In the world of gardening, it’s not uncommon for one major obstacle to dominate an entire growing season. In some years, disease control becomes the primary challenge, with many gardeners finding themselves in a constant battle against various ailments. One particularly insidious disease that can wreak havoc on crops is sclerotinia, also known as white mold.
This highly contagious and often fatal condition affects a wide range of plant species, making it essential to be aware of its signs and symptoms should you encounter it in your own garden.
What is White Mold?
Sclerotinia, caused by the fungal species Sclerotinia sclerotiorum and S. minor, is a widespread rot disease that affects numerous plants. One of its defining characteristics is the cotton-like growth, which may or may not be accompanied by black spots. This resilient fungus can survive in soil for extended periods, with some reports suggesting it can persist for up to five years or more.
Various gardeners refer to Sclerotinia as white mold, cottony rot, watery soft rot, stem rot, and blossom blight, highlighting the disease’s versatility. The majority of damage typically occurs during summer or fall, with infection often beginning in cooler weather before becoming most apparent when temperatures rise. This insidious pathogen can affect a wide range of plants, including vegetables and ornamentals, with hollow-stemmed crops like grass-like species being particularly vulnerable.
In both small-scale gardens and large-scale operations, Sclerotinia’s impact on harvest yields can be substantial, emphasizing the importance of understanding its behavior.
Lifecycle of White Mold
White mold, caused by S. sclerotiorum, can lead to substantial yield losses when it infects plants. Its impact is twofold: not only does it affect crop quality, but it also influences seed production and has the potential to re-infect through seeds of infected plants. The fungus can remain dormant in soil for up to five years or more before a host plant becomes available, at which point it germinates and infects the plant directly through the base. A characteristic feature of S.
sclerotiorum is its ability to produce small, round growths on the soil surface called apothecia, resembling tiny flat mushrooms. These apothecia are capable of releasing spores into the air through the impact of water, rain, irrigation, or even gentle stimuli like mist and fog. The released spores are then carried by wind or gentle currents, potentially landing on new plants. Under favorable conditions, infection can begin shortly after spore release.
Furthermore, at the end of each growing season, remaining spores will persist until the next season arrives, or until they find their next host.
Recognizing the Signs of White Mold
The signs of Sclerotinia infection are multifaceted and may masquerade as symptoms of other diseases. The telltale indications include white mold growths and small black bodies called sclerotia. Typically, the leaves will begin to yellow and develop lesions before wilting, dying, and dropping off the plant. Subsequently, fluffy white fungal growths will emerge on stems and fruit, followed by the formation of black sclerotia – a cluster of fungi that can reach 1/4 to 1/2 an inch in diameter.
In some cases, the initial sign is a soft, wet rot on the stem near the soil, which can cause plants with hollow stems to collapse and die. The white fungal growth will appear at the infected area before sclerotia form. Inside the stem cavity of hollow-stem plants, the sclerotia will also form. Moreover, Sclerotinia infection can affect bulbs like gladioli, causing dry rot on corms and infecting new cormels developing on the main bulb or corm.
On hyacinth bulbs, it leads to a wet rot that renders them mushy and unpleasantly pungent.
Plants Affected by White Mold
Among the many plant species impacted by Sclerotinia, a few stand out as particularly susceptible in the home garden setting. These include
Vegetables and Fruit
In a vibrant display of colors and textures, a variety of vegetables and fruits take center stage. A medley of leafy greens, including lettuce, beans, celery, peas, and chicory, mingle with the lusciousness of tomatoes, cucumbers, and stone fruits. Meanwhile, nasturtium adds a pop of bright color to this diverse and thriving ensemble.
Ornamentals
While the list of affected plants may seem exhaustive at first glance, it’s actually just the tip of the iceberg. The truth is that a staggering array of flora falls prey to the insidious clutches of Sclerotinia, with some estimates suggesting as many as 400 distinct species are vulnerable to its wrath.
And yet, despite this sheer volume of victims, the common culprits remain the likes of dahlia, delphinium, sunflower, daisies, iris, and foxglove – a veritable bouquet of botanical bounty beset by the scourge of Sclerotinia.
Types of Infection of White Mold
While we’ve touched on the severity of Ebola, it’s essential to delve further into the complexities of its infections. There are essentially two distinct forms: primary and secondary.
Basal Stem and Crown Rot Infections of White Mold
The disease’s progression is often initiated in the root system, where it commonly infects S. minor. The fungus resides in the soil and can also spread to the crown, leading to further complications. As the infection advances, a characteristic girdling of the stem just above the soil line becomes apparent. Furthermore, a fluffy, white substance will typically appear on the surface of the stem.
