Black root rot
Rhizoctonia fragaria, Pythium spp., Fusarium spp.
Strawberry black root rot (BRR) is the most common root disease in Minnesota. This disease is a complex problem involving several different pathogens along with a variety of plant stresses. It is common in older strawberry patches or patches stressed by poor growing conditions like soil compaction or poor drainage. The condition is aggravated by root feeding nematodes (soil borne microscopic worms) and winter injury. Plants infected with BRR decline overtime, producing significantly lower yields than uninfected plants.
The first symptoms of BRR are often missed. Infected plants have poor growth and produce fewer and smaller fruit. As the disease becomes more severe, plants are clearly stunted. Plants may wilt and the edges of leaves turn brown or ha a 'scorched' appearance. Plants continue to decline and often die after the high stress of fruit production. In larger patches, disease often starts in low lying areas or areas with poor drainage. Each year the area of infected plants expands.
Plants displaying the above symptoms should be carefully dug up and washed, keeping intact as much of the root system as possible. A healthy plant will have young roots that are creamy white with multiple fine root hairs and older roots will have a dark brown to black woody outside layer but a white interior. Plants with BRR are often described as 'rat tail' because most of the finer feeder roots are rotted away leaving only the thick anchor roots. The remaining young roots have random gray to reddish brown sunken blotches. These lesions can expand to cover large areas of the root. The infected roots are soft and mushy. When touched, the outer layer often falls away, leaving only a thin strand from the core of the root.
One or more of the black root rot pathogens are commonly found in soils. Disease develops when plants are stressed by drought, water-logged soils, winter injury, poor nutrition, and root feeding by lesion nematodes (Pratylenchus penetrans) or insects. Pythium spp., Rhizoctonia spp., and Fusarium spp. are root rotting fungi that infect and rot roots of stressed strawberry plants. This complex interaction of root rotting pathogens, environmental factors and other pests is known as black root rot. Root tips and young feeder roots may be completely rotten and fall off. Infection in older roots is limited to the outer tissues of the root; leaving a white core that is unaffected. The disease commonly occurs in fields with a long history of strawberries where the pathogenic fungi have had significant time to build-up their numbers.
Prevention of black root rot is based on good site selection and proper plant care.
- At this time there are no strawberry cultivars that are resistant to black root rot, however, gardeners should choose a cultivar that is hardy in Minnesota to reduce winter injury and stress on the plant.
- Purchase new plants from a reputable supplier. Roots of young strawberry plants should be white and fleshy.
- For new patches, choose a location where strawberries have not been present for the past 2-4 years.
- Choose a location with good drainage or improve drainage before planting through adding organic matter to soil and redirecting water away from the area. Strawberries can also be planted on raised beds where drainage creates a soil environment less favorable to some root rotting fungi.
- Add organic matter like, high quality compost, peat or straw to the soil prior to planting. This will improve drainage and encourage growth of beneficial microorganisms in the soil.
- Use a soil test to determine optimum fertilizer applications for the site.
- Renovate patches of June bearing strawberries each year after harvest to maintain a healthy vigorously growing patch.
- To avoid winter injury, apply two to three inches of straw in the fall after several frost events below 20 F and above 30 degrees F.
- In existing patches with black root rot, consider starting with new plants in a new location. Do not relocate old plants to the new location as the BRR pathogens will be carried on the roots of infected plants.
There are no pesticides registered for use by home gardeners that are effective in controlling BRR.