Rhizoctonia root rot of wheat

Rhizoctonia root rot occurs in temperate regions and is also called bare patch. It is caused by Rhizoctonia solani and R. oryzae. Field symptoms include bare patches or areas of uneven plant height. Severe infection causes plants to be stunted, appear drought stressed or nutrient deprived, and may result in premature plant death. Mild infection may go unnoticed aboveground. The pathogen “prunes” roots by girdling them, reducing root mass and capacity to transport water and nutrients to aboveground plant tissue. Roots develop characteristically fine, sharp-looking, red-brown points known as spear tip. Girdling can also make roots look pinched or constricted. Symptoms can be observed from seedling stage to ripening.

Roots develop characteristically fine, sharp-looking, red-brown points known as spear tip. Image: M. Burrows. 
Field symptoms of Rhizoctonia root rot include bare patches or areas of uneven plant height. Image: M. Burrows. 

Rhizoctonia root rot is generally more severe in light sandy soils and with direct seeding. Rhizoctonia spp. have a wide host range including a number of grasses and disease may be more severe when the green bridge is not adequately managed.

Practices that encourage residue decomposition may be helpful to reduce pathogen survival in soil debris. Tillage may reduce disease severity, particularly the bare patch disease phase. However, tillage has less of an impact on R. oryzae, which can persist in both direct seed and tilled systems. An effective management practice is to destroy grassy weeds and volunteer host plants that create the green bridge needed for pathogen survival from season to season. The interval between termination of grassy weeds/volunteer hosts and seeding should be a minimum of 2 to 3 weeks, although a greater interval improves disease control. Fungicide seed treatments can be used for early season protection against Rhizoctonia root rot. However, the pathogen can cause infection after fungicide seed treatment protection wears out. Starter fertilizer placed with or directly below seed may help by increasing seedling vigor.

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Gallery images: M. Burrows and J. Marshall