Elevated Genetic Diversity in the Emerging Blueberry Pathogen Exobasidium maculosum.

Jane E Stewart,William O Cline,Kyle Brooks,Phillip M Brannen,Marin T Brewer

doi:10.1371/journal.pone.0132545

Abstract

Emerging diseases caused by fungi are increasing at an alarming rate. Exobasidium leaf and fruit spot of blueberry, caused by the fungus Exobasidium maculosum, is an emerging disease that has rapidly increased in prevalence throughout the southeastern USA, severely reducing fruit quality in some plantings. The objectives of this study were to determine the genetic diversity of E. maculosum in the southeastern USA to elucidate the basis of disease emergence and to investigate if populations of E. maculosum are structured by geography, host species, or tissue type. We sequenced three conserved loci from 82 isolates collected from leaves and fruit of rabbiteye blueberry (Vaccinium virgatum), highbush blueberry (V. corymbosum), and southern highbush blueberry (V. corymbosum hybrids) from commercial fields in Georgia and North Carolina, USA, and 6 isolates from lowbush blueberry (V. angustifolium) from Maine, USA, and Nova Scotia, Canada. Populations of E. maculosum from the southeastern USA and from lowbush blueberry in Maine and Nova Scotia are distinct, but do not represent unique species. No difference in genetic structure was detected between different host tissues or among different host species within the southeastern USA; however, differentiation was detected between populations in Georgia and North Carolina. Overall, E. maculosum showed extreme genetic diversity within the conserved loci with 286 segregating sites among the 1,775 sequenced nucleotides and each isolate representing a unique multilocus haplotype. However, 94% of the nucleotide substitutions were silent, so despite the high number of mutations, selective constraints have limited changes to the amino acid sequences of the housekeeping genes. Overall, these results suggest that the emergence of Exobasidium leaf and fruit spot is not due to a recent introduction or host shift, or the recent evolution of aggressive genotypes of E. maculosum, but more likely as a result of an increasing host population or an environmental change.

Highlights

Many emerging plant diseases are caused by pathogens that have found new hosts or have been introduced to new areas, often as a result of human-mediated movement [1]
A total of 1775 nucleotides were sequenced for 82 isolates of E. maculosum from the southeastern USA, 6 isolates from V. angustifolium in northeastern North America, and an isolate of E. rostrupii from V. macrocarpon collected from New Jersey, USA that was used as an outgroup
A Populations consist of the total population of E. maculosum (n = 88), isolates from Nova Scotia, Canada and Maine, USA (NE, n = 6), isolates from the southeastern USA (SE, n = 82), and isolates from Georgia (GA, n = 68) and North Carolina (NC, n = 14). b Number of haplotypes and total number of individuals included in the analyses shown within parentheses c Haplotype diversity calculated in DnaSP (28) d Segregating sites and number of parsimonious sites shown within parentheses calculated in DnaSP e Watterson’s theta calculated in DnaSP f Nucleotide diversity calculated in DnaSP g Tajima’s D and Fu and Li’s F calculated in DnaSP

Summary

Introduction

Many emerging plant diseases are caused by pathogens that have found new hosts or have been introduced to new areas, often as a result of human-mediated movement [1]. Population genetic studies can elucidate whether emerging pathogens are introduced or native. Pathogens that emerge as the result of a recent adaptation, such as fungicide resistance, often exhibit a reduction in genetic diversity due to positive selection acting on one or a few genotypes that spread rapidly [11]. Emerging pathogens that result from changing environmental conditions or expanding host populations may not show a reduction in genetic diversity [12], and show signatures of recent population expansions. Causes for disease emergence can be understood by establishing the demographic history and genetic diversity of emerging pathogen populations [13]

Objectives

Methods

Results

Conclusion