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Bacterial Leaf Spots

March 19, 2012

Many growers experience leaf spots in the late spring and throughout the summer.  In most instance, the leaf spots are caused by various fungal pathogens; however, in many instances the spotting is caused by bacterial agents, namely Pseudomonas and Xanthomonas.  There are numerous fungicides (including Pageant) that have been proven to be very effective at controlling leaf spot diseases, however most fungicides provide little to no suppression or control of bacterial diseases.

It is often challenging to properly identify bacterial leaf spots as many of the symptoms they express are similar to fungal pathogens. Bacterial diseases develop very quickly and can cause significant crop injury.

Leaf spots caused by Pseudomonas usually appear as small irregular dark brown to black spots at the margins of the leaves, but may also be observed across the entire leaf surface.  These spots appear soft when the leaves are wet and sunken and brittle while the leaves are dry.

Xanthomonas develop leaf spots that initially appear as yellow or tan spots across the leaf surfaces.  As the disease progresses, these spots turn brown and appear angular and are typically confined between leaf veins.  The lesions are typically 1/8-inch wide with irregularly raised edges. Bacterial leaf spots are often confused with the symptoms caused by foliar nematodes and downy mildew.

Under severe infections, growers may observe distortion of the new leaves, complete chlorosis, and abscission of the older leaves.  In some circumstances, the bacterium can move from the leaf through the petiole and into the stem and develop a stem canker. When the flowers become infected, the sepals turn brown to black and often, several inches of the pedicel will die.  

The development of bacterial leaf spots is favored by moisture, high humidity, and high temperatures (60 to 85° F).  There must be free moisture on the leaf surfaces in order for bacterial leaf spots to develop.  Daily rains during warm weather or applying overhead irrigation (particularly late in the day) can lead to the development and spread of bacterial problems.  Bacteria are spread from plant to plant by splashing water.

Controlling bacterial diseases is extremely difficult and it is best to manage them on a preventative basis. When possible use clean, disease free starting materials, provide proper plant spacing, implement sound sanitation practices, do not handle plants when they are wet, and avoid overhead irrigation during times when the foliage will not dry quickly after application.  Using drip irrigation or sub-irrigation systems to irrigate susceptible crops will greatly reduce the occurrence and spreading of bacterial leaf spots.

Once bacterial spots are present, carefully rogue infected plants out of the production area to prevent bacteria from spreading to uninfected plants nearby.  Unfortunately, there is no curative control for bacterial diseases.  Bactericides (copper compounds and streptomycin) will help protect against bacterial disorders preventatively, but offers limited effectiveness once outbreaks have developed.  Use bactericides when the environmental conditions are conducive for disease development.

Paul Pilon
Perennial Solutions Consulting
paul@perennialsolutions.com

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