Ann. appl. Biol. zyxwvutsrq (1997), 130:537-547 Printed in Great Britain 537 z Identification of the determinants of host resistance and pathogenicity in interactions between AZternariu Zinicola Groves zyxwv & Skolko and Linum usitatissimum L. accessions using multivariate analyses By N EVANS*, N MCROBERTS, D HITCHCOCK' and G MARSHALL Plant Science Department, and 'Biomathematics and Statistics, Scotland, Scottish Agricultural College (SAC), Auchincruive, Ayr zyxw KA6 5HW, UK (Accepted 22 April 1997) Summary Multivariate analysis of variance (MANOVA) and canonical variates analysis (CVA) were used to examine differences in host plant resistance and pathogen behaviour in interactions between Alternaria linicola and three genotypes of Linum usitatissimum, previously identified as susceptible, moderately resistant and resistant to the pathogen. Significant differences in pathogen development were found among the Linum accessions at 18, 24, and 40 h after inoculation. At 18 h after inoculation attempted penetration by the pathogen was relatively rare on all three accessions and canonical variates analysis revealed that overall differences among accessions resulted from large differences with respect to a small number of variables associated with successful penetration on the most susceptible accession. At later times after inoculation, when attempted penetration was more common, overall differences among accessions were found to result from smaller absolute differences among a group of variables which characterised the early colonisation of the host tissue. The results from these investigations are discussed in relation to recent research on the ecology of the pathogen and the importance of the timing of host responses to infection in determining host plant resistance. Key words: Altemaria linicola, linseed, plant/pathogen interactions, MANOVA, canonical variates analysis Introduction Alternaria Zinicola Groves & Skolko is a commonly occurring pathogen of linseed (Linum usitatissimum, L.) in north western Europe. The pathogen causes a range of symptoms on its host including poor germination, damping-off of seedlings, leaf spotting on seedling and adult plants, and a form of head blight in the seed capsules. A. linicola is important in the field at the seedling stage and again later in the season when the seed capsules are infected. Seed infection, in addition to resulting in poor germination, can result in loss of yield and a reduction in oil quality (Mercer, McGimpsey & Ruddock, 1989). Recent studies (Vloutoglou, Fitt & Lucas, 1995) have shown that the pathogen can survive over winter in naturally infected seeds and crop debris, thus providing the pathogen with sources of inoculum for Corresponding author; present address: Department of Plant Pathology, Kearney Agricultural Center, University of California, 9240 Riverbend Avenue, Parlier, CA 93648, USA zyxwv 0 1997 Association of Applied Biologists