Extension Master GardenerSM Volunteers of Buncombe County

blight

Southern Blight: Large Variety of Plants Vulnerable to This Disease

With summer coming, so are plant diseases. One that can affect an astonishing variety of plants—from apple trees to zinnias—is Southern blight, caused by the fungus Sclerotium rolfsii.

Southern Stem Blight_Sclerotium rolfsii_leaf wilt on pepper plant; courtesy of Chatham County NC State Extension
Leaf wilt on pepper plant, a sign of Southern blight

What to watch for
Southern blight thrives in hot, humid conditions. Although the fungus can infect many plant parts—including roots and fruits—it typically shows itself as wilt resulting from rotting stems at the base of the plant. Even before the plant wilts, you’ll often see water-soaked spots on the stems.

Common casualties
Some plants are particularly vulnerable. Hostas may succumb in your perennial garden. Many root crops are affected, including beets, carrots, Jerusalem artichokes, parsnips, radishes, sweet potatoes, and turnips. Other victims may be cantaloupe, eggplant, peppers, snap beans, sweet corn, and tomatoes.

SouthernBlight_Hosta_moccasinlanding_CC BY-NC-ND 2.0_Flickr
Southern blight on hosta

How it survives
One reason Southern blight can persist in soils is that it forms sclerotia: hard, rounded bodies that look like mustard or radish seeds. The disease is sometimes called Sclerotial blight for this reason. It can also remain in plant residue in the soil and spread through surface water.

What to do
As with all fungal diseases, prevention is key. Once the plants are infected, fungicides may help prevent the spread of the disease to other plants, but won’t reverse problems on plants already affected. Depending on the crop, fungicides used as soil drenches prior to planting may prevent infection. Different fungicides are recommended for different crops, however.

Sclerotia or hard nodules on pepper plant stem with Southern blight, Sclerotium rolfsii; courtesy of Chatham County NC State Extension
Sclerotia or hard nodules on pepper plant stem with Southern blight

Immediately remove infected plants along with their roots and the surrounding soil. Do not compost! If container plants are affected, do not reuse the potting soil. If Southern blight takes hold in your garden, solarizing for several weeks using plastic during the summer may kill the fungus.

For more information about recommended fungicides:
https://content.ces.ncsu.edu/north-carolina-agricultural-chemicals-manual/disease-control
(See page 498 for Southern stem blight, Sclerotium rolfsii)

Factsheet from the American Phytopathological Society:
http://www.apsnet.org/edcenter/intropp/lessons/fungi/Basidiomycetes/Pages/SouthernBlight.aspx

Article written by Debbie Green, Extension Master GardenerSM Volunteer.

Blight-Resistant Tomatoes: WNC Leads the Way in Research

There are two words that elicit a deep sigh and considerable head-shaking from backyard and commercial vegetable gardeners: tomato blight. 

A costly and deadly pathogen
Of the tomato blights, the deadliest is late blight caused by Phytophthora infestans, which infects some members of the Solanaceae plant family, especially tomatoes and potatoes. Although this pathogen superficially resembles a fungus, it is an oomycete that is more closely related to brown algae. This is the culprit that produced the Irish potato famine in the 1840s, so to say it’s a problem is an understatement!

For the home gardener, late blight is the destroyer of a beloved summer treat—a freshly picked, vine-ripened tomato, bursting with flavor and warm from the sun. The pathogen overwinters in infected potato tubers. Its spores are carried by the wind, which means that it spreads easily. Once it starts, it is next to impossible to stop.

Conditions favorable to blight
Don’t be fooled by its name. Late blight isn’t limited to later in the growing season. It can develop anytime between early and mid-season, depending on weather conditions, and it is a threat wherever tomatoes are grown.

The very things that tomatoes need to grow create conditions favorable for Phytophthora infestans to take hold—plenty of moisture (rain, dew, irrigation, and fog) and temperatures between 64 and 71°F. Under the right conditions, the disease lifecycle can occur in as little as five days and can spread quickly any time humidity is over 80 percent for two days or more. There are many days when conditions are just right in WNC for tomato blight! It is for this reason that NC Cooperative Extension tracks and reports where the disease is appearing and how quickly it is spreading from farm to farm and county to county. You can also follow the disease each year through the USA Blight website (https://usablight.org/).

Prevention and control strategies
Phytophthora infestans is a formidable adversary for any gardener and requires multiple strategies to combat it. Prevention and control are particularly challenging for organic gardeners and farmers. Choosing the right location for tomatoes can help. Full sun, good air movement, reduced leaf wetness, and well-drained soil all help plants stave off the blight for as long as possible. Chemical products may slow down the spread of blight, but need to be applied frequently and on ALL plant surfaces with every application. It’s a lot of work.

WNC, a leader in tomato research and development
There is some good news on the horizon for tomato growers worldwide! Unlike infected potato tubers, tomato seeds do not transmit the blight pathogen. This makes it possible for researchers at Western North Carolina’s Mountain Horticultural Crops Research and Extension Center in Mills River, Henderson County, to develop tomato varieties resistant to Phytophthora infestans and many other tomato diseases.

Dr. Dilip Panthee, Associate Professor of Horticulture at North Carolina State University (NCSU), leads this research. Developing disease-resistant, higher-yielding plants for North Carolina’s $30-million-a-year tomato industry will reduce the global economic impact of diseases like Phytophthora infestans.

