White mold is a disease caused by a fungus called Sclerotinia scletotiorum, and it’s a killer. Unlike eco-friendly saprophytic fungus, it is one of the most damaging soil-borne fungal pathogens that affect hundreds of plants. Once the white mold is in your garden, the disease will typically recur year after year.
You can identify white mold on the stems and leaves of plants between normal green stem tissue. It looks clearly like a fluffy, white, fungal growth, although some infected parts of stems are an off-white to tan color and both dry and brittle. You may also spot hard bits, called sclerotia, that form on the surface and within stems that are affected.
Can the white mold on my soil hurt my plant?
White mold causes plants to wilt stems of plants to rot, and it kills many common flowers, vegetables, and herbs. It can attack anywhere and can be as much of a problem in home gardens as in commercial operations.
Because it is so damaging, the diseases it causes have many different names. These include blossom blight, Sclerotinia blight, drop, cottony rot, crown rot, watery soft rot, and stem rot.
Once it’s killed the plant host, the fungus can grow saprophytically on the dead plant tissue. Saprophytic fungi are, of course, vital to the ecosystem because they decompose nonliving organic matter and they play a major role in the carbon cycle.
It then continues to form sclerotinia on the surface of the plant, in any cavities it can find, in the plant debris, and in the soil around the plant.
The worst part is that it can remain dormant for as long as 10 years. Then it can re-emerge and attack again.
Reasons why soils grow white molds
We know that white mold is caused by the sclerotinia fungus that overwinters in infested plant soil and dead plant material. We also know that it can survive for years.
As Michelle Grabowski, Extension educator at the University of Minnesota, points out, the white mold fungus forms hard, black structures about the size of the tip of pencil lead. This is what we know as sclerotinia, and they allow the fungus to survive in the plant debris and soil for at least 5 years.
But how do the sclerotia get there?
Research has told scientists that when temperatures are cool (51-68 ℉) in spring and summer, and the soil is moist, sclerotia produce tiny mushrooms. These produce and release mold spores that are carried for miles by the wind.
They are also spread by insects and splashes of rain.
When the spores land on old or damaged plant tissue, including petals and leaves, they germinate and start an infection. The infection then moves into the main stem of the plant, eventually girdles it, and the leaves above the infection wilt and die.
It’s a never-ending circle because new sclerotia begin to form within the dead plant tissue.
Robert Harveson, extension plant pathologist at the University of Nebraska-Lincoln, explains how to recognize the symptoms of the disease. He also describes the environmental conditions it favors.
Symptoms of White Mold
White mold disease often appears after plants wilting in a canopy have been soaked by water. They develop water-soaked spots that develop into a mass of rotten tissue that is covered with white fuzzy mold.
Infected stems wilt and the branches above them die. Sclerotia also form within the cottony, white fuzzy growth on other parts of the plant.
Once white mold gets a hold of your plants, you probably won’t even notice the white, fuzzy mold. All you will see is soft, watery rot.
Favorable Environmental Conditions for White Mold
There’s a good possibility that we encourage the growth of white mold, even if we don’t know we are doing so. Harveson cites high plant populations, the excessive use of fertilizer, and high levels of irrigation and/or rainfall as factors that exacerbate diseases caused by sclerotinia.
For instance, if there is free moisture available, the fungal spores will continue to germinate and infect the leaves and stems of more and more plants. When you have a lush canopy of growth, the higher the humidity and less air movement there is, the more the disease will progress.
When we continue to plant susceptible crops the disease gets even worse. So, let’s pinpoint the environmental issues more closely.
If your soil is contaminated, replace it. Also, be aware that weeds can also be hosts to white mold. So get rid of weeds at the same time.
We’ve ascertained that sclerotia thrive in moist conditions. This is why it’s no surprise to learn that it produces little mushroom-like structures.
So, be careful not to overwater.
There’s not much you can do about the rain, but you can take steps to ensure there is good drainage in your garden.
Poor Air Circulation
It’s lovely to have dense canopies formed by plants, but it’s also essential to have good air circulation. If need be, thin out plant growth to improve air circulation.
There is no doubt that good airflow through plant canopies can prevent horrible diseases like those caused by Sclerotinia scletotiorum.
How do I get rid of white mold in my soil?
It’s not easy to get rid of white mold in the soil. If unused potting soil is infected, dump it. But if you spot mold growing in your garden soil, you’ve got some major challenges.
The University of Massachusetts Amherst Extension Vegetable Program warns that crop rotation and deep plowing don’t work very well. The problem is that there is a wide host range of mold growing on your plants.
Also, it has a horrible ability to stick around in the soil for extended periods of time. Unfortunately, when sclerotia are buried deep in the soil, deep plowing can uncover them years later!
Michelle Grabowski has some useful pointers on how to manage white mold. They are certainly worth embracing since once it’s in your garden, the diseases white mold brings with it are very likely to recur.
We’ve already emphasized the need to avoid overwatering and to ensure there is good drainage in your garden. This, in itself, will help to reduce moisture on and around your plants.
But, other steps you can take to reduce moisture are to select plants that have an open, upright form. They will usually dry out more quickly than those that spread out over the ground or grow in dense clumps.
Spacing your plants sensibly will also help to reduce moisture because it will ensure that air movement is more effective. It isn’t rocket science to realize that air will dry plants just as well as sunlight.
Taking this a step further, it is also possible to reduce high moisture by orienting rows of crops in the direction of prevailing winds. But it’s tricky in a home garden.
Clean Up and Remove Infected Plants
If and when you find that white mold has attacked plants in your garden, you need to get rid of it fast.
Start by removing all plants that are infected. Just be sure that you don’t knock any of the sclerotia off when you pull out the plants.
If you have a compost heap that heats up to 148-158 ℉ for at least 21 days, you can compost the infected plants. But we advise, to be safe, that if you have the space, rather burn or bury the infected plants somewhere that you won’t be planting anything anytime soon.
From then on, be sure to remove any withered flowers, dead leaves, and any other plant debris you find.
How to prevent new mold from growing
Grabowski goes on to say that the best way to stop new mold from growing is to opt for plants that are resistant to white mold. It’s a challenge, but it may just be worth the effort. We’re just not certain how many white mold-resistant species there are!
Here are a few other steps that you can take. We have already mentioned some of them.
Don’t ever forget that too much moisture is an invitation to white mold. So, be sure to use well-drained houseplant potting soil in your containers and well-drained soil in your garden beds too.
When you water, do it early in the day so that your plants can dry. It’s also good practice to avoid watering the leaves of plants.
Rather a water at the base, and water deeply.
Proper watering is nothing if you don’t have good drainage. Sometimes you will be forced to install drains to take excess water away from garden beds.
If you have a problem with white mold, whether it seems to be attacking your garden plants and/or the soil in your garden or even your houseplant soil, it might be a good idea to get professional advice. Several universities have plant and diagnostic clinics that will investigate and confirm whether or not your plants have the disease.
Usually, they will be able to make judgment calls if you submit good-quality photographs that show affected parts.