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Drying Up Root and Crown Rot Pathogens

There is an old saying, “too much or too little water can kill or harm plants”. This simple statement speaks volumes in understanding plant disease prevention. Most plants can be affected by some form of root rot, usually caused by species of water molds: Phytophthora and Pythium, or by species of fungi: Rhizoctonia, Fusarium, and Thielaviopsis. These opportunistic, soil-borne plant pathogens infect plant root systems, where they thrive under low oxygen or anaerobic conditions. They take advantage of roots that are injured by excessive soil moisture and the resulting low soil oxygen conditions. Prolonged standing water or compacted, water-soaked soils cause oxygen levels in the soil to drop significantly. These soils with little to no oxygen are referred to being anaerobic, and this condition is favorable for root suffocation and injury, which allows for infection by soil-borne pathogens.

Symptoms of root rot, such as Phytophthora wilt caused by Phytophthora capsici are typically wilted foliage, poor growth, a lack of fruit development, and finally plant death.

Symptoms of root rot, such as Phytophthora wilt caused by Phytophthora capsici are typically wilted foliage, poor growth, a lack of fruit development, and finally plant death.

Symptoms of root and crown rots often begin with wilting of leaves and poor growth. Wilt is caused by the pathogen infecting and damaging the root system, which causes a decrease in water taken up into vascular tissues of the upper plant parts. Splashing water and irrigation runoff can spread these pathogens from plant to plant. The species of water molds are also capable of spread through soil water by motile spores. A healthy root system will have white feeder roots present, but with root rots, these feeder roots will be brown or black and rotting.

Some fungicides are effective, but only at suppressing these diseases, and they are often expensive, which leaves preventative cultural control measures as the better options.

Cultural Controls for Root and Crown Rot:

  • Proper planting site – Planting the right plant in the right conditions is important. Certain plants can tolerate “wetter” conditions. Evaluating your garden areas and determining what plant material best fits the landscape condition will greatly reduce potential problems down the road. Equally important is to ensure that the soil is well drained. Heavy clay soils can be amended with organic matter to improve soil structure, which allows for better internal soil drainage. For a list of plants species that can tolerate wet conditions, see Clemson HGIC 1718, Plants for Damp & Wet Areas.
Root rots, such as Black Root Rot caused by Thielaviopsis basicola, often attack plants, such as Hollies (Ilex spp.) when improperly planted, fertilized, or irrigated

Root rots, such as Black Root Rot caused by Thielaviopsis basicola, often attack plants, such as Hollies (Ilex spp.) when improperly planted, fertilized, or irrigated.
Photo credit Cheryl Kaiser, University of Kentucky, Bugwood.org.

  • Proper planting depth – Proper planting depth is crucial to healthy plant growth. Trees are the most common victims of this problem. A tree planted too deeply results in the root flare being covered by soil or mulch, which prevents proper air exchange in the root system, and this allows for excessive moisture adjacent to the trunk, which can lead to infection. The most common infection of trees and shrubs are crown rots caused by Rhizoctonia and Phytophthora species. Rhizoctonia prefers a moist soil for infection, and Phytophthora, being a water mold, prefers a wet soil. Wounds are not required for these crown rots. For more information on proper tree and shrub planting, see Clemson HGIC 1001, Planting Trees Correctly, and HGIC 1052, Planting Shrubs Correctly.
  • Providing proper irrigation and drainage – Most plants cannot tolerate wet conditions, i.e. standing water. Irrigation practices of providing a “deep watering” infrequently has shown to promote better, and healthier, root growth than irrigating lightly and frequently. Areas with standing water, or where water pools after rain events can be handled by installing French drains, catch basins, or even a drywell. Standing water or excessively moist soils in lawns can lead to a disease infection called Brown Patch or Large Patch in many grass species. Preventing prolonged standing water and reducing the frequency of irrigation are the best preventative measures in alleviating this problem. Vegetable garden drainage can be improved by installing drains, or using raised rows or inground beds to help increase internal drainage in garden soils. For more information on irrigating plants see Clemson HGIC 1810, Landscape Irrigation Equipment, Part 1: Sprinklers & Spray Heads.
Frequent irrigation or prolonged standing water can lead to Brown Patch on Fescue, caused by Rhizoctonia solani.

Frequent irrigation or prolonged standing water can lead to Brown Patch on Fescue, caused by Rhizoctonia solani.
Photo credit William M. Brown Jr., Bugwood.org.

  • Adding beneficial microorganisms– Adding organic matter is not just for improving soil structure, aeration, and drainage. It also introduces beneficial soil fungi and bacteria that feed on the soil organic matter, stimulate plant growth, and protect the roots from soil-borne plant pathogenic disease organisms. Good choices for organic matter include composted manure, garden compost, and organic soil conditioners. For more information on soil conditioning, see Clemson HGIC 1655, Soil Conditioning.

Using these techniques in combination with other sound horticulture practices can help reduce the likelihood of infection by root and crown diseases. Though there is no silver bullet to these issues, taking a proactive approach rather than reactive is always the best medicine.

If this document didn’t answer your questions, please contact HGIC at hgic@clemson.edu or 1-888-656-9988.

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