The polyphagous shot hole borer (PSHB) is an invasive ambrosia beetle that forms a symbiosis with a new, as-yet-undescribed Fusarium sp., together causing Fusarium dieback on avocado and other host plants in California and Israel. In California, PSHB was first reported on black locust in 2003 but there were no records of fungal damage until 2012, when a Fusarium sp. was recovered from the tissues of several backyard avocado trees infested with PSHB in Los Angeles County. The aim of this study was to determine the plant host range of the beetle–fungus complex in two heavily infested botanical gardens in Los Angeles County. Of the 335 tree species observed, 207 (62%), representing 58 plant families, showed signs and symptoms consistent with attack by PSHB. The Fusarium sp. was recovered from 54% of the plant species attacked by PSHB, indicated by the presence of the Fusarium sp. at least at the site of the entry hole. Trees attacked by PSHB included 11 species of California natives, 13 agriculturally important species, and many common street trees. Survey results also revealed 19 tree species that function as reproductive hosts for PSHB. Additionally, approximately a quarter of all tree individuals planted along the streets of southern California belong to a species classified as a reproductive host. These data suggest the beetle–disease complex potentially may establish in a variety of plant communities locally and worldwide.
The invasive Asian ambrosia beetle Euwallacea sp. (Coleoptera, Scolytinae, Xyleborini) and a novel Fusarium sp. that it farms in its galleries as a source of nutrition causes serious damage to more than 20 species of live trees and pose a serious threat to avocado production (Persea americana) in Israel and California. Adult female beetles are equipped with mandibular mycangia in which its fungal symbiont is transported within and from the natal galleries. Damage caused to the xylem is associated with disease symptoms that include sugar or gum exudates, dieback, wilt and ultimately host tree mortality. In 2012 the beetle was recorded on more than 200 and 20 different urban landscape species in southern California and Israel respectively. Euwallacea sp. and its symbiont are closely related to the tea shot-hole borer (E. fornicatus) and its obligate symbiont, F. ambrosium occurring in Sri Lanka and India. To distinguish these beetles, hereafter the unnamed xyleborine in Israel and California will be referred to as Euwallacea sp. IS/CA. Both fusaria exhibit distinctive ecologies and produce clavate macroconidia, which we think might represent an adaption to the species-specific beetle partner. Both fusaria comprise a genealogically exclusive lineage within Clade 3 of the Fusarium solani species complex (FSSC) that can be differentiated with arbitrarily primed PCR. Currently these fusaria can be distinguished only phenotypically by the abundant production of blue to brownish macroconidia in the symbiont of Euwallacea sp. IS/CA and their rarity or absence in F. ambrosium. We speculate that obligate symbiosis of Euwallacea and Fusarium, might have driven ecological speciation in these mutualists. Thus, the purpose of this paper is to describe and illustrate the novel, economically destructive avocado pathogen as Fusarium euwallaceae sp. nov. S. Freeman et al.
Scientific communication is facilitated by a data-driven, scientifically sound taxonomy that considers the end-user's needs and established successful practice. Previously (Geiser et al. 2013; Phytopathology 103:400-408. 2013), the Fusarium community voiced near unanimous support for a concept of Fusarium that represented a clade comprising all agriculturally and clinically important Fusarium species, including the F. solani Species Complex (FSSC). Subsequently, this concept was challenged by one research group (Lombard et al. 2015 Studies in Mycology 80: 189-245) who proposed dividing Fusarium into seven genera, including the FSSC as the genus Neocosmospora, with subsequent justification based on claims that the Geiser et al. (2013) concept of Fusarium is polyphyletic (Sandoval-Denis et al. 2018; Persoonia 41:109-129). Here we test this claim, and provide a phylogeny based on exonic nucleotide sequences of 19 orthologous protein-coding genes that strongly support the monophyly of Fusarium including the FSSC. We reassert the practical and scientific argument in support of a Fusarium that includes the FSSC and several other basal lineages, consistent with the longstanding use of this name among plant pathologists, medical mycologists, quarantine officials, regulatory agencies, students and researchers with a stake in its taxonomy. In recognition of this monophyly, 40 species recently described as Neocosmospora were recombined in Fusarium, and nine others were renamed Fusarium. Here the global Fusarium community voices strong support for the inclusion of the FSSC in Fusarium, as it remains the best scientific, nomenclatural and practical taxonomic option available.
1 The ambrosia beetle morphologically identified as Euwallacea fornicatus consists of several cryptic species that exhibit large differences in the DNA sequences of several nuclear and mitochondrial gene regions. 2 Based on these differences, we suggest that there are at least three different species each with distinct phylogeography. 3 Members of this cryptic species complex have invaded many areas outside their native range and cause substantial damage to both agriculture (avocado in particular) and other tree species. 4 Three of these cryptic species have invaded the USA: two species in California and a third species in both Florida and Hawaii. 5 Identification of their native range allows directed search for their natural enemies that may be used in biological control of these tree pests.
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