Breeding resistance in strawberry cultivars for Fusarium oxysporum f. sp. fragariae

Abstract

An alternative method of disease management is needed to combat the high incidence of Fusarium wilt of strawberry, caused by Fusarium oxysporum f. sp. fragariae (Fof), in Queensland and Western Australia following the phase-out of methyl bromide fumigation. The use of Fusarium wilt resistant cultivars as part of an integrated disease management plan could play an important part in reducing losses to this disease.The variations in the virulence that may exist among Fof strains, and the variation in susceptibility among strawberry genotypes to Fof strains, are important aspects in screening for resistance to ensure disease exposure and expression are adequate for assessment. As no research has been done regarding the genetic variability of Australian Fof strains, 25 isolates of F. oxysporum, obtained from the major strawberry production areas in Australia, were characterised using pathogenicity testing, vegetative compatibility groups (VCGs) based on complementation testing of mutants, and molecular techniques using translation elongation factor-1 alpha (EF-1α) and mitochondrial small subunit (mtSSU) ribosomal DNA gene analyses. Disease development response determined from pathogenicity tests showed significant variation among isolates; from these tests, four pathogenic isolates were chosen for use in cultivar evaluations and screening for Fof resistance. Two distinct VCGs were identified which closely corresponded to two of the ten lineages identified by partial EF-1α sequence phylogenetic analysis.An evaluation of cultivar resistance to isolates of Fof showed significant differences, ranging from highly resistant to highly susceptible. Cultivar × isolate interactions performed by fitting mixed models with smoothing splines determined responses ‘over time’ as significant. The isolates from Western Australia were distinct from the Queensland isolates and were more virulent to the dominant strawberry cultivar grown in that region.To further develop a screening technique suitable for use in the screening process for Fof resistance and for large scale screenings, two conidial-suspension methods (root dip and injected) as well as three conidial concentrations within the root dip, and two incubated seed-carrier (ryegrass and millet) inoculation methods were compared. Disease incidence and severity ratings were used to determine Fof infection and disease development. No significant differences were observed among conidial concentrations. Ryegrass inoculation proved to be a very effective method that produced higher disease rating than the other treatments. A modified root dip method incorporating ryegrass seed was subsequently used in the resistance screening.Experiments were performed to identify individual breeding values and determine the inheritance of the resistance trait in strawberry to provide sufficient knowledge to allow for future development of Fof resistant strawberry cultivars. Best performing genotypes were determined using a general linear mixed model (GLMM), incorporating a pedigree. Variation in disease response of the screened population implies a quantitative effect. The estimate of the narrow sense heritability (0.49) suggests that the screened population would be responsive to phenotypic recurrent selection. While this study is based on the breeding values from one breeding population, information gained suggests Fof resistance can be incorporated into commercially suitable genetic background.