It seems that in some years, our biggest challenge in the garden is trying to control all of the diseases that affect our crops. In fact, some years it feels like all you are doing is battling diseases. One disease, in particular, is prolific and serious, and that’s sclerotinia.
Sclerotinia, or white mold, is something you don’t want to become familiar with. It attacks many different species and can be deadly. Let’s dive right in so you’re well informed should you need to deal with it.
What is White Mold?
Sclerotinia is a rot disease of many plants caused by fungal species Sclerotinia sclerotiorum, and S. minor. Common signs are a cotton-like growth with or without black spots. The fungus can survive in the soil for long periods of time. Reports suggest up to five years or more.
Some gardeners call Sclerotinia white mold, cottony rot, watery soft rot, stem rot, and blossom blight.
The most common time to see white mold is summer or fall, and this is when most of the damage is done. Usually, the infection starts in cooler weather but is most noticeable when it’s warm.
White mold infects many plants including vegetables and ornamentals. It’s particularly damaging in plants with hollow stems, such as grass-like crops.
Whether in a small garden or larger operation, white mold can cause significant harvest loss, so knowing as much about it as possible is key.
Lifecycle of White Mold
White mold can result in a significant yield loss when it strikes. Not only does it affect the crop of your plants, but it also affects the volume of seeds produced and is likely to infect again through the seeds of infected plants.
It can sit dormant in the soil for up to five years or more before a host plant comes along. Then, it will germinate and infect the plant directly through the base.
S. sclerotiorum produces small round growths on the soil surface called apothecia. They are like tiny, flat mushrooms.
Spores are ejected from the apothecia from the impact of water, rain, irrigation, or even from gentle stimuli like mist and fog. The spores are carried by the wind or on gentle currents where they land on plants.
In the right conditions, the infection begins soon after the spores are released.
At the end of the growing season, new spores will remain until the next season, or until they find their next victim.
Recognizing the Signs of White Mold
The symptoms are varied and often look like the symptoms of other diseases. The most telling signs are the white mold and little black bodies called sclerotia.
Often, the leaves begin to yellow and will develop lesions. The leaves will eventually wilt, die and drop off the plant. Then you will see the fluffy white fungal growth on stems and fruit, followed by the black sclerotia, which is a mass of fungi.
Sometimes the first sign will be a soft, wet rot on the stem near the soil. This can cause the plant to collapse and die, especially for the plants with hollow stems.
The white fungal growth appears on the plant at the infected area, before the sclerotia form. These black seed-like growths can be 1/4 to 1/2 an inch in diameter. Some may be smaller.
In hollow stem plants, the sclerotia will form inside the stem cavity, as well.
On bulbs of flowers like gladioli, dry rot will set in on the corms and infect the new cormels developing on the main bulb or corm. On other bulbs like hyacinths, sclerotinia causes a wet rot to set in, making them mushy and smelly.
Plants Affected by White Mold
There are literally hundreds of plants affected by sclerotinia. Here are the most common in the home garden:
Vegetables and Fruit
Those are the common ones, but there are many more. Some reports say up to 400 different plants are affected by sclerotinia, if not more.
Types of Infection of White Mold
Let’s look a little deeper into the two types of infections of this disease.
Basal Stem and Crown Rot Infections of White Mold
This is when the disease starts in the root system. This is more often than not infected with S. minor. The fungus resides in the soil and infects the crown as well.
As the disease progresses, you will likely see a girdling of the stem just above the soil line. On the stem surface, there will be a fluffy, white substance.
If this infects lettuce, S. minor creates the girdling before the plant browns and starts a watery rotting, before eventually dying.
Arial Infections of White Mold
This is where the infection begins in the upper part of the plant. It generally starts on the stem, flower, fruit, or foliage. Water-soaked spots appear first that gradually spread out and grow.
The stems become girdled, before rotting and dying. A thick white, fluffy mold develops, including making the infected parts of the plant look bleached.
How White Mold Spreads
You could be forgiven for thinking that when a plant is infected with white mold, it will infect all of those around it. This isn’t always true. Plant-to-plant spread can occur, but once the plant is diseased, it’s not contagious in that spores aren’t produced from that infection that season.
Severe infections normally develop from spores formed the previous season.
When the environment is right next season, white mold spores germinate. This is when the soil is shaded by the canopy of thick foliage or by crowded rows that keep the soil moist.
When the temperature inside this canopy or foliage is between 40-60ºF, the spores in the first two inches germinate, forming the mushrooms that release spores.
The spores can be blown at least 160 feet to land on host plants where a new infection begins. The process that follows will be the source of future infections.
How to Manage White Mold
There are various ways in which to manage an outbreak of white mold and prevent it as well.
Plenty of space between plants and rows is one of the best preventative methods for many diseases, not just white mold.
You must have good airflow, especially in plants and crops with thick canopies. You can inadvertently create an environment desirable to white mold if airflow is restricted due to thick foliage and crowded rows.
Over-fertilizing or using a fertilizer with too much nitrogen for a particular crop can also create an environment likely to harbor an outbreak of white mold.
If the fertilizer causes the plant to create more foliage than is natural for it, this can cause the microclimate around the plants and rows to increase in humidity and thus increase the severity of an outbreak.
Make sure to use the correct fertilizer and the amount recommended by the manufacturer.
In areas where white mold has been a problem in the past, avoid animal manures as a form of fertilization. Manure often produces quick, lush growth of foliage which white mold loves.
3. Plant Timing
Cooler weather combined with excessive moisture is often a catalyst for an outbreak of white mold.
If you are able to, and your crop can handle it, plant in spring to late summer. Just ensure you provide the crop with sufficient moisture for this warmer period. This warmth can help to inhibit the growth of white mold.
If there’s a lot of rain in the coming forecast, put off your planting until it has passed, if at all possible.
4. Weed Control
Many weeds are host to the disease, especially broadleaf types. Keeping these weeds down in conjunction with sufficient airflow will help to stop an outbreak, or at least lessen the effects and duration.
5. Biological Control
There are several fungi identified as mycoparasites to Sclerotinia species. There are commercial products released that contain these fungi.
Products containing Coniothyrium minitans are effective against the disease, as are those containing the fungus Unocladium oudemansii (U3 strain).
Similarly, products that contain the microbe Streptomyces lydicus WYEC 108 can be used to treat this disease, as can products with Bacillus amyloliquefaciens, such as Bonide Revitalize.
6. Crop Rotation
This is beneficial for a couple of reasons. Planting non-host crops in between host crops reduces the viability of the spores over time. Also, many of the spores may germinate while the non-host is present.
Without the constant host infection, the spores aren’t continually returned to the soil for reinfection next season.
Use non-host crops such as corn or wheat.
Although many of us shy away from chemical intervention in the garden, sometimes you may have to use it when necessary.
There are a number of fungicides available that can have an effect on white mold. They are different in terms of names, timings, and application rates, so ask your local experts. Copper fungicide is always a good bet.
8. Resistant Cultivars
While there aren’t a ton of them out there, there are resistant cultivars that breeders have bred. Look for resistant canola plants and brassicas, as well as some soybeans, in particular.