Fusarium wilt of strawberry is caused by Fof and is a major soil-borne disease of strawberry worldwide. This disease quickly kills infected plants by impeding water and nutrient movement in the vascular tissues of the plant. Since the introduction of the fumigant methyl bromide in the 1970s, and the use of disease-free transplants, losses due to fusarium wilt and other soil-borne disease have been minimized. However, with the phase out of methyl bromide and no commercially viable equivalent on the market soil-borne diseases have recently caused major losses in the Australian strawberry industry.
Fusarium oxysporum is associated with a group of pathogens causing crown and root diseases of strawberry. These include: Colletotrichum species, Macrophomina phaseolina, Phytophthora species, Pythium species, Rhizoctonia fragariae, and Verticillium species. Crown and root diseases of strawberry are considered a serious problem to strawberry production with 10% of plant death in QLD and up to 50% in the Perth district of WA (Fang et al., 2012; Golzar et al., 2007; Phillips and Golzar, 2008) being attributed to Fof. In WA, high incidences of plant decline and death were reported in strawberry crops in 2005 in both fumigated and nonfumigated soils. In some fields ‘Camarosa’ and ‘Gaviota’ suffered up to 60% mortality (Golzar et al., 2007). As a result, surveys were implemented to identify the causes and severity of the problem. Fof was identified as a predominant pathogen responsible for much of the plant deaths. In 2005, Fof was isolated from 80% to 90% of the samples tested (Phillips and Golzar, 2008). In 2008, a survey undertaken to determine the severity of crown and root diseases in WA showed that plant decline and death occurred in both nonfumigated and fumigated fields. Fusarium oxysporum was the dominant pathogen isolated from crowns. Up to 41% of the isolates tested were F. oxysporum, isolated either as the sole species or one of several species isolated from infected crowns and roots (Fang et al., 2011).
Queensland is the biggest producer of strawberries in Australia, growing mainly for the domestic market. In 2014–15, up to 30% of isolations undertaken from plants showing root and crown disease at the Department of Agriculture and Fisheries, QLD, have shown F. oxysporum (A. Gomez, unpublished data).
An increase in disease outbreaks of fusarium wilt in the past few years, plus the search for alternative disease management has placed emphasis on the development of resistant cultivars. Knowledge of the diversity of the pathogen is critical to ensure that potential new cultivars are tested against a wide a range of pathotypes. However, information on the genetic variation of Fof across Australia is unknown. An assessment of a broad range of Australian isolates and their pathogenicity would allow the identification of highly virulent or diverse strains and give some indication of the diversity of Australian isolates. In addition, virulent isolates can be used to challenge potential breeding lines.
Molecular techniques and VCG testing have been used to resolve genetic variation amongst Fof isolates. Vegetative compatibility tests are used for characterizing variation in genetically isolated asexual populations. Isolates belonging to the same VCG, confirmed by the ability to form a prototrophic heterokaryon, can exchange genetic information and are often clonally derived from populations of a forma specialis (Correll et al., 1987; Kistler et al., 1991). In Korea, Hyun and Park (1996) used random amplified polymorphic DNA (RAPD) polymerase chain reactions (PCRs) to separate 24 isolates into two distinct clades and assigned 32 isolates into four major VCGs. Nagarajan et al. (2004) also used RAPD and restriction fragment length polymorphism of the intergenic spacer region of ribosomal DNA to study variation amongst isolates in Korea. They identified eight distinct clusters, and noted dissimilarities between isolates from different sites. Nagarajan et al. (2006) identified three major VCGs and one incompatible group among 22 isolates, and noted a relatively high correlation existed among VCG and RAPD, and virulence.
A number of studies have shown the nuclear translation EF-1α gene region can reveal genetic relationships amongst formae speciales of F. oxysporum. O’Donnell et al. (1998) demonstrated that EF-1α could resolve relationships within F. oxysporum. Fourie et al. (2009) determined the relatedness amongst VCGs of F. oxysporum f. sp. cubense and other formae speciales and nonpathogens using EF-1α data. These studies reveal that the EF-1α gene regions can separate isolates of F. oxysporum.
The first objective of this study was to evaluate pathogenicity and variation amongst F. oxysporum isolates from strawberry in Australia by means of pathogenicity testing, VCGs, and partial sequencing of the EF-1α gene region. The second objective was to evaluate cultivar responses to Fof. This information will help us understand the diversity of isolates and their potential use in screening cultivars and breeding lines.
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