It was Panthee’s work on tomato breeding in his native Nepal that grabbed the attention of NCSU. “You’ll find tomatoes growing on every continent except Antarctica. We tend to associate them with Mediterranean food, but tomatoes are a favorite ingredient world-wide,” Panthee said in an April 2015 interview. “The varieties we are creating are important for two reasons. First, we are reducing the need for farmers to use pesticides, and we know that is good for the environment. Second, we are creating fruits with better food quality, both in flavor and appearance. That means greater availability of nutritious fresh tomatoes, particularly in developing countries.”

Disease-resistant tomato varieties
Working with NCSU professor emeritus Randy Gardner (the man behind ‘Mountain Pride’ and many more tomato varieties), Panthee developed ‘Mountain Merit,’ a high-yielding, fresh-market cultivar that is resistant to three tomato plant problems—late blight, tomato spotted wilt virus, and root-knot nematodes. Panthee also developed ‘Mountain Majesty,’ a large tomato with improved fruit color and resistance to tomato spotted wilt virus. Both are commercially available.

Panthee’s latest creation is ‘Mountain Rouge,’ a pink heirloom-type hybrid that is also resistant to early and late blight, as well as root-knot nematodes. “The flavor of this tomato is excellent,” Panthee says. “And its heirloom appearance makes it very appealing to commercial growers and backyard gardeners.”

Because the Mountain tomato series (‘Mountain Pride,’ ‘Mountain Rouge,’ ‘Mountain Merit,’ and ‘Mountain Majesty’) was developed here in Buncombe County, it is well-suited to local growing conditions. You’ll find one or more of the cultivars in the series throughout the planting season at area garden centers.

Learn more
To learn more about the important tomato breeding work being done at the Western North Carolina’s Mountain Horticultural Crops Research and Extension Center in Mills River, go to http://mountainhort.ncsu.edu/fletcher/programs/tomato/. Better yet, plan a visit in late summer 2017 for Tomato Field Day, talk to the experts, and taste the fruits of their labors.

Article written by Janet Moore, Extension Master Gardener Volunteer.

Resources
http://www.apsnet.org/edcenter/intropp/lessons/fungi/oomycetes/pages/lateblight.aspx

Late Tomato Blight

The first symptoms of late blight on tomato leaves are irregularly shaped, water-soaked lesions.
The first symptoms of late blight on tomato leaves are irregularly shaped, water-soaked lesions.

Tomatoes should be reaching their prime, and if you have lush green plants with ripening fruit, enjoy your harvest.  If your plants aren’t looking so good, however, they may have one of the many tomato diseases that sometimes thwart even the most experienced vegetable gardeners. One of the most serious of these is late tomato blight, which typically affects plants later in the growing season, but has been confirmed on plants in West Asheville as of early July this year.  This disease can spread spores as far as 20 miles, so identifying the disease and destroying the affected plants may spare others’ tomatoes from developing the symptoms.

Rain and high humidity are both favorable conditions for the spread of late blight. Once your plants are infected, they can’t be cured; fungicides containing copper or chlorothalonil are currently the only treatments proven effective to prevent late blight. If you spray your plants, follow label directions.  Take care to cover all leaf surfaces;  use eye protection and other precautions to prevent contact with the spray.

The good news is there are less devastating diseases that are not late blight, so getting a positive diagnosis is important.  The first signs are irregular dark spots that look water-soaked on the newer leaves at the top of the plant, often with a lighter-colored “halo” around them.   As the spots enlarge, the leaves shrivel and die. You may also see white cottony growth on the underside of the leaves.  Both ripening and green fruits are also affected, with greasy looking spots that turn brown and leathery.

During humid conditions, white cottony growth of P. infestans may be visible on the underside of affected leaves.
During humid conditions, white cottony growth of P. infestans may be visible on the underside of affected leaves.

There are many images available online to help you determine if you have late blight; one of the most comprehensive series of photos is this one prepared by Dr. Meg McGrath of the Long Island Horticultural Research and Extension Center: http://www.longislandhort.cornell.edu/vegpath/photos/lateblight_tomato.htm#images.   If you are unsure about the diagnosis, you can bring photos and samples of your plants to the Master Gardener Clinic at the Buncombe County Extension Office.

If you do have late blight, bag up your plant as soon as you can, but do it on a sunny day to reduce further spread of the disease.  You can salvage already ripened fruit or green tomatoes that don’t show symptoms;  these are safe to eat.  Take care to look for affected plants other than tomatoes, especially potatoes and petunias.  Weeds in the nightshade family may also show symptoms and should be removed and bagged.

Infected fruit are typically firm with spots that eventually become leathery and chocolate brown in color.
Infected fruit are typically firm with spots that eventually become leathery and chocolate brown in color.

If you lose your plants, there is always next year. There is ongoing research on less susceptible varieties and some evidence that ‘Defiant,’ ‘Mountain Merit,’ ‘Mountain Magic,’ and ‘Plum Regal’ tomato varieties are resistant to late blight.   Consider planting these varieties next season if you want to increase your chances of a blight-free crop.

 

 

See the North Carolina State University fact sheet for more details: http://www.ces.ncsu.edu/wp-content/uploads/2013/06/Tomato_late_blight_ki.pdf

Article written by Debbie Green, Extension Master Gardener Volunteer.

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