Potato Southern Blight Control – Managing Southern Blight On Potatoes

By: Mary Ellen Ellis

Potato plants with southern blight can be quickly destroyed by this disease. Watch for early signs and create the right conditions for preventing southern blight and minimizing the damage it causes to your potato crop.

About Southern Blight of Potatoes

Southern blight is a fungal infection that can affect multiple types of vegetables but which is commonly seen in potatoes. The fungus that causes the infection is called Sclerotium rolfsii. This fungus lives in the soil in masses called sclerotia. If there is a host plant nearby and conditions are right, the fungus will germinate and spread.

Signs of Potato Southern Blight

Because the fungus survives as sclerotia in the soil, it begins to infest plants right at the soil line. You may not notice this right away, but if you are concerned about the infection, check the stems and the tops of roots of your potato plants regularly.

The infection will begin with white growth at the soil line that turns brown later. You may also see the small, seed-like sclerotia. As the infection surrounds the stem, the plant will decline rapidly, as the leaves yellow and wilt.

Managing and Treating Southern Blight on Potatoes

The right conditions for southern blight to develop on potatoes are hot temperatures and after a rain. Be on the lookout for the fungus after the first rain that comes down following a hot period of weather. You can take steps to prevent the infection by keeping the area around the stems and soil line of your potato plants clear of debris and by planting them in a raised bed.

To prevent an infection from coming back the next year, you can till the soil under, but be sure to do it deeply. The sclerotia will not survive without oxygen, but they need to be well buried under the soil to be destroyed. If you can grow something else in that part of the garden that is not susceptible to southern blight the following year, this will also help.

Fungicides may also help reduce losses from an infection. In severe cases, especially in commercial farming, the fungus spreads so quickly that the soil has to be fumigated with fungicides.

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How to Rescue Potatoes From Potato Blight

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Potato blight, a fungal disease caused by the pathogens Phytophthora infestans and Alternaria solani, results in shriveled, brown leaves and lesions on the plant stems. In its advanced stages, the disease spreads to the tubers. Tubers develop a brownish-red decay that starts growing beneath the skin. The decay soon causes the potatoes to rot. Blight typically develops during times of high humidity and when temperatures are between 55 and 85 degrees Fahrenheit. Taking precautions can help prevent potato blight, but if your plants are already infected, it is possible to treat the disease and save some of the harvest.

Causes of the disease

To get a good potato crop, it is necessary to study the characteristics of the plant disease. Thanks to this information, you will know exactly how to get rid of late blight on potatoes. The causative agent of this disease is the pathogenic fungus of the same name, which belongs to the category of simple fungi - oomycetes.

Late blight is a dangerous disease for plants belonging to the genus nightshade: peppers, tomatoes, and eggplant. This disease also affects buckwheat, raspberries, strawberries and castor oil.

The spread of the disease occurs through phytospores. They are washed off from the terrestrial part of the plant. Through the soil capillaries, the fungus enters the tubers and stem.

Texas Plant Disease Handbook

Solanum tuberosum

Bacterial Wilt or Brown Rot (bacterium – Pseudomonas solanacearum): Stems discolor (at first only on inside) and plants wilt and die. Tubers with a dark, vascular ring may decay. Avoid infested fields and plant seed from northern states. Discard tubers with dark eyes or with sticky ooze on surface.

Blackleg (bacterium – Erwinia carotovora subsp. atroseptica): The lower parts of affected stems develop an inky black , slimy , foul-smelling , soft rot. Leaves near tops of vines curl upward and become stunted. Aerial tubers usually form in leaf axils. Severely affected plants die. Tubers from affected plants show black stem-end discoloration and sometimes a severe black soft rot in storage. Plant certified, healthy seed in well-drained soil. Cut seed tubers with a disinfected knife. Treat seedpieces and cure properly before planting.

Early Blight (fungus – Alternaria solani): Mycelium and spores of Atlernaria survive in crop residue. Frequent rain and heavy dews are needed for heavy infection. First symptoms are oval to angular dark-brown to black target spots on the leaves (See Photo). Spots often have a yellow area around them. The lowest or oldest leaves are infected first. Leaves are often killed and yields are reduced. Tuber infection may occur with brown-black sunken spots, from to inch in diameter. The underlying tissue shows a brown, corky, dry rot more than inch deep. Control with 2 year rotation, good fertility and foliar fungicides [see table below].

Early and Late Blight Fungicides:
Chlorothalonil Chlorothalonil + sulfur
Copper hydroxide Mancozeb
Metalaxyl + chlorothalonil Metalaxyl + mancozeb (late blight only)

Late Blight (fungus – Phytophthora infestans): Water-soaked spots enlarge rapidly and turn brown to black. Very humid conditions may produce white mold on the underside of the leaf. Under cool, wet conditions blight may attack petioles and stems, ruining a field in a few days. Blight spores can infect tubers at harvest or while in the ground. Late blight has been verified in the Winter Garden, but has not been reported on the High Plains. Some of the isolates were shown to be metalaxyl resistant. Use fungicide applications [see table above] and delay harvest about 2 weeks until vines are killed by chemical treatment.

