Identify and stop diseases on greenhouse tomato seedlings

Bacterial diseases can infect tomato seedlings long before symptoms become obvious, said Mich. St. Univ. plant pathologist Mary Hausbeck. She said bacterial speck, bacterial spot, and bacterial canker are three pathogens that can wreak havoc on greenhouse-grown seedlings.
Symptoms of both bacterial speck and bacterial spot appear as dark blotches or spots on the leaves. These blotches or spots are sometimes nearly black in color, but they can also be light brown. A yellow halo can surround the blotches or spots, but may not always be present.
Symptoms of bacterial canker look different than other bacterial diseases. This disease causes a light browning along the mid-vein of the leaves and extends down the petioles. Growers can miss seeing bacterial canker infection because it can mimic other disorders including plant stress from drying. Bacterial canker symptoms are easy to overlook because they do not appear to be “disease-like” compared to bacterial speck and spot.
Bacterial speck, bacterial spot and bacterial canker can be introduced to greenhouse tomato seedlings via the seed. Bacteria can occur on the seed coat, but the bacterial canker bacterium 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. Once the bacterium moves to the seedling, it reproduces rapidly when conditions are warm and wet.
Bacteria can move between plants by splashing water. Overhead watering can disperse the bacteria from diseased transplants to adjacent healthy transplants. At a high humidity these bacteria can become encapsulated within tiny airborne water droplets and may move within the greenhouse via water aerosols.
Bacteria can also be introduced onto tomato seedlings through plant residue left in the greenhouse from a previous season’s diseased tomato plants. Old foliage under benches or packed down in aisles may serve as a potential source of bacteria that can infect new seedlings.
In general, a plant wound or a natural plant opening is needed so the bacteria can gain entrance into the plants. The wound can be as small as a broken plant hair or other microscopic abrasion. Natural plant openings are found along the leaf edge. Bacteria can move along the leaf surface and move into the leaf through these openings. This can result in a dark fringing along the outer edge of the leaf.