IACR-Long Ashton Research Station, University of Bristol, Long Ashton, Bristol BS18 9AF, UK

Background and objectives
Sphaerellopsis filum, (S/f.), is a mycoparasite of rusts. It has been recorded on some 350 species in 30 rust genera. We are interested in its potential for biological control of willow rust, (Melampsora spp. on Salix), which causes a serious disease in willow when grown in short-rotation (3-5 ;year harvest cycles) coppice (SRC) for renewable energy and fibre. The crop can be sustained commercially only with low inputs and so we are studying natural means of rust suppression avoiding fungicide use. The mycoparasite first came to our attention in 1990 naturally suppressing a rust epidemic in a mature plantation of S. ;viminalis Bowles Hybrid [1]. It overwinters and is widely encountered in its asexual, splash-dispersed form within stem pustules and cankers caused by infection of Bowles Hybrid with a stem-infecting form of rust (SIF). Its sexual phase, (Eudarluca caricis), is ascosporic and airborne, but has not been found on willow rusts or, until recently, on rusts of other hosts in Britain. The most damaging forms of rust in SRC are heteroecious, (alternating mainly on Larix spp.). They exist as different pathotypes infecting leaves of many Salix species and clones. In order to parasitize leaf rust, S.f. must spread from the ecological niches of SIF to adjacent plantings of clones which become infected with leaf-infecting rust. We describe ecological studies on the distribution and spread of S.f., provide evidence for the existence of pathogenic variation and, using AFLP, give an insight into the genetic structure of the S.f. population.

Materials and methods
Records of rust and mycoparasite development were made from May-July in established, adjacent plantings (c. 48X20 ;m/plot) of S. ;viminalis Bowles Hybrid (in which S.f. was endemic) and S. ;burjatica Korso at Long Ashton. Rust uredinia per leaf and stem and S.f. pycnidia per uredinium were counted on leaves and stems. S.f. isolates from rust on willow and other host plants were inoculated on to leaf discs in vitro simultaneously or following inoculation with rust. AFLP was developed for S.f. and used to fingerprint isolates from SRC plantations.

Results and conclusions
Because the mycoparasite is indigenous in the overwintering cankers on Bowles Hybrid, and splash dispersal of the asexual form from this primary source is relatively inefficient, stem rust in Bowles Hybrid plantings was colonized earlier, more rapidly and more intensively than rust (leaf-infecting) on Korso. Further, in the Korso planting, leaf rust developed more rapidly than S.f. so that rust colonization occurred too late for suppression to be effective. Without external intervention, there appears to be little opportunity to exploit the mycoparasite for biological control in those monoclonal plantings which support only leaf-infecting forms of rust. However, plantation designs involving cional mixtures, which contain a regular component of clones susceptible to SIF, are more likely to allow splash dispersal to effect rapid colonization of developing rust by S.f. throughout a plantation. A model has been constructed to predict the rate of S.f. spread in plantations with different proportions of Bowles Hybrid. The sexual stage of S.f. has recently been discovered on blackberry, (Rubus sp.), but not yet on willow in Britain [2]. Studies with different isolates show that S.f. exhibits specificity to different rust hosts, and that isolates from rusts other than from willow are occasionally able to infect willow rust. AFLP analysis of isolates taken from different regions of a Bowles Hybrid plantation indicate little genotypic variation in some regions but a great deal in others, suggesting a heterogeneous source. The implications of these findings will be discussed.

1. Morris RAC, Royle DJ, Whelan MJ, Arnold GM, 1994. Proceedings, Brighton Crop Protection Conference: Pests and Diseases, pp. 63-68.
2. Yuan ZW, Pei MH, Hunter T, Royle DJ, 1998. Mycological Research (in press).