Pink Rot of Tubers (fungi – Phytophthora parasitica and P. cryptogea): Diseased tubers are spongy and initially discolored around the point of stolon attachment. Later, they become discolored around the buds and lenticels. The internal tissues appear cream-colored when cut, but turn salmon pink after 15 – 20 minutes. They gradually become darker, turning black after about 1 hour. Diseased potatoes have been observed in the Texas High Plains primarily in furrow irrigated fields with clay soils. Control includes selecting sites with good soil drainage and avoiding excessive irrigation late in the growing season.

Ring Rot (bacterium – Corynebacterium sepedonicum): Resembles brown rot, but stem is not discolored. Use certified seed from areas where ring rot is not permitted.

Scab (fungus – Streptomyces scabies): Rough, corky areas that may be round or irregular occur on tubers. Injury does not extend far into the potato but the appearance is objectional. Scab is less prevalent in acid soils than in alkaline. Avoid alkaline soil amendments such as manure, lime, or ashes. Avoid severely infested fields. Apply heavy irrigation at the time of tuber or early root formation. Use a 4 to 6 year rotation under irrigation. A 3 to 4 year rotation is satisfactory under dry land conditions. Use certified seed of scab resistant varieties. Treat seedpieces (See in the chapter on “Seed Treatment“). Wire worm damage, commonly called “deep scab”, is not a disease. It differs in that the pits are deep and tissue has been removed by insect feeding. Sulfur may be used in home gardens to make the soil more acid.

Scurf and Stem Rot (fungus – Rhizoctonia sp.): Small, hard black bodies adhere to the surface of the tuber. Below ground part of stem turns brown. Aerial tubers are sometimes formed. Rotate crops. Use certified seed. Treat seed as for scab.

Curly Top, Several Mosaics, Leaf Roll, Spindle Tuber (viruses): Plants may be stunted and off-color. Foliage is mottled or leaflets tend to roll up. Tubers are often small, and in case of spindle tuber, elongated. Several viruses can infect potato without causing noticeable symptoms. However, yields can be reduced drastically. Use of seed certified to be true to variety and free of disease agents is the best way to control tuber-borne viruses. Control insects.

Spotted Wilt (virus – Tomato Spotted Wilt Virus): This virus is vectored by thrips insects. Early symptoms are dead spots on upper leaves and death of the upper plant parts. Early leaf symptoms may somewhat resemble early blight, but spotted wilt symptoms will usually be in the top of the plant rather than on lower leaves. Plant seed certified to be free of disease agents. Control broadleaf weeds in and around fields 4 to 5 weeks before planting.

Other Diseases: Potatoes are damaged severely by other diseases including Verticillium Wilt, Fusarium Wilt, Charcoal Rot, Southern Blight, Root Knot Nematodes, and Aster Yellows (also called Purple Top).

Lewis Ginter Botanical Garden

Mustard greens growing as a cover crop in the Kroger Community Kitchen Garden, part of an experiment to combat southern blight.

Soil is vital and essential because it sustains life. You cannot have a healthy garden without healthy soil. The rear portion of the Kroger Community Kitchen Garden (KCKG) was hit last year with southern blight, a common soil-borne, plant disease. Southern blight affects all parts of the plant and the soil surrounding the plant. Even when the affected plants are gone, southern blight remains in the soil for a long time and continues to infect any susceptible host plant that comes along, making it almost impossible to start new crops without first addressing the underlying cause. While we tried to solarize the soil (covering up the planting beds with clear plastic to heat the soil over time) and kill the fungus that causes southern blight we knew we needed a longer-term solution. That’s where the story of this remarkable project with mustard greens and other cover crops begins.

We reached out to out to Dr. Steven Rideout, director of Virginia Tech’s Eastern Shore Agricultural Research Center , to ask for his expertise on this subject. Rideout’s focus of study is southern blight, and in September 2016 he began to help us to test organic methods that will eliminate this contagious disease. Our objective is to “cleanse” the soil by taking advantage of the natural gas released by certain cover crops, such as mustard greens (Brassica juncea) and rapeseed (Brassica napus), to suppress growth of some soil-borne diseases, including southern blight. The whole process is called biofumigation and is a method that has been scientifically proven to result in a more enhanced microbial biomass in the soil and eventually higher crop yield. It is an organic, sustainable, pre-plant treatment alternative that can improve the quality of the soil by natural processes. Horticulturist Chelsea Mahaffey has been working with Rideout to implement this work at the Garden. “This is more sustainable,” said Mahaffey. “By using organic methods to build the soil quality and limit southern blight, we are maintaining our mission of sustainability to the earth instead of reaching for chemicals.”

