Common Root Rot and Fusarium Foot Rot of Wheat

Cause and Occurrence

Common root rot of winter wheat is caused by the fungus Bipolaris sorokiniana. Fusarium foot rot, also known as dryland foot rot, is caused by fungi in the genus Fusarium. These diseases are most common in dryland winter wheat, especially in no-till and continuous wheat cropping systems. Several species of Fusarium are usually involved in the same disease but the most common Fusarium species causing foot rot are Fusarium graminearum and F. culmorum.

Species of Fusarium may look similar but a combination of morphological and molecular tools helps to identify and distinguish them. These fungi exist abundantly in the soil, on cereals, and on other grass hosts. These two pathogens can act synergistically with other Fusarium species in a disease complex that causes more severe root and crown rot diseases. Infection of wheat heads results in contamination of grain. If this grain is used as seed, seedling blights may occur. The fungi also cause leaf spots and/or blotches. In winter wheat, the diseases caused by these fungi occur throughout the growing season.

Symptoms

Symptoms of common root rot include dark brown to black lesions on roots, subcrown internodes, and stem bases. Lesions may coalesce, forming large areas of dead tissue in the crown. Discoloration of the subcrown internode (Figure 1) is diagnostic of common root rot. Infected plants may be stunted and/or chlorotic and occur randomly or may be seen in irregular patches in the field (Figure 2). Primary or secondary roots may appear brown or blackened.

The most common symptom of Fusarium foot rot is a dark brown lesion around the node of mature plants. In dry areas, the whole stem base may become girdled by a dark brown lesion (Figure 3). A diagnostic symptom of Fusarium foot rot is a cottony pink mycelium that may appear on affected stem bases. If the disease is severe, plants may mature early, produce shriveled grain, have white heads which may be void of kernels and appear bronze or bleached, or die prematurely. Occasionally one or more tillers on a plant may die. Scattered pockets of chlorotic (Figure 4), dead and dying plants (Figure 2) may be seen in affected wheat fields.

Disease Cycle

Bipolaris sorokiniana overwinters mainly as mycelium in infested wheat host debris and as conidia (asexual spores) in the soil. Mycelium (pl. mycelia) consists of strands of interwoven, largely microscopic, tubular hyphae (filaments) that make up the vegetative body of a fungus. Fusarium spp. overwinter as perithecia (sexual fruiting structures) and chlamydospores (thick- or double-walled asexual spores) in host debris. Initial infections occur on coleoptiles, subcrown internodes, and primary and secondary roots. Only these initial infections are responsible for root and foot rotting during the growing season.

If infections caused by B. sorokiniana progress above the soil line, secondary conidia are produced and dispersed by wind. They land and initiate lesions on leaves and tillers, causing a disease known as spot blotch, characterized by distinct, elongate brown-black lesions that are most frequent on lower leaves and most noticeable after heading. During wet weather, Fusarium spp. can also cause ash-colored or brown lesions on leaves.

Favorable Environmental Conditions

Common root rot and Fusarium foot rot are favored by drought and moderate to warm temperatures. Stress caused by dry seedbeds, loose seedbeds, wind, freezing, or damage from Hessian flies predisposes wheat plants to the two diseases. Severity of both diseases is higher in no-till and continuous wheat crop- ping systems. In contrast to common root rot and Fusarium foot rot that are favored by dry conditions, take-all, a disease which also affects roots and is caused by the fungus Gaeumannomyces graminis var. tritici, is favored by wet and poorly drained soils.

Cultural Management

• Seed adapted cultivars for the geographic area.

• Seed into a firm, mellow seedbed (loose seedbeds promote disease).

• Maintain a balanced soil fertility (avoid excessive ni-trogen fertilization, particularly by reducing the amount of fall-applied nitrogen).

• Controlweedsinsummerfallowland(weedsdepletesoil moisture and some weeds may act as alternate hosts to pathogens, predisposing plant roots to infection in the fall).

• Use certified, fungicide-treated seed for best results (bin-run seed is at higher risk).

• Seed at the recommended date (Figure 5) for your geographic area (early planting or extended, warm fall weather promotes disease).

• Seedwhenthesoiltemperatureatseeddepthis55-60 ̊F.

• Use the above cultural practices recommended for crown and root rot to also reduce the risk of winter injury.

• Rotate crops to reduce Fusarium foot rot inoculum; avoid planting wheat following corn or wheat.

Management with Seed Treatment Fungicides

Seed treatment fungicides (Table I) provide an early window of protection in the fall against common root rot and seedling blights caused by Fusarium spp. It is best to use certified seed that has been treated with fungicides and tested for germination and variety purity. When selecting a seed treatment product, pick one that has activity against common bunt and loose smut as well as common root rot and Fusarium spp. Uniformly coat the seed when applying the seed treatment product. Commercial seed treating equip- ment does the best job of uniformly coating the individual kernels. For drill box application, fill the drill box one-third full of seed, sprinkle one-third of the fungicide over the seed, and mix with a paint paddle. Repeat until the proper amount of fungicide has been added and mixed. Read and follow all label directions for mixing and application.

Acknowledgments

The authors would like to recognize the contributions of John E. Watkins, the original author of the material presented in this publication.

This publication has been peer reviewed.

Index: Plant Diseases
Wheat
2010, Revised June 2015