If left untreated, this infection can have devastating consequences for lettuce plants, causing them to brown and succumb to watery rotting before ultimately dying.
Arial Infections of White Mold
The infection process typically initiates at the top of the plant, manifesting on the stem, flower, fruit, or foliage. Initially, water-soaked spots emerge, gradually spreading and expanding. As the disease progresses, the stems become girdled, ultimately leading to rot and decay. A characteristic white, fluffy mold develops, imparting a bleached appearance to the affected areas of the plant.
How White Mold Spreads
While it’s possible for white mold to spread from plant to plant, an infected plant isn’t necessarily contagious in terms of producing spores that can infect others. In fact, severe infections typically develop from spores formed during a previous growing season. The environmental conditions necessary for germination and subsequent infection occur when the soil is shaded and moist, with temperatures ranging from 40-60°F.
Under these circumstances, the spores within the top two inches of soil will germinate, producing mushrooms that release new spores. These airborne spores can travel up to 160 feet before landing on a host plant, where they begin a new cycle of infection.
How to Manage White Mold
To effectively address the issue of white mold, several approaches can be employed to mitigate its impact and even prevent future occurrences. These strategies encompass a range of methods, from proactive measures to prompt interventions, all aimed at minimizing the effects of this fungal growth.
Spacing
Proper spacing between plants and rows is a crucial aspect of disease prevention, extending beyond the control of white mold. In fact, this strategy can be effective against numerous diseases. Moreover, maintaining good airflow is vital, particularly in crops or plants with dense canopies, where restricted air circulation can inadvertently foster an environment conducive to white mold.
By ensuring sufficient space and promoting healthy ventilation, you can significantly reduce the likelihood of disease onset.
Fertilizing
When it comes to preventing an outbreak of white mold, using the right fertilizer is crucial. Applying too much nitrogen or using a product not suited for the specific crop can create an environment conducive to the development of this disease. When plants produce excessive foliage due to over-fertilization, the surrounding microclimate becomes more humid, increasing the likelihood of a severe outbreak.
To mitigate this risk, it’s essential to use fertilizers according to the manufacturer’s guidelines and avoid using animal manures in areas where white mold has been a recurring issue. Manure can stimulate rapid growth, which inadvertently encourages the proliferation of white mold.
Plant Timing
When cooler temperatures and excessive moisture converge, they can create an ideal environment for white mold to thrive. To minimize this risk, consider planting during spring or late summer when conditions are generally drier. Providing adequate moisture during these warmer periods can actually help inhibit the growth of white mold. If heavy rainfall is forecasted in the near future, it may be best to delay planting until the weather improves, if possible.
Weed Control
When it comes to preventing fungal diseases from taking hold, a key factor is keeping weed populations under control, particularly broadleaf species. By maintaining a low density of these weeds and ensuring adequate air circulation, you can significantly reduce the likelihood of an outbreak or at least mitigate its impact and duration.
Biological Control
Among the various fungi identified as mycoparasites to Sclerotinia species are several commercially available products. Notably, formulations containing Coniothyrium minitans have been shown to be effective in controlling the disease, while those featuring Unocladium oudemansii (U3 strain) have also demonstrated efficacy.
Additionally, products incorporating Streptomyces lydicus WYEC 108 and Bacillus amyloliquefaciens, such as Bonide Revitalize, can also be used to treat Sclerotinia-related diseases.
Crop Rotation
By strategically incorporating non-host crops between host crops, you can effectively diminish the viability of the fungal spores over time. This approach also encourages germination of some spores while the non-host is present. Additionally, breaking the continuous cycle of host infection allows for a natural reduction in reinfection rates the following season. Consider utilizing non-host crops like corn or wheat to achieve this beneficial outcome.
Fungicides
When it’s necessary to intervene with chemicals in the garden, many of us may feel uncertain about where to start. When it comes to tackling white mold, there are several fungicides available that can be effective. However, each one has its unique characteristics, such as name, timing, and application rate, making it crucial to consult with local experts for personalized guidance. A reliable option is copper-based fungicide, which often proves to be a tried-and-true solution.
Resistant Cultivars
When it comes to finding crop varieties that can withstand the pressure of pests and diseases, breeding for resistance has been a key strategy. Fortunately, there are some notable examples of resistant cultivars out there. In particular, look for canola plants and brassicas that have been bred with resistance in mind. Soybeans are another example of crops that have benefited from this approach.
