How To Get Rid Of Sclerotium Fungus In My Garden

A Southern Blight Strategy
Bill Meyer, Woodbury, CT

T he fungus called Southern Blight has a variety of common names, but the scientific name is Sclerotium rolfsii. It was named for a Peter Henry Rolfs, who first found it in tomato fields in Florida in 1892. It has been a significant crop pest in warmer parts of the world, but is generally controlled these days, mainly by use of preventative fungicides.
In the mid-90's, Southern Blight started to become a problem in many hosta gardens. It was generally thought of as a warm-climate pest, but gardens in even some of the colder parts of the U.S. have been dealing with repeating yearly infections. The occurrence of infections has been rising steadily since then and as of this writing (August, 2010), it has become a frequent complaint of gardeners who collect plants from many sources, both gardens and nurseries.

Hosta under attack - note white threadlike hyphae.

Sclerotium rolfsii is a parasitic fungus that will attack over 500 different species in a wide range of genera, including hostas, which it usually damages severely and sometimes kills completely. If it gets established in the garden it can be a serious problem that is difficult to eliminate if it is not understood.
The life cycle of Sclerotium rolfsii begins in early summer when warm temperatures are well established. It germinates and begins feeding on decaying plant material at or near the surface of the soil. It uses the food from the decaying material to fuel its search for a living plant to parasitize, growing white strands of fungal material called hyphae. These hyphae are part of the overall vegetative mass of the fungus which is called the mycelium.
When it finds a host plant, it penetrates the stem by excreting a chemical called a lectin. Once it is inside the host plant, it begins to grow rapidly, and produces a large mass of hyphae around the base of the plant on top of the soil. This process occurs over a period of 4-9 days. This is then followed by production of a large number of seed-like bodies called sclerotia, which begin as white as the fungus and mature to a yellowish brown. When mature they resemble mustard seed. The fungus then dies, having succeeded in reproduction. The sclerotia then germinate the following summer, starting the process over again.
In the above picture, the fungus has penetrated the petioles and is destroying the plant tissue. As it feeds on the hosta, it begins to grow more hyphae. The second picture (below) shows the developing sclerotia. Sclerotia are seedlike in that they will germinate and start the fungus going again, but their structure is much different. They are formed from multiple layers of the mycelium, with soft white ready-to-grow material in the center surrounded by hardened brown layers that let them survive harsh conditions.

Common Strategies

In my experience, once Southern Blight gets into a garden, it can reappear almost anywhere in subsequent years. Here in our garden it would hit one hosta, then another a hundred feet away in a different part of the garden without appearing anywhere in between. For this reason, preventative measures would have to include the entire garden to be sure they are effective.
Drenches with fungicides are basically the only way to prevent the fungus from getting started in the garden. This will be expensive for larger areas and may be unattractive to many for the sheer amount and expense of chemicals used. It will be effective if reapplied at the intervals specified on the label, as the fungus is not especially hard to kill.
A variety of fungicides will work for this purpose, but products with PCNB as the active ingredient like Terraclor are considered best in farming. Recently a new product has come onto the market with the active ingredient tebuconazol. It is sold as Bayer Advanced Disease Control, and it is possibly more effective than PCNB products. Other products based on both PCNB and tebuconazol are available but these two are the most common, both in retail and professional markets. Both are effective, and both work very well to kill the fungus.

Sclerotia - the real problem

The actual process of drenching the entire garden is labor-intensive, expensive, and not something I would recommend for several reasons. It will work, but it is better suited to the growing of annual crops. For those who want to try it anyway, I recommend keeping the fungicides off of hosta leaves as the PCNB products will leave a persistent dry whitish residue and the Bayer product is an oil which will turn all blue leaves green.

My Strategy

The life cycle of Sclerotium rolfsii gives a clear understanding of what we are dealing with as hosta gardeners. In many ways it closely resembles an annual weed, and my control strategy is based on this similarity. What I did here in our garden was to start watching for the fungus to attack a hosta. This is usually very clear if you watch for it. Around the middle of June when the weather has warmed up completely, begin watching for any hosta leaves that have fallen over and are lying on the ground. Several things can cause this, and they should be ruled out before treating for Southern Blight.

