Black Root Rot of Strawberries
Black root rot is a common fungal disease of strawberries in Utah. Several fungi attack the roots of strawberries and eventually cause the death of some or all of the plants in the patch. Often, above ground symptoms are not apparent until after the roots are severely damaged by rot.
HEALTHY ROOTS
Two types of roots compose the root system of a healthy strawberry plant. The first are structural roots. These are hard and woody, and are covered by a bark-like tissue. The structural roots support the plant and the feeder rootlets. As their name suggests, these roots "feed" the plant by absorbing water and nutrients from the soil. A glistening white or cream color and extensive growth indicate healthy feeder rootlets. A healthy root system is necessary to produce a good yield of strawberry fruit.
SYMPTOMS
Plants infected with black root rot are often stunted and appear wilted: the fruit is small and the plants initiate few runners. As the severity of the disease increases, the leaves begin to turn brown and die from the edges inward. Affected structural roots are entirely rotted and dark or have dark brown, sunken areas along their length. The small feeder roots are usually absent or few in number. Affected plants usually die within the year, especially during hot weather.
CAUSE
Black root rot is caused by a complex of soil inhabiting fungi, including Pythium sp. and Rhizoctonia fragariaea, and possibly plant parasitic nematodes. The symptoms may be caused by a single agent or a combination of several pathogens. Adverse environmental conditions add to the problem by stressing strawberry plants and making them more susceptible to infection by the pathogens. Conditions that may contribute to black root rot are waterlogged soils, drought, nutrient deficient soils, winter injury, and chemical or salt damage. Black root rot may also be compounded by Verticillium wilt (Verticillium albo-atrum) which causes general decline and death.
CONTROL
Present control measures for black root rot and Verticillium wilt are based on preventing infection rather than curing already diseased plants.
Always begin with disease-free planting stock. Check to be sure that all plants have extensive, white rootlets. Do not use plants from a patch that has decline symptoms even if the new runner plants look healthy.
Plant in a light, well-drained soil if possible.
Avoid stressing strawberry plants. Keep the plants uniformly irrigated, but not waterlogged.
Apply nitrogen fertilizer in the late summer.
Cover plants with a coarse organic mulch to reduce winter injury.
If space is limited, the planting bed can be fumigated with a product like Vapam (Sodium methyl dithiocarbamate) which will kill the disease-causing organisms. This must be a pre-plant treatment applied 3 weeks before planting time. Fumigant type fungicides need to be purchased and applied by a certified pesticide applicator. Be sure to follow all directions on the fumigant label.
Sherman V. Thomson/Extension Plant Pathologist
Scott C. Ockey/Plant Disease Diagnostician
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use. Always read and follow the label.