Collar Rot

We’ve probably all lost seedlings to damping off disease. Nascent stems, healthy only the day before, suddenly appear pinched and broken, never to recover. When lesions appear further down the stem at the soil line, it is called collar rot.

Collar rot of apple tree (William M. Brown Jr., Bugwood.org) CC BY 3.0

Collar rot gets its name because of the lesions that form a collar around where the root system meets the stem. In embryonic plants, this area between the first root (radicle) and the first stem (hypocotyl) is called the collet. In more mature plants, this area is called the crown.

Vulnerable stems and seedlings

In the case of both damping off disease and collar rot, pathogens enter delicate new stems as they emerge from the earth, scratched by soil particles. These tiny wounds provide points of entry. Insect and herbivore feeding, garden tools, and rubbing due to overcrowding and improper pruning can also create these wounds. Wounds heal quickly, but sometimes pathogens get in. When they do, problems begin.

Alternaria black mold on pepper plants (Virginia Tech Learning Resources Center, VPISU, Bugwood.org) CC BY 3.0

Collar rot symptoms

As fungi, bacteria, and other pathogens enter a plant, lesions start forming in a band, or collar, around the lower portion of the main stem. At the same time, vascular bundles become blocked and the disease-causing agents begin multiplying.

​​Collar rot pathogens

Collar rot looks similar to damping off disease because the same pathogen may be responsible. Collar rot can also be a symptom of several other infections. Trees infected with fireblight tend to be more susceptible to collar rot, but we don’t know why. As you can see, collar rot isn’t a specific disease. Instead, it is a symptom of attack by several different pathogens:

• Alternaria fungi are responsible for carrot decay, early blight of potato, parsley crown decay, tomato stem cankers, and leaf blight in many fruit and nut trees.
• Calonectria infects papaya.
• Phytophthora causes root rot in citrus and other fruit and nut trees, soybean root rot, potato late blight, blight and fruit rot of beans, cucumber, eggplant, melons, squash, tomatoes, and watermelon. Its name means 'the plant destroyer’.
• Rhizoctonia fungi are responsible for belly rot of cucumbers, black scurf of potatoes, eyespot, Rhizoctonia blight, root rot, and several turfgrass diseases. The name translates from the Latin for ‘root murder’.
• Sclerotium causes Sclerotial blight in eggplant and Swiss chard, southern blight of beans, onions, tomatoes, and hundreds of other crops, tomato blight, and white rot of onions.

Rhizoctonia blight of bean plant (Howard F. Schwartz, CSU, Bugwood.org) CC BY 3.0

Sclerotial blight of Swiss chard (Scot Nelson) Public Domain

Collar rot prevention

Collar rot occurs in gardens and containers where the soil is infected. You can prevent the infections that cause collar rot with these tips:

• Only buy certified disease-free plants.
• Put all new plants into quarantine.
• Maintain good soil structure and drainage.
• Space plants with mature sizes in mind.
• Avoid working with plants when they are wet.
• Avoid overhead watering.
• Sanitize garden tools frequently.
• Top dress the soil with aged chicken manure and rye-vetch green manure.
• Manage insect pests.
• Avoid mechanical injuries to plants.
• Remove infected plants and throw them in the garbage.
• Employ crop rotation.

I hope collar rot never occurs in your garden.