Hosta petioles sometimes are chewed through above the soil line by slugs or some mammal or broken by falling branches or careless gardeners or their pets. Damage from voles or mice, or diseases that kill the rhizome of the plant can also lead to leaves collapsing. To see if Southern Blight is the cause, look carefully at the base of the leaf. The petiole tissue will be soft and rotting right where it meets the ground, not just below ground, and there will be some of the white hyphae noticeable. If the attack of the fungus has been caught at this stage, there will be none of the seed-like sclerotia present.

The three most commonly used fungicides.

It will normally take from four days to a week or so before the first leaf drops to the ground and for the fungus to mature and produce its sclerotia. This is by far the best time to treat the problem. If the sclerotia have formed it becomes much more difficult. Prior to the sclerotia forming, Southern Blight is very much like an annual weed in bloom. Like all such weeds, if it is allowed to set "seed" it will surely be back again next year. Although it is possible for Southern Blight sclerotia to survive for several years in the soil, they will normally germinate the following summer, so eliminating it before it can set "seed" is the key to ridding the garden of the pest once and for all.
If Sclerotium rolfsii is caught prior to setting its little progeny, a simple drench with either of the products mentioned above will kill it quickly and effectively. Once should be enough, but it can't hurt to watch the plant closely for another week or two for any signs the fungus still lives. It does not go very deep into the soil so just drenching enough to penetrate an inch into the soil should be enough. If you can kill it every time it appears before any sclerotia have set, the odds are very good it will not be back next year. If it does show up again the next year, repeating the process should finish it off.
If it has set sclerotia and by the time you discovered the problem and there are hundreds of little brown mustard-seed like bodies nestled in a large flat mass of white mycelium, all efforts must first be directed at cleaning up the sclerotia as completely as possible. Any that escape will germinate the following year and start the problem up again.
Carefully remove the affected leaves of the hosta and place them into a plastic trash bag. Then clean up all the soil around the base of the plant. Some have suggested a hand-held vacuum cleaner to pick up the sclerotia. When the sclerotia have been cleaned up as thoroughly as possible, scrape off the top inch or so of soil and add it to the trash bag. Only when you are sure you have done all you can to clean up the sclerotia is it time to drench with some fungicide.
It is not necessary to dig up the hosta at this point, as little can be done to improve its condition. Usually the affected plant will recover to some extent, but the loss of too many leaves will set the plant back considerably. Sometimes the hosta will die, possibly from some damage the fungus did. In the Southern Blight attacks I've dealt with, about one fourth of the hostas died, while some survived only as small pieces. The damage done is considerable if it is not caught early.

Alternatives

Stopping sclerotia formation is the key to getting Southern Blight
out of the Garden permanently

Keep fungicides on hand to treat when it appears

Some will suggest the use of bleach to drench and clean up the affected hostas. Bleach will also kill Sclerotium rolfsii very effectively, but bleach is a caustic chemical that will also do further damage to the hosta in the process. As the plant has already been badly damaged by the fungus itself, the use of a caustic chemical that does further damage is counterproductive, in my opinion. The fungicides mentioned seem to do no damage to plants when used as directed, so they are the better choice for saving the plant.

For those who are opposed to any chemical controls in the garden, there seem to be no good alternatives. I would suggest being vigilant and digging the hosta before the sclerotia form. Cut any petioles that have been penetrated by the fungus below the attacked area as near to the rhizome as possible. Then let the plant dry in the sun for several hours. Pouring boiling water into the soil where the plant was may eliminate the fungus in the soil if the soil is warmed enough.

After several years of battling outbreaks of Southern Blight here in the garden, I believe I have been successful in eliminating it from the garden. There is always the chance I will bring it back in on a new plant, so I continue to keep an eye out for any leaves that are lying down. Sclerotium rolfsii can be eliminated in the garden with some vigilance and minor use of fungicides, but it can return with new plants at any time. Keeping one of the above fungicides available and ready to use should be a part of hosta gardening life, once Southern Blight has shown up in the garden.

Update: Terraclor has been difficult to find lately, but remains a good treatment option for Southern Blight. A newer product called Prostar is easier to find and also very effective.

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