Michelle Grabowski, University of Minnesota Extension Educator
Several kinds of blight are common on tomatoes grown in Minnesota. Almost every gardener has struggled with the fungal diseases Septoria leaf spot or early blight on their tomato plants at one time or another. This year bacteria are the primary pathogens being isolated from tomato leaf spots.
There are two bacteria that result in leaf spots on tomato; bacterial speck caused by Psuedomonas syringae pv. tomato and bacterial spot caused by Xanthomonas campestris pv. vesicatoria. Bacterial speck and spot can cause spots to form on the leaves, stems and fruit of tomato plants. The leaf spots caused by bacterial speck and spot look identical but the two pathogens can be distinguished by differing types of fruit spots that form later in the season.
Leaf spots are dark brown to black, small (about the size of a pencil tip) and have a yellow halo around them. Often the center of these leaf spots dry up and drop out leaving tiny holes throughout the leaf. If a leaf is infected with many leaf spots, the leaf may turn partially or completely yellow, and may even fall off. In past years, only minor infections from bacterial spec and spot have been observed in Minnesota. This year’s weather conditions seem to favor the pathogens, resulting in much more severe infections than previously seen.
Some varieties of tomato seem to be more sensitive to the bacterial pathogen than others. A few varieties, like Brandywine tomatoes, have been observed with larger leaf spots (up to 3/8th of an inch across). Infections with larger leaf spots could easily be confused with one of the common fungal leaf blight diseases. If gardener’s are unsure which pathogen they are dealing with, they should use the UMN Extension Online Diagnostic Tool ‘What’s wrong with my plant?’ before making any management decisions.
On the tomato fruit, bacterial speck infections are small (the size of a pencil tip) raised black spots. These spots start on green fruit and may develop a green halo as the fruit turns red. In contrast, bacterial spot fruit infections are larger (the size of the eraser end of the pencil) dark brown to black, raised and often corky appearing. Bacterial spot infections may be surrounded by a white halo. In both cases, the infection on the tomato fruits remains fairly superficial and does not result in fruit rot.
Both bacterial speck and spot can come into the garden on infected seed or transplants. The bacteria are then easily transferred from plant to plant through splashing water, strong winds or on a gardener’s hands and tools. The bacteria survive Minnesota’s harsh winters in plant debris.
Luckily in most cases infection with bacterial speck and spot do not result in significant yield loss. Although the fruit with raised corky bacterial spots would not be considered marketable at the grocery store, smart gardeners know the fruit can still be enjoyed once the superficial spots have been cut away. Infected tomatoes should not be used for canning, however, because the disease may have changed the pH of the fruit.
To reduce the spread of the disease, gardeners should avoid working in tomato plants when the leaves are wet. Under moist conditions the bacteria reproduce and easily stick to a gardener’s hands and tools. Waiting until plants are dry for chores like staking, pruning and weeding will reduce the spread of the bacteria. In addition, providing good air movement around the plants by staking or caging tomatoes, pulling weeds, and spacing plants far apart will allow leaves to dry quickly. Allow 2 years to pass before planting tomatoes or peppers in the same location. Do not save seed from infected plants.