Understanding Sclerotinia Stem Rot (White Mold): A Common and Destructive Plant Disease

Overview

Sclerotinia stem rot, also known as White mold, is a harmful disease caused by the fungus Sclerotinia sclerotiorum. This disease thrives in prolonged wet and cool weather conditions, leading to the formation of white, fluffy fungal growth on infected tissues.

Key Characteristics of the Pathogen

One notable characteristic of the Sclerotinia sclerotiorum fungus is its ability to produce two distinct structures: black resting structures called sclerotia and white fuzzy growths of mycelium. These structures are found on the plants it infects and can be referred to as cottony rot, watery soft rot, stem rot, drop, crown rot, and blossom blight.

Hosts Affected

The pathogen has a wide host range, affecting many broadleaf crops and other plants, except for corn and small grains. Some examples of susceptible plants include cabbage, common bean, citrus, celery, coriander, melon, squash, soybean, tomato, lettuce, cucumber, carrots, onions, peas, and pumpkins.

Disease Cycle

The lifecycle of Sclerotinia sclerotiorum is considered monocyclic, meaning it does not produce secondary inoculums. The fungus survives in soil or stems in the form of hard black masses called sclerotia, which can remain viable for more than five years. Under favorable conditions, some sclerotia germinate while others remain dormant.

The germinated sclerotia near the soil surface develop into small tan-to-gray mushroom-shaped structures called apothecia. These apothecia produce ascospores that spread through air currents, causing infections. Additionally, the pathogen can contaminate seed lots and infect seeds, contributing to its spread.

Symptoms

Sclerotinia sclerotiorum can invade various tissues, including stems, foliage, flowers, fruits, and roots. The majority of its life cycle occurs in the soil, and symptoms typically start at the soil surface. However, the fungus can also be transported through the air.

Although symptoms may vary across host species, there are some common characteristics. Water-soaked lesions with irregular shapes appear on fruits, stems, leaves, or petioles. These lesions enlarge, and a cottony mycelium covers the affected area. The fungus continues to spread, resulting in a soft, slimy, water-soaked mass. The presence of numerous black sclerotia within the cottony mycelium is a reliable diagnostic sign of the fungus.

“Dry” lesions may develop on stalks, stems, or branches, clearly demarcating healthy and diseased tissues. These lesions grow, encircling the plant part and causing the distal portions to turn yellow, then brown, and eventually die. Typically, the base of the plant is girdled, leading to the death of the entire plant. Sclerotia can form within stem pith cavities, fruit cavities, or between tissues.

Disease Management

Chemical Methods

Systemic fungicides are effective in controlling the disease. Some recommended fungicides include:

  • CHANCETYL ELITE 800WDG
  • DOMAIN 250EC
  • GEARLOCK TURBO 250WP
  • MILLIONAIRE 690WDG
  • PYRAMID 700WP
  • TRINITY GOLD 452WP
  • COLINIZER 440WP
  • GREEN COP 500WP

Non-chemical Approaches

Implementing non-chemical strategies can also aid in disease management. Consider the following measures:

  • Reduce irrigation frequency during flowering when using irrigation systems.
  • Utilize certified or disease-free planting materials.
  • Practice long-term crop rotation with non-host crops like corn or small grains to minimize white mold incidence.
  • Opt for resistant or tolerant plant varieties whenever possible.
  • Avoid planting highly susceptible varieties in fields with a history of the disease.
  • Choose short-statured varieties that are less prone to lodging, reducing the potential for disease.
  • Maintain proper spacing between plants, as narrow rows increase the risk of Sclerotinia stem rot.
  • Implement effective weed control, as several weeds can act as hosts for the fungus.

Helpful Tips!

  • Enhance the efficacy of foliar sprays by incorporating INTEGRA 3ml/20l, a sticker, spreader, and penetrant.
  • Ensure timely control of the disease to minimize its spread.
  • Rotate different chemical treatments throughout the crop season to prevent resistance buildup in the fungus.
  • Pay attention to proper crop nutrition to support overall plant health and resilience.

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