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Black Root Rot of Strawberries

Black root rot of strawberries is a very complex and serious disease that has been reported in strawberry fields around the world. Several pathogenic organisms have been associated with the disease; however, the soil-borne fungus Rhizoctonia fragariae is probably the most frequently isolated pathogen from strawberry roots exhibiting symptoms of black root rot. Several studies conducted in Connecticut clearly show that the root lesion nematode together with Rhizoctonia cause more severe black root rot symptoms than either organism alone. Other studies conducted in California have shown that water mould pathogens such as Pythium interact with Rhizoctonia to cause severe black root rot symptoms. Environmental factors have also been associated with this disease include soil compaction, excessive water and drought.

Black root rot affects primary feeder and perennial roots of strawberries (Figure 1). Often symptoms are not evident until a year or two after planting. Infected roots develop black lesions on the surface which eventually extend into the interior tissue resulting in the entire infected root becoming black and rotted inside and out. Symptoms are often difficult to diagnose because the epidermis of strawberry structural roots, which eventually develop into water conducting perennial storage roots, turn black naturally and maybe mistaken for black root rot. However, these healthy perennial roots are black on the outside but white inside. Infected plants with compromised roots appear wilted and stunted particularly around fruiting time when there is a lot of demand from the roots for water and nutrients. Other root rot diseases such as red stele and Verticillium wilt can also cause similar above ground symptoms, so never assume that wilted stunted plants are caused by black root rot. Regardless of the cause, compromised root systems of strawberry plants grown in a perennial productions system often results in low vigour, increased winter injury and eventually crown death.

Figure 1. Black root rot developing on primary and feeder roots of strawberry

So what can growers do to manage this disease? Once the disease has become established and plants exhibit symptoms, there is nothing growers can do to reverse the damage. Management of black root rot begins with avoiding the disease and keeping plants healthy and vigorous from transplanting through to harvest. Always inspect the roots of transplants obtained from nurseries and never plant strawberry transplants that have root rot symptoms. Fumigating fields prior to planting and avoiding planting in fields with compact soils will reduce the risk of black root rot developing. Planting cover crops such as pearl millet and oriental mustard that are not host to Rhizoctonia and suppress root lesion nematodes will also help reduce the risk of black root rot developing. Choose well-drained fields and avoiding planting strawberries in the same field for 2-3 years. Some strawberry cultivars such as Brunswick, Bounty, Cabot and Cavendish have been found to be tolerant to black root rot and should be selected for planting in fields with a history of the disease. OMAFRA specialists and a University of Guelph Researcher have teamed up to investigate new methods for controlling black root rot in strawberries.