How to identify and treat sclerotinia rot

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It seems that some years our biggest challenge in the garden is trying to control all the diseases that affect our crops. In fact, some years it feels like all you’re doing is fighting disease. One disease in particular is prolific and serious, and that is sclerotinia.

Sclerotinia, or white mold, is something you don’t want to be familiar with. It attacks many different species and can be deadly. Let’s dive right in so you’re well informed if you have to deal with it.

Contents

What is white mold?

Sclerotinia is a rotting of many plants caused by fungal species Sclerotinia sclerotiorumand S. minor. Common signs are cotton-like growth with or without black spots. The fungus can survive in the soil for long periods. 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 that’s when most of the damage is done. Usually the infection begins in cool weather, but is more noticeable in warm weather.

White mold infects many plants, including vegetables and ornamentals. It is particularly damaging to plants with hollow stems, such as herbaceous crops.

Whether in a small garden or a larger operation, white mold can cause significant crop loss, so it’s essential to know as much as you can about it.

White mold life cycle

White mold can cause significant yield loss when it strikes. Not only does this affect the harvest of your plants, but it also affects the volume of seeds produced and is susceptible to re-infection via seeds from infected plants.

It can remain dormant in the soil for up to five years or more before a host plant appears. 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 look like tiny flat mushrooms.

The spores are ejected from the apothecia under the impact of water, rain, irrigation or even mild stimuli like mist and fog. The spores are carried by the wind or by light currents where they land on the plants.

Under the right conditions, 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.

Recognize the signs of white mold

Symptoms are varied and often resemble symptoms of other diseases. The most telltale signs are white mold and small black bodies called sclerotia.

Often the leaves begin to turn yellow and develop lesions. The leaves will eventually wilt, die and fall off the plant. Next, you will see the white, fluffy fungal growth on the stems and fruits, followed by the black sclerotia, which are a mass of fungus.

Sometimes the first sign will be a soft, wet rot on the stem near the ground. This can lead to plant collapse and death, especially for plants with hollow stems.

The white fungal growth appears on the plant at the infected area, before the formation of sclerotia. These black seed-like growths can be 1/4 to 1/2 inch in diameter. Some may be smaller.

In hollow-stemmed plants, sclerotia also form inside the stem cavity.

On flower bulbs like gladioli, dry rot will take hold on the bulbs and infect new bulbs growing on the main bulb or bulb. On other bulbs like hyacinths, sclerotinia causes wet rot, 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 vegetable garden:

Vegetables and fruits

ornamental plants

  • Dahlia
  • Delphinium
  • Sunflower
  • daisies
  • Iris
  • Foxglove
  • Peony

These are the most common, but there are many more. Some reports indicate that up to 400 different plants are affected by sclerotinia, if not more.

Types of white mold infection

Let’s take a closer look at the two types of infections of this disease.

Basal stem and crown infections caused by white mold

This is when the disease begins in the root system. This is most commonly infected with S. minor. The fungus resides in the soil and also infects the crown.

As the disease progresses, you will likely see a girdle of the stem just above the soil line. On the surface of the stem, there will be a white, fluffy substance.

If it infects the lettuce, S. minor creates girdling before the plant turns brown and begins a watery rot, before eventually dying.

Airborne white mold infections

This is where the infection begins in the upper part of the plant. It usually starts on the stem, flower, fruit or foliage. Waterlogged spots appear first which gradually expand and grow.

Stems become girdled, before rotting and dying. A thick, fluffy white mold develops, especially giving infected parts of the plant a bleached appearance.

How White Mold Spreads

You could be forgiven for thinking that when a plant is infected with white mold, it will infect everyone around it. This is not always true. Spread from plant to plant can occur, but once the plant is diseased it is not contagious as long as spores are not produced from that infection that season.

Severe infections normally develop from spores formed the previous season.

When the environment is good next season, white mold spores germinate. This is when the ground is shaded by the canopy of thick foliage or by tight rows that keep the ground moist.

When the temperature inside this canopy or foliage is between 40 and 60°F, the spores in the top two inches germinate, forming the fungi 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 deal with white mold

There are different ways to handle a white mold outbreak and prevent it as well.

1. Spacing

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 air circulation, especially in plants and crops with heavy canopies. You can inadvertently create a favorable environment for white mold if airflow is restricted due to thick foliage and crowded rows.

2. Fertilize

Over-fertilizing or using a fertilizer that contains too much nitrogen for a particular crop can also create an environment that could harbor a white mold outbreak.

If the fertilizer causes the plant to create more foliage than is natural for it, this can lead to an increase in the humidity of the microclimate around the plants and rows and thus increase the severity of an outbreak.

Be 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 rapid, lush foliage growth that white mold loves.

3. Plant Schedule

Cooler weather combined with excessive humidity is often a catalyst for a white mold outbreak.

If you are able and your crop can handle it, plant from spring to late summer. Just be sure to provide the crop with enough moisture for this warmer period. This heat can help inhibit the growth of white mold.

If there is a lot of rain in the forecast ahead, delay your planting until it has passed, if possible.

4. Weed control

Many weeds are host to the disease, especially the broadleaf types. Keeping these weeds in conjunction with ample airflow will help stop an outbreak, or at least reduce its effects and duration.

5. Biological control

There are several fungi identified as mycoparasites Sclerotinia species. There are commercial products that contain these mushrooms.

Products containing Coniothyrium minitans are effective against disease, as are those containing the fungus Unocladium oudemansii (strain U3).

Similarly, products containing the microbe Streptomyces lydicus WYEC 108 can be used to treat this condition, as can products containing Bacillus amyloliquefaciens, such as Bonide Revitalize.

6. Crop rotation

This is beneficial for several reasons. Planting non-host crops between host crops reduces spore viability over time. Additionally, many spores can germinate in the presence of the non-host.

Without the constant infection of the host, the spores are not continuously returned to the soil to be reinfected next season.

Use non-host crops such as corn or wheat.

7. Fungicides

Although many of us shy away from chemical intervention in the garden, sometimes you will need 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, deadlines and application rates, so ask your local experts. Copper fungicide is always a good bet.

8. Resistant Cultivars

Although there aren’t a ton of them, there are resistant cultivars that breeders have selected. Look for resistant canola and cruciferous plants, and some soybeans in particular.

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