Research Update – Rhizoctonia Belly Rot of Cucumbers

By Elaine Roddy, OMAFRA and Cheryl Trueman, University of Guelph – Ridgetown Campus

Rhizoctonia belly rot has been identified in certain Southwestern Ontario cucumber fields since 2011. The pathogen, Rhizoctonia solani, thrives under hot, humid weather conditions and is more commonly associated with growing conditions in the Southern United States.

Belly Rot on Cucumber Fruit
Figure 1. Belly Rot on Cucumber Fruit

Under suitable conditions, symptoms appear in as little as 24 hours on the underside of the fruit, or where the blossom-end rests on the soil surface or on decaying plant material. Irregular-shaped, yellow-brown discoloured patches develop on fruit (Figure 1). These patches do not usually penetrate below the surface of the skin. Lesions later develop a scab-like appearance and fruit fly maggots may occasionally be found feeding on the damaged area.

Research at the University of Guelph Ridgetown Campus sponsored by the Ontario Cucumber Research Committee, is evaluating the efficacy of foliar fungicides applied at different crop development stages for reducing damage to this disease. Fungicides and biofungicides were evaluated in growth chamber screening trials over the winter months using greenhouse mini-cucumbers. Quadris Top and Fontelis, which are both registered for other fungal diseases on cucumber, reduced disease severity in the growth chamber studies. They are now being evaluated under field conditions along with one experimental product.

This disease may be difficult to control with standard foliar fungicides because protective sprays are unlikely to reach the lower surface where the fruit infection occurs. The field evaluation at Ridgetown includes a variety of fungicide application timings including the 1-3 leaf stage, vine tip-over, and different fruit development stages.

Since standard foliar fungicide applications may not be a silver bullet solution, the tolerance of different cucumber cultivars to belly rot is also being evaluated in two other trials at Ridgetown Campus. The incidence and severity of belly rot on five common pickling cucumber cultivars grown in inoculated field soil will be assessed in the first trial. In the second trial, healthy field-grown cucumber fruit from four cultivars will be collected and evaluated for disease development after exposure to inoculated media in a growth chamber.

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