Janna Beckerman, Associate Professor, Department of Botany & Plant Pathology
The major function of the root is to anchor the plant to the soil, and to absorb water and nutrients for the plant. The below ground aspect of the plant is often overlooked in plant health, as the roots are rarely observed in their entirety even though the structure of the root system profoundly impacts above-ground plant health. As a result, these problems are frequently underdiagnosed. Dry and droughty weather forces a plant’s root system to work harder—if it is rotted, it may not work as well as it should, resulting in vague symptoms of slow(er) growth, decline in crown, smaller leaves that may or may not be chlorotic, heavier seed crops (in landscape plants), and the simple description “It just doesn’t look good.” Often, secondary insects and opportunistic fungi attack these plants, and are blamed for the overall poor health. Plant death results in the plant not being able to take up sufficient water or nutrients to support the branch, leaf, flowers, or fruit development.
Examining the root of an infected annual is easy-- simply pop it out, rinse and examine the roots for signs of rot that include poor root growth, or yellow, brown or blackened roots. Unfortunately, examining the roots for established plants, particularly trees and shrubs, is difficult, and may not be possible in all situations without a tree care professional. After removing the soil from the base of the tree, look for the following problems:
- Excessive mulch
- Planted too deeply or shallowly
- Significant root or root flare damage
- Stem girdling, or root girdling root
- Few main roots or roots on only one side of tree or shrub
- Feeding damage
- Root rot
The primary causal agents of woody plant root disease are fungi like Rhizoctonia, Cylindrocladium, Thelaviopsis, and Fusarium (to name but a few!), and the pseudofungi, like Phytophthora and Pythium. Although very different organisms, they are all able to survive on dead and dying tissue. Some of these fungi are opportunists, attacking those plants suffering from poor site, drought, flooding, or other abiotic disorders commonly found in the urban landscape. A diagnosis of the root rot is necessary to develop a good management plan, including the right fungicide, good cultural controls, and what other nearby plants may be susceptible. Knowing a site has an established history of root rot should discourage landscapers, planners, or homeowners from planting future susceptible plants (e.g., a Phytophthora- infected sites should not be planted with Rhododendron, Azalea, Pieris, or similar plants) in the same area.
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Root rots not only affect the roots, but cause discoloration in the above-ground portion of the plant as well.
Rhododendron are especially susceptible to Phytophthora root rot. Although the infect probably occurred during a very wet period, symptoms often develop during drought, when the plant is challenged to provide water.