Introduction

Gaeumannomyces graminis is pathogenic on many members of the Poaceae family, but is most commonly a problem on wheat, barley, rye, oats and turf grass. G. graminis var. tritici is the form of the fungus that attacks wheat and related species, whereas G. graminis var. avenae attacks oats and G. graminis var. graminis is problematic on turf and other grasses. Since the pathogen takes all of the grain and leaves an obvious ‘white head’ that is empty of seed, the disease it produces is commonly known as Take-all.

Host Range and Distribution

This pathogen has a worldwide distribution under temperate conditions or in dry areas where irrigation is used. Take-all development is favored by moist, cool soils ranging from 12° to 20°C. Severity of take-all is increased where alkaline, poorly drained, compacted soils are present. Soils deficient in nitrogen, phosphorus, and/or copper can increase disease severity. These conditions are present in the Pacific Northwest of the US, but the disease occurs in other parts of the US as well.

G. graminis has a wide host range within the Poaceae family. Winter wheat is one of the major hosts for the pathogen, and it occurs wherever winter wheat is grown. The disease is most common where wheat is grown continuously or without adequate rotation, but it can occur in wheat the first year out of sod.

Isolation

G. graminis is a slow-growing fungus, causing difficulties when trying to isolate the fungus from host tissue. Following good protocol assists in this process. Host tissue should first be washed under tap water for one hour, then surface disinfested with 1% silver nitrate for thirty seconds followed by rinses with sterile distilled water (1). The tissue should be plated out on a semi-selective medium, such as SM-GGT3 which contains PDA amended with L-DOPA, antibiotics, and fungicides. The L-DOPA turns black in the presence of hyphae from G. graminis (1). Isolations can be made from perithecia on the host tissue by spreading asci and ascospores onto PDA and incubating at 20° to 25°C (1).

Ascospores are not released into the soil matrix, which makes isolations from soil difficult (1). Isolation from soil is thereby obtained through planting susceptible hosts in the suspect soil at 15° to 20°C and –10kPa for 3-4 weeks, then removing them and following the same isolation techniques from host tissue as above (1). Identification

G. graminis is an ascomycetous fungus, in the class Pyrenomycetes, order Diaporthales (2). This fungus produces a perithecial ascocarp within the host tissue, composed of stromatal and host tissues (2). The ascospores are multiseptate and filiform, and conidia may act as spermatia (2). Several characteristics have been determined that separate this genus from the other fungi in the Diaporthales, including its (necrotrophic) nutritional relationship with plant roots, prominent hyphopodia, and anamorphs that are Phialophora-like hyphomycetes (2).

Symptoms

Infected plants appear stunted or off-color at the beginning of the season, but usually are attributed to problems related to nutrition or soil moisture. The root system will appear dark brown to shiny black with lesions (Figure 1), which would not be seen with other problems. These plants will die prematurely within circular patches in the field. Before normal maturity, diseased plants will appear bleached or straw-colored. Seeds are not produced in the heads of infected plants. Instead, the fungus causes a whitening of the head, often referred to as a whitehead (Figure 2). Sometimes there is also a crown dry rot that includes dark brown to black mycelium at the base of the plant. These obvious symptoms appear after heading.


Fig. 1. Infected roots (courtesy Missouri Extension Service)	Fig. 2. Whiteheads (courtesy Oregon State Univesity)

Ecology and Life Cycle

Survival occurs in the field saprophytically in crop residue, on grassy weeds, and on volunteers. Through tillage practices, growers can move the pathogen on grassy weeds from nearby ditches. After a field is planted, the fungus moves saprophytically from the old residue onto the roots of closely emerging seedlings under high moisture conditions. It infects the seedlings’ root cortical cells. The pathogen can then spread from plant to plant through root bridges and “runner” hyphae. The fungus also produces aerial ascospores that may play a role in initiating new infections.

If the pathogen is present in a field at harvest, it is recommended that the field lay fallow or have another crop planted to allow G. graminis to die out in the field. The pathogen has a low survival rate and is usually only a problem where monoculture is practiced.

Fig.3 Lifecycle of G. graminis on wheat

Links to Other Sites

Take-all – Gaeumannomyces graminis: Alberta Agriculture, Food and Rural Development

Take-all of Wheat: Pest Management & Crop Development, University of Illinois Extension

Kentucky Plant Pathology: Take-all of Wheat

The Take-all Forum: Monsanto

Selected References

1. Mathre, D.E. 1992. Gaeumannomyces. Pages 60-63 in: Methods for research on soilborne phytopathogenic fungi. Singleton, L.L., Mihail, J.D., and Rush, C.M., eds. APS Press, St. Paul, MN.

2. Alexopoulos, C.J., Mims, C.W., and Blackwell, M. Introductory Mycology. 4^th Ed. John Wiley & Sons, Inc. 1996.