Ann. appl. Biol. (2002), 141:45-59 Printed in Great Britain *Corresponding Author E-mail: dthackray@agric.wa.gov.au © 2002 Association of Applied Biologists 45 Effect of strain-specific hypersensitive resistance on spatial patterns of virus spread By D J THACKRAY 1,2 *, L J SMITH 1,2 , Y CHENG 1 , J N PERRY 3 and R A C JONES 1,2 1 Centre for Legumes in Mediterranean Agriculture, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia 2 Plant Pathology Section, Crop Improvement Institute, Department of Agriculture, Locked Bag 4, Bentley Delivery Centre, WA 6983, Australia 3 Division of Plant and Invertebrate Ecology, IACR-Rothamsted Experimental Station, Harpenden, Herts, AL5 2JQ UK (Accepted 8 May 2002; Revised Version Received 3 May 2002) Summary Spatial patterns of spread were compared between strains of Bean yellow mosaic virus (BYMV) that differ in causing systemic necrotic (hypersensitive) or non-necrotic symptoms in narrow-leafed lupin (Lupinus angustifolius). Both types of BYMV were spread naturally by aphids from adjacent infected pasture into a large lupin block (natural spread site), or from clover plants introduced as virus sources into two field experiments with lupin. Cumulative spatial data for plants with disease symptoms from a range of times in the growing period were assessed using Spatial Analysis by Distance IndicEs (SADIE). At the natural spread site, with non-necrotic BYMV, the extent of clustering of plants with symptoms increased gradually over time, while with necrotic BYMV there was less clustering and no increase over time. In both experiments, for the type of BYMV that was introduced into a plot, there was a gradual increase in clustering, but with this being greater with non-necrotic BYMV. In the second experiment, there was also significant clustering of plants with symptoms of non-necrotic BYMV in plots without introduced non-necrotic foci but not for necrotic BYMV in plots without introduced necrotic foci. When clustering data for plants with newly recorded symptoms was tested for spatial association between successive assessment dates, association was positive for both BYMV types though stronger for the non-necrotic type, declining as the temporal lag increased. Generally, association was strongest for assessments 2-3 wk apart, corresponding approximately to the period for BYMV to move systemically in plants and for obvious symptoms to appear in shoot tips. Contour maps for local association between dates showed that the strongest spatial associations were from coincidence of infection gaps rather than infection patches. The combination of information from clustering and association analysis showed that spread of non-necrotic BYMV is less diffuse, with considerably more localised infection surrounding the infection sources. This work demonstrates how spatial virus spread can be diminished when hypersensitive (necrotic) resistance is deployed, and the limitations associated with employing hypersensitivity that is strain specific. Key words: Hypersensitivity, strain specificity, resistance, Lupinus angustifolius, BYMV, potyvirus, necrotic, non-necrotic, spatial spread, SADIE, clustering, association, pattern Introduction Bean yellow mosaic virus (BYMV, family Potyviridae, genus Potyvirus) causes a serious disease of narrow-leafed lupin (L. angustifolius) in Australasia, the USA, southern Africa and Europe (Jones & McLean, 1989). It is spread in a non- persistent manner by many aphid species (Edwardson & Christie, 1991; Jones, 1993; Berlandier et al., 1997). In the grainbelt region of south-west Australia, BYMV spreads to lupin crops from annual pastures dominated by subterranean clover (Trifolium subterraneum) (McKirdy et al., 1994; Jones, 1994a, 1996). The more common, necrotic types of BYMV strains cause severe systemic hypersensitive resistance reactions in infected narrow-leafed lupin plants resulting in plant death (Jones & McLean, 1989; Jones, 1993, 1994b, 1997; Cheng & Jones, 2000). Because plants are rapidly killed, the window of opportunity for aphids to acquire and spread virus from plants infected with necrotic BYMV within a crop is small (Jones, 1993). Typically infections are concentrated at the edges of a crop, especially the windward edge(s) and in thin areas within the crop. Elsewhere, infected plants are mostly isolated or in pairs and tend to be scattered