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 III. Control of Bacterial Spot

 R. E. Stall and P. L. Thayer2/


 In order to obtain maximum benefit from a virus control program, plants
should be protected as much as possible from other diseases. One of the most
important of these is bacterial spot (caused by Xanthomonas vesicatoria (Doidge)
Dows). Therefore, it might be well to review what we know about bacterial spot
and what can be done to control it.


 Source and Spread of the Bacterium

 Bacterial spot is most commonly introduced into the field by diseased seed.
In certain areas of the country there is evidence that the bacteria can over-
season on plant refuse, however, there has been no proof of this occurring in
Florida. The number of diseased seed in a lot may be very small, but under the
proper conditions the bacteria can spread from one plant over the entire field.

 For spread to occur there must be water present to act as a carrier of the
bacteria, and there must be some method of transporting the water. Wind blown
rain may serve in this capacity. Another important carrier is farm implements
brushing from one dew or rain covered plant to another.

 Once the bacterium has been deposited on a healthy plant it moves to the
inside of the leaf through tiny openings in the leaf surface called stomata or
through openings in the edge of the leaf called hydrathodes. Inside the leaf
the bacteria grow and increase in number for 5 to 7 days before there is any
outside indication of the disease. Bacterial build-up inside the leaf results
in death of leaf cells and consequently we see a spot of dead tissue called a
lesion. As the bacteria continue to increase in the leaf a viscous material is
secreted around the bacterial cells and some of them are forced to the surface
of the leaf. This is evidenced visually by the greasy appearance of the spot
lesion. It is at this stage of development that a drop of water falling on the
lesion can spatter bacteria to the next plant or wind driven rain and farm imple-
ments moving through wet plants can carry bacteria over most of the field.

 Once the bacterial spot pathogen is established in a field its seriousness
 depends to a large extent on the weather. A thunder shower accompanied by heavy
 winds can spread the spot pathogen to almost every plant in a field. Periods
 of rainy, overcast days tend to favor spread and development of the disease. On
 the other hand periods of dry sunny weather and light dews tend to suppress
 disease development.


 Methods of Control

 Obviously the time to begin control of bacterial spot is with disease free
 seed. California seed should be free of the bacterial spot pathogen, however,
 if this seed cannot be obtained, seed treatment with bichloride of mercury will
 eradicate the bacteria. This chemical can be purchased in the form of 7.3 grain
 blue tablets or as a white powder. The use of tablets is convenient, since one
 tablet dissolved in one quart of water gives the desired 1 to 2000 dilution. If


 5/ Assistant Plant Pathologists, Indian River Field Laboratory, Ft. Pierce, and
 Everglades Experiment Station, Belle Glade, respectively.