Garden Variety: Take steps to protect plants from root rot

Root rot diseases are common among all kinds of plants, and this year’s excessive rainfall is making it easier for many of the causal organisms of these diseases to proliferate. Because root rot diseases often kill the plants they infect and control after infection is difficult, prevention and early management are key to keeping plants healthy.

Prevention of root rot diseases takes a little understanding of the organisms that cause them and how plants grow. Most root rots are caused by fungi and water molds, both of which need adequate moisture to grow. Few of these fungi and water molds are aggressive, but they will attack weakened, stressed plants.

Many species of plants become stressed when they are in dense or poorly drained soil because water fills the air space in the soil and suffocates the roots. As root tissue dies, plants have a harder time moving water and nutrients to the top of the plant to maintain cellular function. Parts of the top of the plant die as a result of the root loss, and eventually the whole plant may die.

The fungi and water molds that cause root rots are common in soil around the world. They may also be introduced by water, wind, other microorganisms and other plants. They may be present in the soil without causing harm to nearby plants, but when a plant is stressed and starts losing tissue, the root rot organisms can easily move in and colonize in the plant tissue. As the plant becomes weaker, the pathogen continues to grow.

Signs of stress vary with plant species but may appear as a few yellow leaves or wilted appearance. Sometimes this prompts gardeners to water plants because they think the plant needs more, but that only makes the problem worse.

If plants show signs of stress and root rot is suspected, pull the plant out of the pot or dig around it to inspect plant roots. Roots that are infected with root rot diseases typically appear brown and soft. The outer layer of the root may separate from the white, inner layer like a snake shedding its skin. This is called sloughing. Mold or white spiderweb-like fungi may be visible on the surface of the plants’ roots or near the crown of the plant.

Trees are harder to diagnose but are susceptible. In extreme cases, mushrooms may appear on roots or on the crown of the tree.

The best way to prevent and treat this type of stress is to improve soil drainage.

Prevention is easiest and is best achieved prior to planting. Add compost or organic matter to the soil and mix it in before planting. In areas that need a lot of improvement, building a raised bed or berm (mounded planting area) may be easier than amending existing soil. In container plantings, use high quality potting mix rather than garden soil.

In existing planting beds, add compost to the soil surface and mix in carefully to avoid damaging plant roots. In lawns and around mature trees, use a core aerator (or have a lawn care company core aerate the area) to reduce compaction. Fall, especially September and October, is one of the best times to core aerate lawns. Spread compost across the lawn after core aerating for additional improvement.

Pay close attention to plants’ water needs and avoid planting species that are especially susceptible to root rots in areas that are poorly drained. Avoid overwatering. Most outdoor plants need about one inch of water per week. For container plants (indoor and outdoor), stick a finger into the soil or potting mix to check for moisture below the surface even if the surface looks dry. Water only when the soil or potting mix is dry a few inches below the surface. Make sure containers have drainage holes.

If soil drainage and water cannot be controlled and plants succumb to root rot, remove them as soon as possible. Avoid replacing with the same species and make soil improvements prior to replanting in the space.

For container plants, dispose of soil along with the plant. Sterilize the entire pot with a bleach solution or other sanitizer before reusing the pot.

The most common causes of root rot are the water molds phytophthora and pythium and the fungus rhizoctonia.

— Jennifer Smith is a former horticulture extension agent for K-State Research and Extension and horticulturist for Lawrence Parks and Recreation.

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