker symptoms do not appear to be “disease-like” compared to bacterial speck and spot.

Disease spread
In general, bacteria are spread via splashing water between plants and require a wound or a natural opening to gain entrance. A wound can be as small as a microscopic abrasion. Bacteria can move into a leaf via natural openings along the leaf edge. This can result in a dark fringing along the leaves’ outer edge.
 
Bacterial speck, bacterial spot and bacterial canker can be introduced to a tomato seedling via the seed. Bacteria can occur on the seed coat, but the bacterial canker bacteria can sometimes be inside the seed.
 
When the seed coat stays attached to the cotyledons following emergence, any bacteria on the seed coat can move to the seedling. They reproduce rapidly when conditions are warm and wet.
 
Bacteria move between plants by splashing water. Overhead watering allows bacteria to move readily from diseased transplants to adjacent healthy transplants. When the greenhouse humidity is especially high, the bacteria can become encapsulated within tiny airborne water droplets and may move within the greenhouse via water aerosols.
 
Bacteria can also be introduced to tomato seedlings via diseased plant residue left in the greenhouse. Old foliage may serve as a source of bacteria to infect a new group of seedlings.

Start control early
Bacterial control is best begun even before there is a sign of disease. Start with tomato seed that has been tested for bacterial pathogens and commercially treated. Growers can treat the seed themselves including using a hot water treatment. Although this treatment can be effective, the high temperature can hurt germination. Hot water treatment is not used widely but may be an option for those who are looking for relief from bacteria and are growing transplants for the organic industry and have limited options.

Developing problems
Problems can develop as seedlings develop. Alternaria and Botrytis can cause problems as the seedlings grow and the lower leaves age, overlap and develop a canopy. Alternaria is also known as early blight and first appears on the oldest leaves mimicking a bulls-eye target with concentric rings in the spot/blotch. Botrytis is everywhere within the environment and is best known as grey mold because of its fuzzy grey spore masses that are produced on diseased leaves and stems. It is not uncommon for Botrytis to infect older tomato transplants via the cotyledons which are clinging to the stem. The pathogen then progresses into the stem and may constrict and girdle it.
 
Botrytis and Alternaria require the leaves stay wet for at least four to six hours so their spores can germinate and grow into the plant and cause disease. If the relative humidity is high during the day and the temperature drops even a degree or two, the moisture in the air can condense onto the foliage as a fine film and provide enough water for the spores to germinate and infect the plant.
 
Botytis is primarily a greenhouse problem but may also occur in low and high tunnels if there is inadequate ventilation and air movement. Once plants are moved outside, there is better air circulation and Botrytis becomes less of a problem.
 
Alternaria is a common problem in field production where it can overwinter in tomato plant debris. It can also occur on seedlings in the greenhouse.
 
Botrytis and Alternaria spores move on air currents within the greenhouse. Since they are common pathogens in the environment, the distance they might travel is not considered to be of great importance.

Reducing disease infection
Keeping the relative humidity lower than 85 percent and providing good air circulation limits disease. Water that accumulates under benches and tight plant spacing can contribute to pockets of high relative humidity that favors Alternaria and Botrytis.