Root rot on Plants: Causes, Treatment & Prevention

Symptoms

Start with the roots, where an early symptom of infection appears. Small brownish lesions encircling the fine feeder roots are an early symptom of infection. In affected plants, the fine roots turn black, brittle, and dead. The roots can also turn black and become mushy. Dark cankers may appear on the lower trunk and on the main roots.

Above ground, the leaves wilt, discolor, stay undersized, and drop prematurely. You may see yellowing and wilting of the foliage across the whole plant or just part of the canopy. Twigs and branches die back as well. Wilting, yellowing, and dieback of twigs and branches are the signs that show in the canopy.

The trunk can show signs too. Dark sap, whether black, brown, or reddish, may ooze from darkened bark on the trunk. A brown or reddish streak, stain, or canker may also be visible in the wood under the bark.

Taken together, the symptoms run from small lesions on feeder roots to dying branches and dark sap on the trunk.

Causes

Root rot depends on wet conditions around the roots. The pathogens require high soil moisture to infect the roots of a host plant. They thrive in areas of excess soil moisture and poor drainage. Moist, soggy soil at moderate temperature is another condition listed for the disease. Overwatering or damaged roots can also lead to the problem.

Some cases come from a soil-borne fungus. Rosellinia fungi, most commonly R. necatrix, are among the fungi behind the disease. Over 300 species of woody plants can be infected by Rosellinia species.

The pathogen can also reach your garden in several ways. Infested nursery stock can introduce it to a new site. Water can spread the pathogen between plants. Waterborne spread is a separate route from infested nursery stock.

If your soil stays soggy or drains poorly, those are conditions linked to root rot. Overwatering and damaged roots are listed together as one cause.

Diagnosis

The roots give the most direct evidence of root rot. Small brownish lesions on feeder roots point to infection, as do roots that are black, brittle, dead, or mushy. Leaf symptoms are only part of the picture. Wilting, yellowing, and premature leaf drop can appear on the whole plant or on part of the canopy. Check the trunk for dark cankers, darkened bark with oozing sap, and brown or reddish streaks in the wood under the bark.

Then think about the conditions around the plant. Ask whether the soil is soggy or drains poorly, since excess soil moisture and poor drainage are where the pathogen thrives. Ask whether the plant has been overwatered or its roots damaged. Ask whether the plant came from a nursery, because infested nursery stock is one way the pathogen arrives.

Root, trunk, and wood signs are the ones that point to infection, while leaf changes alone do not settle it.

Treatments

Cultural steps come first. Manage irrigation well, since good water management is the most important factor in reducing the threat of root and crown rot. Improve drainage where soils are compacted, drain poorly, or are often soggy. Avoid overcrowding, overwatering, and overpotting.

Keep foliage dry rather than wetting the leaves. Provide ample air circulation around plants. Keep tools and pots sanitary.

Plant only nursery stock certified to be free of Phytophthora spp., and inspect the roots before planting, rejecting plants with decayed roots or crown rot. Start with pathogen-free nursery stock from a reputable source. Remove and dispose of dead or infected plants, trench around the site to break root grafts between neighboring plants, and avoid replanting the site with known host species.

Prune and discard infected plant parts as soon as a problem is detected, and discard severely affected plants.

There is also one chemical option. Certain pesticides help control some Phytophthora species, and phosphonates are the active ingredient named for this purpose. Larger plants, those bigger than medium-sized shrubs, are difficult to treat effectively in landscapes. The product label is the legal instruction, so follow it for the crop and site being treated.

Prevention

Prevention is mostly about the conditions your plants live in. The pathogen thrives where soil stays wet and drains badly. Before you buy, look over a plant's roots and reject any with decayed roots or crown rot. Choose nursery stock certified free of Phytophthora, from a reputable source.

Good water management is the most important factor in lowering the risk of root and crown rot. Avoid overcrowding and overwatering, and keep leaves dry instead of wetting them. Leave room between plants for air to move, and avoid overpotting.

Keep your tools and pots sanitary. Where a host plant has died, avoid replanting the site with known host species.

Plants most often affected

Over 300 species of woody plants can be infected by Rosellinia species. Hosts include apple, avocado, ceanothus, cotoneaster, holly, pear, poplar, privet and viburnum. That makes a large group of woody plants possible hosts for this disease. If you grow one of the plants named here, watch its roots and trunk for the signs described above.

Root rot on Plants

At a glance