Mustard greens are grown as cover crop at the Kroger Community Kitchen Garden as part of a sustainable way to heal the soil after it was infested with southern blight. These mustard green plants will be plowed under the soil and chopped into small pieces and then will be quickly incorporated into the soil to create a biofumigation plot. The plants will act as natural deterrent to weeds, pathogens and diseases.

How it Works:

Mustard greens are a good friend of gardeners. Besides harvesting the leaves and seeds for salads, recipes, prepared mustard and spices, gardeners plant mustard greens as a cover crop. Mustard green plant tissues, from both the roots and the stems, contain a chemical compound and an enzyme in the plant cell that behaves as a soil biofumigant when integrated into the soil, which can suppress nematodes and pathogenic fungi. When the crop is chopped into small pieces and then quickly incorporated and sealed into the soil, the chemical is exposed to the enzyme and it breaks down into a powerful organic chemical compound that serves as a fumigant. The most amazing aspect of this whole process is that the fumigant occurs naturally and is derived from plant. It’s an organic and environmentally-friendly, sustainable treatment for southern blight, which is why we were interested in Virginia Tech’s experiment.

The Plan:

We’ve been working on this treatment plan for one year, but as part of a three-year partnership, we still have two more years to go and a lot to learn. Lewis Ginter Botanical Garden has supplied the area for Virginia Tech team to carry out the research and we’ll take care of planting the cover crops and turning them into the soil at the end of the growing season. Virginia Tech will give us feedback from their observations as we continue this work and refine it based on our results.

For the experiment, the area with southern blight will be divided into three plots. We will plant a different type of cover crop on each plot: the first with mustard greens, the second with rye, and the third with a legume (most likely vetch). “Mustards are known to have biofumigant properties, and we’re testing out the others,” said Mahaffey. “We are looking at the effectiveness to treat southern blight with all three.”

When these plants start to flower with their yellow and purple blooms respectively, it’s a sign that they are ready for the most critical stage of the experiment— to be tilled into the soil and covered with clear plastic to create the biofumigation plot. The objective is to combine solarization and biofumigation into the treatment of soil-borne diseases: the plastic cover will allow solar energy to heat up the soil to create unfavorable conditions for pests, and the crushed plants will transform into a powerful natural compound that can really make a difference. Then we will pump water under the plastic to ‘flood’ the soil, which we hope will suppress the fungus with the anaerobic conditions created by the water. “Anaerobic conditions means without oxygen,” explained Mahaffey. “Like most living things, without oxygen means no life.”

Since there hasn’t been much research on organic practices to counter southern blight, we hope that through this partnership, Lewis Ginter Botanical Garden can become a resource on this topic and give back to the agriculture community through outreach where we share with local farmers and other botanical gardens about a “green” way to heal soil after a bought of southern blight. Who knows, these studies may even show a way that biofumigation can act as a preventative measure. “We hope to find organic means of treating southern blight to give farmers, large and small, and homeowners viable options,” said Mahaffey. “This could give us important options for treating our gardens in sustainable manners.

Meanwhile, we continue to grow produce in the front part of the Kroger Community Kitchen Garden. With a 2,700-pound yield so far, we have been sending large quantities of lettuce, cucumbers, tomatoes, squash, and other produce to FeedMore, which turns them into delicious, nutritious meals for vulnerable populations in Richmond and across Central Virginia. Besides donating to FeedMore, the produce we grow (smaller harvests of radishes and green beans and the like) also go to LAMB’s Basket, a local foodbank serving area residents. Through the KCKG, we give back to the community with fresh, sustainable, and healthy foods that can make a difference.

With Lewis Ginter Botanical Garden’s focus on education and connecting people and plants, we strive to turn as many opportunities as we can into learning experiences. In the past year we’ve spent 1,700 hours in the KCKG educating the community on sustainable practices and agriculture with classes, walk-and-talks, tours, and teaching service learning students. We couldn’t accomplish all this without our dedicated volunteers, who have spent over 1,000 hours watering, weeding, planting and harvesting all of those vegetables. To our wonderful volunteers, if you are reading this blog post, please know that we appreciate you immensely and are deeply grateful for your contribution.

Phuong Tran is a PR & Marketing intern at Lewis Ginter Botanical Garden this spring. It's very likely that you will find her somewhere in the Garden crouching on the ground and looking through her camera lens for some good shots. When she is not taking pictures, she may also be mistaken for a wandering child. Don't worry, she's not lost, just deep in thoughts over stories to tell about all these gorgeous flowers.

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