Be on the look-out! Garden diseases to watch for in May and June

With summer swiftly approaching, we are seeing quite a few more diseases out and about in the landscape and in the home garden. We wanted to take a few minutes and go over some common diseases you should look out for in your own vegetable garden. 

Tomato Spotted Wilt Virus 

A few days ago we received the second TSWV sample of the season on greenhouse tomato. TSWV is a virus spread by at least 7 different kinds of insects called thrips. TSWV has a very broad host range that includes a variety of ornamental plants, along with tomatoes, peppers, tobacco, and peanuts. Early symptoms of this disease include cupping or bronzing of the foliage and pale ring-spots/ mottling on the fruit. As the disease progresses, necrotic spots/lesions can be seen on the foliage, stems, petioles, and fruit. Infected plants are usually severely stunted and new growth is often deformed. Infected plants will not recover and should be removed from the garden. Detailed information can be found here: 

Early TSWV symptoms on tomato (Photo: E.C. Lookabaugh)

Ringspot symptoms on pepper (Photo: E.C. Lookabaugh)

Symptoms on tobacco (Photo: E.C. Lookabaugh)

Fruit Symptoms (Photo: F.J. Louws)

Septoria Leaf Spot of Tomato 

Septoria leaf spot is a very destructive fungal disease of caused by the fungus Septoria lycopersici. Usually this disease shows up on the lower foliage after the first fruit set. Necrotic spots can be seen on the foliage. As the spots age, the centers turn gray and tiny black dots (fruiting bodies of the fungus) can be seen in the center of the spots. The fungus survives the winter on infected tomato debris or nearby weeds. Controlling this disease in the home garden can best be achieved by removing all crop debris at the end of the growing season or by tilling it under the soil. In commercial situations, control can be achieved through the use of resistant cultivars and fungicides. 

Septoria foliar symptoms (Photo: E. C. Lookabaugh)

Southern Blight 

Southern blight is a serious and frequent disease in the Piedmont and Coastal Plain regions of North Carolina. This disease is caused by the fungus Sclerotium rolfsii, which attacks many vegetable crops including tomato, bean, cantaloupe, carrot, pepper, potato, sweetpotato, watermelon, and several field crops such as peanut, soybean, and tobacco. This disease is easily recognized by the white fan-shaped growth of the fungus at the base of the plants. Over time, tiny round tan to brown sclerotia are formed on soil and infected plants. These sclerotia can survive in the soil for MANY years. Rotation is not very effective because this pathogen has more than 1,000 reported hosts. Corn and some other members of the grass family are not hosts and are safe to plant in problem areas. In gardens, planting on a raised bed filled with sterile soil is the best way to avoid contact with native soil that may contain the pathogen. The disease is more active in warm, wet weather and can be seen every year in North Carolina. Watch for a more comprehensive post on this disease coming soon! 

Southern Blight (Photo: Kurt Taylor)

Southern Blight up close (Photo: F.J. Louws)

Southern Bacterial Wilt 

Here in North Carolina, southern bacterial wilt is one of the most commonly diagnosed diseases of tomatoes in the home gardens and production fields. This disease is found throughout the Piedmont and Coastal Plain regions of the state. It is caused by the soilborne bacterial pathogen Ralstonia solanacearum, and is most commonly found on tomatoes, peppers, potatoes, tobacco and other members of the nightshade family. Brown discoloration of the vascular tissue in stems and leaves is a distinctive symptom of bacterial wilt. The discoloration is caused by bacteria colonizing the plant’s vascular tissue, plugging it up. The plant loses its ability to conduct water, which results in yellowing and wilting, especially during the hottest part of the day. Infected plants quickly collapse and die. Diseased plants should be removed and susceptible species should not be planted back into infested areas. More detailed information on this disease can be found here: 

Complete collapse caused by bacterial wilt (Photo: F.J. Louws)

Vascular discoloration (Photo: M.J. Munster)

Root-knot Nematodes 

Root-knot nematodes (Meloidogyne sp.) attack a wide variety of vegetable and field crops, including tomato, peanuts, spinach, carrots, and many others. Usually root-knot nematodes are more common in eastern North Carolina where we have more sandy soils. Symptoms are worse is hot, dry summers. The most obvious symptoms are galls and swellings on the roots and stunting and yellowing of the above-ground portion of the plants. Control can be achieved through the use of resistant cultivars and crop rotation.

Root-knot nematodes on tomato, notice galls on roots (Photo: F.J. Louws)

Female nematodes under dissecting scope (Photo: F.J. Louws)

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:

http://www4.ncsu.edu/~clrivard/TubeGraftingTechnique.pdf

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!