Understanding Plant Diseases, Pt. 3: Diagnosing Root Diseases
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Root rots are one of the most significant causes of loss to ornamental and vegetable plants in the coastal region. In addition to being fatal to plants, root rots may also diminish the quality and production of other plants in your ornamental or vegetable gardens. And unlike leaf diseases, which may vary in intensity from year to year, soil borne pathogens may persist in the soil for many years, even if a suitable host plant is not present. These factors alone make root rots one of the most frustrating and expensive diseases to manage in commercial and home landscapes.
Though root rots are generally untreatable, knowing the symptoms of root rot can help you identify a problem and take necessary precautions to isolate the problem. Many fungi cause root rot and control is next to impossible, primarily due to the fact that once symptoms are noted above ground, the fungi has already done serious damage to the root system below ground.
Oftentimes the first signs of root disease will be that your plants have reduced growth or vigor. This is most apparent in a bed where you have multiple plants of the same species that look healthy and are growing normally. A sudden yellowing or browning of the lower leaves, followed by leaf drop are signs of root disease. Other things to look for include ‘flagging’ of leaves, which may also be a sign of drought stress. Cutting off limbs from the plant may reveal bluish-black streaking inside the cambium layer. Though this does not apply only to root rot, this can help you better determine where a disease is present in the plant.
Further inspection below ground may reveal dark brown or black roots with only a few or no white roots. Further, only the larger pieces of the root system will be present, with no smaller fibrous roots. A diseased root system will have limp or brittle roots that slough off easily from the main part of the plant. Healthy roots should be strong and feel crisp to the touch.
Phytophthora root rot
A serious and widespread problem to many ornamental plants is Phytophthora root rot. This diseases affects many plants that are grown in our area including, Camellia japonica, dogwood, azaleas, junipers, aucuba, to name a few. Symptoms include flagging or wilting of leaves, and interveinal chlorosis, which may be confused with nutritional deficiencies. Roots may appear reddish-brown in coloration, and are often brittle and limited to the upper portion of the container or landscape bed. Choosing disease-resistant varieties is the only way to reduce the occurrence of Phytophthora in landscapes.
Another common root disease that affects many woody ornamentals and vegetables as well is Verticillum wilt. This disease will lie dormant in the soil for many years waiting to colonize unhealthy root tissue. Once it infects root systems, look for heavy seed production (woody ornamentals), smaller-than-normal leaves, and browning along the margins of leaves. Wood under the bark of the tree will have discolored streaks varying from green to black in coloration. There is no treatment available for Verticillum wilt in home landscapes, though resistant cultivars exist. Refer to a trusted nursery grower or vegetable seed guide to determine which are resistant to this disease.
Southern stem blight
Southern blight spreads rapidly through vegetable and fruit gardens, and affects a wide range of plants, ranging from beans and cantaloupes, to tomatoes and watermelons. Though not technically a true root disease, stem blight may persist in the upper 2-3 inches of soil and causes yellowing and wilting of leaves on plants. For plants like watermelons, the runners may be affected and tomatoes and peppers usually rot away at the soil surface. White moldy growth is evident on affected stem tissues and adjoining surface soil. Control is limited to prevention and sanitation, though these are arguably your greatest tool in managing southern stem blight.
Diagnosing ornamental and vegetable plant diseases in our area is difficult. But while it can be frustrating, the next step to diagnosing diseases in your landscape is to bring in a sample to your local extension office. If you suspect root or leaf diseases in your landscape, a sample can be properly diagnosed through the Plant Disease and Insect Clinic in Raleigh for a $20 fee. If you suspect a disease in your landscape plants, contact your extension agent and they can help you process plant samples for diagnosis.