Cooperative Extension Service
The University of Georgia College of Agricultural and Environmental Sciences
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Mila Pearce
Extension Plant Pathologist
Pythium Root Rot
Black Root Rot
Myrothecium Crown Rot
Cercospora Leaf Spot
Downy Mildew
References
The fungus Pythium is probably the most common pathogen associated with root rot disease in landscape bedding plants. Pythium is inclusive of all soil types and has a global distribution. Disease severity depends on environmental stress, susceptibility and health of the host.
Aboveground symptoms include in stunting, wilting, and chlorosis with eventual death (Fig 1). Roots are necrotic, often starting at the root tip, but soon entire root systems are glassy, brown and shriveled. The cortex is easily sloughed off leaving only the vascular center intact (Fig 2). The base of the plant may also become necrotic as the fungus moves through the roots several inches to cause crown rot.
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| Figure 1. Wilting, chlorosis and collapse of plants. | Figure 2. Cortex sloughed off from necrotic root. |
Since Pythium is ubiquitous, virulence depends on the health of the plant and environmental conditions. Excessive soil moisture and high fertility are conducive for disease development. Pythium may be spread by soiled tools or contaminated plant material, but most often by splashing water.
The best management strategy is exclusion of the fungus by ensuring all plants are healthy and not contaminated before planting. Do not plant pansies too close together or apply too much mulch. Promptly remove any diseased plants and discard. Apply irrigation only as needed, and the soil drain properly. Selective groups of chemicals may be applied if the disease is detected early. Consult your local county extension agent for a list.
The fungus Thielaviopsis can infect a broad range of ornamental plants including geranium, petunia, snapdragon, vinca and pansy.
Black root rot can be easily confused with Pythium root rot. Aboveground symptoms include chlorosis and stunted or poor growth (Fig 3). The roots are discolored resembling the decayed roots of Pythium infected plants. However, upon closer inspection, the roots are distinctly black in color (not brown) and lesions are numerous but separate (Fig 4). Roots do not become mushy and slough off as those affected by Pythium.
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| Figure 3. Chlorosis and stunted growth. | Figure 4. Distinct black and sparse lesions. |
Thielaviopsis can survive in many different soil conditions and is stimulated by exudes from root growth. A suitable soil pH for fungal germination ranges from 5.0 to 8.5. However, root rot is more severe in plants grown in neutral and alkaline soils and disease is reduced at pH levels below 5.5. Optimal temperatures for root rot are 56 -64 degrees F. Factors that induce plant stress such as drought stress, temperature extremes, and over-fertilization also increase disease.
The best management strategy is exclusion of the fungus by ensuring all plants are healthy and not contaminated before planting. Do not plant pansies too close together or apply too much mulch. Promptly remove any diseased plants and discard. Apply irrigation as needed. The soil should have a 5.6 pH with low phosphorus and high ammonium nitrogen. Selective groups of chemicals may be applied if the disease is detected early. Consult your local county agent for a list.
Myrothecium is fungus that is widely distributed with a broad range. Although considered a weak pathogen that invades injured or stressed tissues, it can be a serious problem when plants are injured due to transplanting in flower beds.
Much like Pythium or black root rot, above-ground symptoms result in stunting, poor growth, chlorosis, plant wilt, and eventual death (Fig 5). However, unlike Pythium and black root rot, a distinct characteristic of Myrothecium is sporodochia. Sporodochia can be seen on the lower stem or just above the soil line. They appear as small black dots and may have white fringes surrounding them (Fig 6). Roots do not appear to be necrotic, but the plants are easily pulled away from the roots because of the crown decay.
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| Figure 5. Wilting and death of plants. | Figure 6. Sporodochia at base of plant. |
Moderate temperatures of 65-85 degrees F favor disease development, but hot intermittent temperature extremes that stress the plant are of particular concern. Injury from transplanting and excessive chemical application also favors disease development. Spores are dispersed primarily by splashing water.
The best management strategy is exclusion of the fungus by ensuring all plants are healthy and not contaminated before planting. Avoid wounding pansies during transplanting. Do not plant pansies too close together or apply too much mulch. Promptly remove any disease plants and discard. Apply irrigation only as needed. Selective groups of chemicals may be applied if the disease is detected early. Consult your local county agent for a list.
Cercospora leaf spot is the most common disease of pansy in the southeastern United States.
Irregular purple lesions appear first on the lower leaves which have a feathered margin. The upper leaves soon develop small purple flecks as the disease develops (Fig 7). In the advanced stages, the leaf spots develop tan centers with purple borders (Fig 8). Overall plant health and vigor are compromised.
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| Figure 7. Irregular purple lesions. | Figure 8. Tan centers on advanced lesions. |
Cercospora favors warm temperatures and wet conditions and is most often seen in late spring and fall. The disease is mainly spread by splashing water; therefore, the disease usually starts near the base of the plant and works upward. The pathogen is able to over-winter in the infected foliage and debris.
The best management strategy is exclusion of the fungus by ensuring all plants are healthy and not contaminated before planting. Do not plant pansies too close together or apply too much mulch. Remove severely infected plants to reduce disease spread. Discard or destroy all plant debris. Avoid overhead irrigation and use only as needed. Selective groups of chemicals may be applied if the disease is detected early. Consult your local county extension agent for a list.
Once confined to the Pacific Northwest, downy mildew caused by the fungus-like organism Peronospora can cause serious losses across the United States in a wide range of hosts including snapdragon, viola, salvia, and pansy.
Light colored blotches with gray-purple spores below appear on leaves (Fig 9). The congregation of spores beneath the leaf has a gray, fuzzy texture (Fig 10). Leaves may curl under and become distorted to resemble aphid damage.
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| Figure 9. Chlorosis on top of leaves. | Figure 10. Gray fuzzy growth due to sporulation. |
Downy mildew favors wet and cool weather conditions. It is most severe when nights are cool and days are warmer with high humidity. Since downy mildew grows within the plant tissue and not on the surface, it can escape notice until the conditions are ideal for sporulation.
The best management strategy is exclusion of the fungus by ensuring all plants are healthy and not contaminated before planting. Infected plants should be removed and discarded as quickly as possible. Do not plant pansies too close together or apply too much mulch. Selective groups of chemicals may be applied if the disease is detected early. Consult your local county extension agent for a list.
Alabama Cooperative Extension Service http://www.aces.edu/pubs/docs/A/ANR-1214/
Barnes, L.W. Texas A&M University, Department of Plant Pathology http://plantpathology.tamu.edu/directory/faculty/barnes.asp
Chase, A.R. Downy Mildew Control. Western Connection. Volume 1, Issue 6. February1999.
Daughtrey, M., Wick, R., Peterson, J. 1995. Compendium of Flowering Potted Plant Diseases. American Phytopathological Society Press, St. Paul, Minn.
All photos were generously given by
Dr. Larry W. Barnes.
Bulletin 1281/June, 2005
The University of Georgia and Ft. Valley State University, the U.S. Department of Agriculture and counties of the state cooperating. The Cooperative Extension Service, the University of Georgia College of Agricultural and Environmental Sciences offers educational programs, assistance and materials to all people without regard to race, color, national origin, age, gender or disability.
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