Strawberry Black Root Rot

Strawberry black root rot and crown rot are caused by multiple soilborne fungi that can severely affect plant vigor and productivity. If strawberries are planted in contaminated soil, loss in plant vigor becomes evident within a few months of planting and plant mortality rises during harvest – especially under conditions of environmental stress. In a perennial strawberry production system, disease severity increases each year leading to increased yield variability and eventual productivity loss.

Causes

Although a few fungal pathogens and nematodes have been associated with black root rot, Rhizoctonia spp. and Pythium spp. are documented as causal agents. Colletotrichum gloeosporioides, Phytophthora cactorum and Fusarium spp. infections also can quickly move to the crown resulting in crown rot and plant wilting. Lesion nematodes may predispose roots to black root rot-causing fungi that will develop into black cortical lesions. These lesions may girdle the whole root resulting in loss of function and mass giving the root a rat-tail appearance (Figure 1).

For U-pick strawberry production, growers prefer to grow strawberries in suitable plots year after year. As a result, soilborne pathogen populations build up over time causing significant disease problems and yield reduction. This is a much more significant problem for growers who utilize perennial mattedrow system with limited crop rotation options.

Management

With the phasing out of methyl bromide, soil fumigation techniques involving methyl bromide alternatives (synthetic chemicals) are not as effective and very difficult to apply due to new regulations. It emphasizes the importance of treating/keeping plants infection-free beginning with the nursery-produced planting stock. Options for sustainable management of black rot root and crown rot include:

• Rotation is the first line of defense. Growers should select a new piece of land for each new planting. The strawberry black root rot problem is similar to apple replant disease in that it manifests itself when susceptible plants are placed into soil previously occupied by a related species.
• Pre-colonizing the plant root system with beneficial microbes will boost plant vigor and keep the harmful microbes at bay. Strawberry plug plant producers can treat the tips and planting mix with a commercially available product suspension, such as Serenade, Terragrow, etc., before plug setting. Before planting, fruit growers can dip the plug or the plant’s bare root system in product suspension overnight to facilitate root system colonization. Studies indicate that probiotic bacteria-treated strawberry plants have higher vigor and less root disease.
• Biofumigation (the use of biologically active plant substances to control soilborne pests and diseases) of plots with high glucosinolate containing mustard cover crop, such as ‘Caliente 199’ or mustard meal, can provide both microbial suppression, as well as adding organic materials to the soil. If biofumigation cover crop is grown to the optimum stage and tissues are softened then immediately incorporated into the soil, it can suppress soil fungi and nematodes in addition to increasing soil biomass.
• Anaerobic soil disinfestation on selected plots can be done in three steps: 1) Incorporate organic material to provide carbon (C) source to activate soil microbes. Options for C source include rice bran at 8 tons/acre, mustard meal at 1 ton/acre or any legume or grass cover crop biomass; 2) These C sources are mixed into the soil with a walk-behind tilling machine, and then the area is covered with oxygen-impermeable tarp; 3) Soil is irrigated to saturation to create anaerobic conditions. This stimulates the anaerobic decomposition of the added organic material and enhances the diffusion of by-products in the soil. Accumulation of toxic/suppressive products deriving from the anaerobic decomposition kill pathogenic microbes. This process takes at least three weeks to complete.

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• Figure 1 photo credit: Frank Louws, NCSU

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