Urocystis agropyri (flag smut of wheat).

U. agropyri causes flag smut on leaves of species in the family Poaceae. As a pathogen of grasses, it appears to have a wide host range (Mordue and Waller, 1981) and a worldwide distribution (UK CAB International, 1991). However, some authorities do not include the pathogen on wheat [Triticum aestivum], identified as Urocystis tritici, within U. agropyri. Rossman et al. (2006) place U. agropyri in the category of a "Threat to Major Crop Plants" and wheat and wheat straw imports are restricted in North America (Anon., 2005; CFIA, 2008). If the widespread species includes the wheat pathogen, then it is already present on all continents with agriculture and in major wheat-growing areas (Purdy, 1965), so it has already been introduced and may be difficult to exclude from additional areas. Both smuts are seed- and soil-borne, causing systemic infections that can be perennial in weeds and graminaceous crops, including turfgrasses. The spore balls are windborne (Purdy, 1965) and prevention of spread among wild grasses on land is not amenable to control.

First Report of Flag Smut of Wheat Caused by Urocystis agropyri in the United Kingdom

An isolated occurrence of flag smut of wheat, caused by Urocystis agropyri (G. Preuss) J. Schröt., was confirmed in the county of Essex, UK, in autumn-sown winter wheat (Triticum aestivum L.) cv. Riband grown from certified seed. The origin of the infection is unknown. Symptoms were first observed on 22 May 1998. The presence of U. agropyri was confirmed on the basis of the macroscopic symptoms on the host and the morphology of the pathogen (2). Long gray-black streaks parallel with the veins were observed on leaf blades, especially the flag leaf, and on leaf sheaths. The streaks consisted of lines of fungal sori developing between the leaf veins, giving a striped appearance. Eruption of the sori through the epidermis of the leaves had caused them to fray, exposing dark brown to black powdery spore masses. Microscopic examination revealed spore balls measuring 20 to 28 μm in diameter containing 1 to 3 spores per ball. Spores were dark brown measuring 12 to 17.5 μm in diameter and were surrounded by light brown sterile cells measuring 7 to 7.5 μm. Affected plants were stunted to about half the height of healthy plants. Some affected plants tillered excessively and in some cases the ear failed to emerge from the boot. Some diseased leaves were twisted and, where the host epidermis had ruptured to expose the spore masses, the affected parts had the appearance of being covered in black soot. All of the symptoms are typical of flag smut on wheat. Flag smut has been observed on grasses in every continent except Antartica. However, the distribution of the pathogen on wheat is more limited. Flag smut of wheat is known to occur in at least some of the countries of the European Union as well as (e.g.) the USA and Australia. This finding is significant because it had been considered that U. agropyri on wheat had reached the limits of its ecoclimatic zone, since it requires specific environmental conditions for infection and disease development, particularly a mild winter and an arid summer (1). The climatic conditions in Essex between September 1997 when the affected crop was planted and May 1998 when symptoms were first observed were considerably drier and warmer than normal, thus favoring the pathogen. This is the first report of U. agropyri on wheat in the UK. References: (1) R. F. Line. 1998. Pages 49–60 in: Bunts and Smuts of Wheat: An International Symposium. No. Am. Plant Prot. Org., Ottawa. (2) J. E. M. Mordue and J. M. Waller. 1981. Urocystis agropyri. CMI Descriptions of Pathogenic Fungi and Bacteria, No. 716. Kew, England.