Bacterial Speck of Tomato

Published: June 1, 2012

Since a number of cases of bacterial speck have been confirmed in Missouri this spring it would be good to review the management of this troublesome disease of tomato. Bacterial speck along with bacterial spot are diseases with the potential to cause significant crop loss. Although caused by different microbes, bacterial speck and bacterial spot are nearly identical in their epidemiology, symptoms and management. This article will address bacterial speck which seems to the more prevalent of the two in Missouri this spring.

Bacterial speck is caused by the bacterium Pseudomonas syringae pv. tomato. Symptoms of the disease appears as dark brown to black lesions of various sizes and shapes on leaves, fruit, and stems. Chlorotic (yellowish) tissue surrounding leaf lesions is common. Lesions frequently are observed near leaf margins, causing (eventually) marginal necrosis . Lesions on immature fruit are slightly raised and small, varying in size from tiny flecks that develop into raised black spots on mature fruit. The fact that lesions appear on tomato fruit adds to the disease’s potential to cause economic loss. Although seldom more than a few cells deep, fruit lesions often render the fruit unsalable.

The pathogen responsible for bacterial speck thrives in cool, moist environmental conditions. This is a primary reason why the disease usually is most severe on early plantings. The disease will develop rapidly at 75°F. However, it can develop at temperatures as cool as 63°F. At 89°F pathogen populations do not develop to the point that typical symptoms are evident. The pathogen is spread via contaminated seed, mechanically (e.g. working with plants, especially when the foliage is wet) and by "driving" rains.

There is a direct correlation between the amount of time tomato leaves remain wet and the likelihood that populations of the pathogen will build to levels sufficient for the production of visible symptoms. If leaves remain wet for at least six hours bacterial speck development is promoted in most instances when temperatures are conducive for infection.

Once established in a planting, the control of bacterial speck is difficult. Therefore, an IPM approach to the management of the disease is recommended. This begins with the use of disease-free transplants as a method of preventing the disease. Growers producing their own transplants should be certain seeds they use are known to be free of pathogenic bacteria. Additional methods of cultural control include crop rotation and strict sanitation. Rotate tomato plantings with crops other than tomato and pepper to avoid carryover of bacteria from year to year. The control of weeds also is important in areas to be planted to tomatoes the following year. Horse nettle and other weeds that are members of the Solanaceae (Nightshade) family can act as hosts of the pathogen that causes bacterial speck. Always remove and destroy diseased plant debris or incorporate it into soil soon after harvest is completed.

It is important to note that this disease is caused by a bacterium and not a fungus. Unfortunately, there are very few pesticides that are effective against bacterial diseases. Fixed copper compounds (e.g. Kocide®) applied during cool, rainy weather at 10 to 14 day intervals is helpful. Remember, copper acts as a protectant and does not cure an established disease population. Therefore it must be applied before conditions become conducive for infection to be effective. The use of mancozeb products (e.g., Dithane®) as a tank mix with fixed copper compounds may allow more copper to become available on the leaf surface, thus making it more effective, especially in cases where resistance to copper has been observed.

Tanos®, a relatively new, broad-spectrum fungicide, has been shown to suppress bacterial speck while at the same time controlling other problematic foliar diseases of tomato. Additionally, Actigard® and Serenade® MAX are both labeled for bacterial speck suppression on tomato. It is speculated that Actigard® may help to lessen the symptoms of bacterial speck by inducing the plant to increase its resistance to disease. Serenade® MAX is a biological product with organic clearance derived from a strain of the bacterium Bacillus subtillus. It often used in conjunction with copper compounds in the battle against bacterial speck.

Finally, bacterial speck and spot are not inevitabilities in the production of tomatoes. They do, however, represent diseases that can result in significant loss should they occur and due vigilance is appropriate. Growers with a history of these diseases on their tomatoes especially must be watchful and take appropriate measures to control them.