Take Control Of Bacterial Spot And Spotted Wilt In Peppers

Bacterial spot symptoms appear as small, circular lesions on the leaves and fruit. Photo credit: Dan Egel

Bacterial spot symptoms appear as small,
circular lesions on the leaves and fruit.
Photo credit: Dan Egel

Both bacterial spot and spotted wilt can be devastating to pepper crops and have been known to cause severe economic losses if control measures are not applied in time.
While bacterial spot, caused by the bacterium Xanthomonas campestris pv. Vesicatoria, tends to thrive in warm and humid climates, spotted wilt is vectored by several different species of thrips, and is found widely in the Southeast U.S. as well as in greenhouses across the country.

Dan Egel, Associate Professor at the Department of Botany and Plant Pathology at Purdue University, offers preventive control measures for both of these diseases so you can tackle them in time and save your crops from potential losses.

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Bacterial Spot Symptoms
On leaves, symptoms of bacterial spot appear as lesions, which are small, yellow-green in color, circular in shape, surrounded by a yellowish halo. According to Egel, the lesions can appear water-soaked around the edges under wet conditions. As the lesions mature, the yellow extends beyond the area of the lesions, and the centers turn brownish-black in color and appear sunken.

On fruit, the lesions tend to be raised like pimples and are green, circular, approximately 1/8th of an inch in diameter, and eventually turn brown, he says.

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Ideal Conditions
Bacterial spot has been known to overwinter in crop residue, soil, on seeds, and on other wild host plants. The pathogen thrives in warm, rainy, wet, and humid conditions. Egel mentions that it is exacerbated if it rains frequently and if prolonged periods of leaf wetness are experienced.

He adds that a hard rain might allow water droplets to infiltrate the leaf surface permitting bacteria to enter the leaf, as well as allowing bacteria to spread more easily from leaf to leaf, and from plant to plant.

For this reason, Egel suggests avoiding any overhead irrigation as it might cause spread of the disease, and suggests drip irrigation as an alternative. However, if you must use overhead irrigation, he says don’t do it at night to avoid prolonged leaf wetness.

Take Precautions
Starting out with clean transplants is key, Egel says. “If you grow your own transplants, you want to make sure where you grow them is clean and sanitized between generations, and you want to monitor those seedlings for symptoms of disease as they come up. If you are suspicious of any contamination, you want to make sure to get an official diagnosis,” he explains.

If you purchase transplants, he suggests inspecting them closely when you receive them and to make sure you’re purchasing from a company that has an adequate testing procedure.

He also mentions the availability of resistant varieties, but says because there are a number of races, to be sure you are covering all that are likely to be present in your region. Furthermore, he says to choose your varieties wisely, as some resistant varieties vary in quality and yield.

“Buy varieties that you can sell, and you want to try to get as many resistance genes in there as you can and still be able to sell and market those peppers,” Egel says.

Chemical Control
To manage bacterial spot chemically, Egel suggests using a copper-based product such as a copper sulfate or a copper hydroxide and a mancozeb product.

He suggests mixing both together and applying the mixture to the plants soon after transplanting before any symptoms appear and on a weekly basis.

“The chemicals (combined) will act as a shield. They tend to protect the leaves and the fruit, but they don’t go into the plant in any curative way, so you have to get those on before the disease starts,” he says.
He cautions, however, to get an official diagnosis before any conclusions are drawn.

Spotted Wilt Symptoms
Moving onto spotted wilt, Egel says the first symptoms of the disease appear as a ring spot on the fruit along with distorted foliage. One of the earliest symptoms is a bronzed appearance on infected leaves along with a general wilting of the plant. However, the symptoms might vary widely depending on the stage of plant growth at the time of infection.

When the plant is infected at a later stage of growth, it’s possible that only a part of the plant might become infected because the virus does not move easily to mature parts of the plant. At later stages, the leaves curl and are pale green to yellow in color. Fruit are bumpy and display spots and necrosis.

Monitoring For Thrips
Cornell University research indicates western flower thrips, tobacco thrips, onion thrips, and T. setosus are the most important vectors of spotted wilt because of their extensive distribution and wide range of hosts.
“If you see thrips, you need to start monitoring immediately and follow recommendations for whatever state you’re in for controlling them,” Egel says.

Egel also notes greenhouses provide an ideal environment for reproduction.

“It’s warm in [greenhouses], and it’s easier for them to overwinter. Rain might tend to wash them off, so any type of protected environment is more likely to favor thrips than field conditions,” he says.

He also has noticed a higher incidence of spotted wilt in greenhouses where ornamentals are grown alongside peppers or tomatoes and suggests observing nearby plants to see if thrips have taken a liking to those plants. It also might be possible to eliminate plants in the portion of the greenhouse that was infected so the thrips don’t spread, he notes.

Thrips populations do have a wide distribution in non-greenhouse environments, and the virus can be transmitted to the field from infected seedlings from greenhouse-grown transplants.

Egel also explains that if you have winter wheat or a similar crop, thrips might spread after harvesting to feed on nearby fields.

“They don’t have any place to go and sometimes they’ll end up in an adjacent field. Either avoid cutting down, or at the very least monitor very closely after the cutting to figure out what’s happening. Always be aware of other crops growing next to yours.”

Control Measures
Similarly to bacterial spot, the first step in controlling spotted wilt is making sure you’re working with virus-free and thrips-free transplants from the start. Closely monitoring all incoming transplants for signs of infection is the way to do this, Egel says.

“Also, be sure to get rid of any plants that have the disease. Burn or bury them and get them far away from the production area. Avoid planting next to a field that has traditionally had a problem with thrips,” he says.

Using varieties resistant to spotted wilt is another control strategy, and Egel notes observing which varieties thrips prefer and staying away from those is another valid strategy.

Foliar insecticidal treatments also may be recommended for suppression throughout the season, and it’s suggested to rotate the class of insecticide used to ensure no resistance is built.

Above all, Egel says to talk to an entomologist first.

“If you start applying chemicals for thrips and they aren’t there, you might end up killing beneficial insects,” he says.

Because there is no current cure for the virus itself, proper monitoring and control of thrips is crucial to keep spotted wilt at bay.

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