First report of a Labyrinthula spp. causing rapid blight of Agrostis capillaris and Poa annua on amenity turfgrass in the UK

In August 2004 turf samples showing watersoaking and discoloration of Agrostis capillaris and Poa annua (Fig. 1) were received from a golf course in Scotland. The watersoaked patches increased in size and number during periods of warm dry weather to around 25 cm diameter, and the affected sward continued to discolour to red-brown over several weeks (Fig. 2). Microscopic analysis of affected tissues revealed fusiform cells (4 to 5 x 15 to 20 μm) in all samples which are typical of those described for the genus Labyrinthula (Pokorny, 1967). Based on field symptoms and microscopic characteristics, the cells were described as a Labyrinthula spp. (Martin et al., 2002; Olsen et al., 2004). Although commonly referred to as marine net slime moulds, the taxonomy of Labyrinthula is uncertain. Although presently classified in the kingdom Chromista, they are not closely related to other included members. Labyrinthula species are single-celled organisms that occur in aggregations in culture and which exhibit a distinctive gliding motion within a colonial network of slime filaments.

The Labyrinthula spp. was isolated by placing leaf and root tissues with symptoms on to 1% horse serum water agar (HSA), using saline water prepared by diluting saltwater (Tropic Marin® SEA SALT). The cells formed visible colonies (Fig. 3) on this medium within 3 days and were morphologically identical those previously described on cool-season turfgrasses (Olsen et al., 2003).

Three-week-old A. capillaris seedlings and P. annua plants were inoculated with the Labyrinthula spp. isolated from affected turf. The plants were grown in plastic cups filled with autoclaved vermiculite, and nutrients were supplied using Hoagland's solution. Grasses were inoculated by placing colonised agar plugs in contact with the leaves and irrigated with saline water (electrical conductivity [EC] = 3.5 to 4.0 dS/m). Controls were prepared in which grass plants had been inoculated with clean agar plugs and irrigated with saline water. Two replicate experiments were prepared for each grass. Leaf tissues of Labyrinthula spp. inoculated plants showed symptoms within 7 days. The fusiform cells, observed by light microscopy, were re-isolated from all symptomatic plants back on to 1% HSA. No symptoms or any fusiform cells were identified on or within the control plants. This is the first report of a Labyrinthula species causing damage to Agrostis capillaris and Poa annua turf in the UK.