Tuesday, July 19, 2011

My Tomato Plants Are Wilting... And Then They Die!

Tomato Wilting Symptoms (Photo by F.J Louws)
Every summer the Plant Disease and Insect Clinic receives samples from clients who tell us that their tomato plants “wilt and then die.”  This can be very discouraging for a home gardener, and devastating for a commercial grower who has a field of plants going down.  If you observe these symptoms, be prepared for some bad news!

In the piedmont and coastal plain regions of our state the most common cause of wilting and death in tomatoes is bacterial wilt.  The destructive pathogen Ralstonia solanacearum is able to wipe out home gardens and whole fields of tomatoes.  It also attacks many members of the nightshade family: tomatoes, peppers, potatoes, and eggplant, along with zinnias, geraniums, sunflowers, and marigolds.  Ralstonia causes Granville wilt in tobacco (also a relative of tomatoes), the subject of some of the earliest work on plant diseases at NC State. A bulletin published in 1903 warned growers that curing Granville wilt was “hopeless.”

The most obvious symptom of bacterial wilt is wilting, followed by sudden collapse of the entire plant.  Basically, bacteria plug up the water conducting tissue in the stem. The plant is unable to transport water from the soil to the leaves and the plant wilts and dies.  

Tomato Collapse (Photo by F.J. Louws)
Burley Tobacco Wilting (Photo by H.D. Shew)
Ralstonia can survive in most soils for years without a host present. When susceptible plants are planted in infested soil, it is only a matter of time before the plant becomes infected.   A heavy rain or watering can help bacteria move through soil to the roots of susceptible plants. A few large black roots are often found where the infection got started. This is especially evident in infected tobacco plants.   
Black Roots Where Infection Took Place
(Photo by H.D. Shew)
If you suspect bacterial wilt, there are quick and easy diagnostic checks you can do yourself.  The first thing you need to do is check for vascular browning inside the stem.  Tomato stem tissue should be very light green.  The easiest way to check for vascular discoloration is to cut the stem, as shown in the image below. 

Tomato Stem: Note Brown Discoloration
(Photo by M.J. Munster)
Vascular Browning (Photo by E.C. Lookabaugh)
Next, check for bacterial ooze. The easiest method is to cut the stem off at the base.  Take the stem cutting and hang it over a glass of water (shown below).  After a few minutes, you should be able to observe a cloudy stream of bacteria oozing from the cut tissue.  It is very important not to disturb the glass of water during the test.  

Bacterial Stream Test (Photo by M.J. Munster)
Note White Bacterial Streaming (Photo by M.J. Munster)

Sometimes, it can be hard to see the bacteria. If you have access to a microscope, take a thin slice of brown vascular tissue and place it in a drop of water on a glass microscope slide.  Make 2 straight cuts across the tissue (cutting the tissue into 3 pieces).  Put a cover slip on top and look through the microscope using the 10x objective.  Within seconds, you should be able to see dark brown plumes of bacteria oozing from the slices.  

Slice Vascular Tissue on Glass Slide
(Photo by E.C. Lookabaugh)

Micrograph of Bacterial Ooze
(Photo by E.C. Lookabaugh)
If you don’t see any bacteria, you probably don’t have bacterial wilt. Fusarium wilt can cause vascular browning and is common in the Piedmont. Most modern varieties of tomatoes are resistant to Fusarium wilt, but we see it often in heirloom varieties. Southern blight or high soil salts are other potential causes of browning. Another wilt disease, Verticillium wilt, is present in many mountain areas, and has other distinctive symptoms.

Arrow Pointing to Bacterial Ooze on Tobacco Stem
(Photo H.D. Shew)
Arrow Pointing to Bacterial Ooze on Tomato Stem
(Photo by E.C. Lookabaugh)
Much as in 1903, if your plants have bacterial wilt, there is nothing you can do to save them. Infected plants will not recover and should be removed and destroyed. Replanting with new plants will not solve the problem because they will likely become infected and die too. The bacteria can persist in the soil for years, so it is best not to plant susceptible plants back into that area. Rotation with non-host plants, for example, sweet corn, for at least three years is recommended. 

Growing tomatoes in containers with potting soil is one option if your garden is infested. Be sure that the containers are not in contact with the native soil and be careful not to spread infested soil to new locations. 

For commercial growers, some soil treatments and the use of resistant rootstocks and grafting can be an option for the following year.  More information on root grafting can be found here:

For a look at the history of the Department of Plant Pathology and Granville wilt in North Carolina agriculture, see http://www.cals.ncsu.edu/plantpath/FromLabtoField.pdf

Special thanks to Dr. David Shew for helping with this posting!