Abstract
Strawberry (Fragaria×ananassa) is one of the most important berry crops in the world. Root rot of strawberry caused by Rhizoctonia spp. is a serious threat to commercial strawberry production worldwide. However, there is no information on the genetic diversity and phylogenetic status of Rhizoctonia spp. associated with root rot of strawberry in Australia. To address this, a total of 96 Rhizoctonia spp. isolates recovered from diseased strawberry plants in Western Australia were characterized for their nuclear condition, virulence, genetic diversity and phylogenetic status. All the isolates were found to be binucleate Rhizoctonia (BNR). Sixty-five of the 96 BNR isolates were pathogenic on strawberry, but with wide variation in virulence, with 25 isolates having high virulence. Sequence analysis of the internal transcribed spacers of the ribosomal DNA separated the 65 pathogenic BNR isolates into six distinct clades. The sequence analysis also separated reference BNR isolates from strawberry or other crops across the world into clades that correspond to their respective anastomosis group (AG). Some of the pathogenic BNR isolates from this study were embedded in the clades for AG-A, AG-K and AG-I, while other isolates formed clades that were sister to the clades specific for AG-G, AG-B, AG-I and AG-C. There was no significant association between genetic diversity and virulence of these BNR isolates. This study demonstrates that pathogenic BNR isolates associated with root rot of strawberry in Western Australia have wide genetic diversity, and highlights new genetic groups not previously found to be associated with root rot of strawberry in the world (e.g., AG-B) or in Australia (e.g., AG-G). The wide variation in virulence and genetic diversity identified in this study will be of high value for strawberry breeding programs in selecting, developing and deploying new cultivars with resistance to these multi-genetic groups of BNR.
Highlights
Strawberry (Fragaria6ananassa) is one of the most economically important berry crops in the world, with production reaching about 4.4 million tons in 2010 [1,2]
This paper reports studies conducted to (i) characterize the nuclear condition of Rhizoctonia spp. isolates recovered from diseased strawberry in Western Australia; (ii) determine their pathogenicity and relative virulence; (iii) define their genetic diversity based on ITS1-5.8S rRNA-ITS2 sequence analysis; (iv) determine if there is an association between genetic diversity and virulence; and (v) determine the phylogenetic relationships of these isolates with those from strawberry and other crops across the world
Nuclear condition and pathogenicity Nuclear staining of each Rhizoctonia isolate showed two nuclei per cell in the hyphae of all the 96 Rhizoctonia isolates tested (Fig. 1). This shows that binucleate Rhizoctonia (BNR) was the predominant Rhizoctonia recovered from diseased strawberry in Western Australia
Summary
Strawberry (Fragaria6ananassa) is one of the most economically important berry crops in the world, with production reaching about 4.4 million tons in 2010 [1,2]. Rhizoctonia spp. are soil-borne fungal pathogens with a worldwide distribution [14,15] They can cause severe damage to a wide range of economically important agricultural and horticultural crops [16,17,18]. The phasing-out of some broad-spectrum pre-planting fumigants due to environmental and health concerns has fostered keen interest in developing alternative non-chemical ways to manage root rot and other soil-borne diseases more effectively and sustainably [23,24,25,26]. It is accepted that identifying and deploying resistant cultivars is the most cost effective and environmentally sustainable strategy to control soil-borne disease on strawberry [24,25,26,27]. If novel management strategies, including the breeding of Rhizoctonia-resistant cultivars, are to be developed, it is essential to understand the genetic diversity within Rhizoctonia and how this diversity pertains to the virulence of isolates
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