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.

Shot hole borer (SHB)-Fusarium dieback (FD) is a new pest-disease complex affecting numerous tree species in California and is vectored by two distinct, but related ambrosia beetles (Euwallacea sp. nr. fornicatus) called polyphagous shot hole borer (PSHB) and Kuroshio shot hole borer (KSHB). These pest-disease complexes cause branch dieback and tree mortality on numerous wildland and landscape tree species, as well as agricultural tree species, primarily avocado. The recent discovery of KSHB in California initiated an investigation of fungal symbionts associated with the KSHB vector. Ten isolates of Fusarium sp. and Graphium sp., respectively, were recovered from the mycangia of adult KSHB females captured in three different locations within San Diego County and compared with the known symbiotic fungi of PSHB. Multigene phylogenetic analyses of the internal transcribed spacer region (ITS), translation elongation factor-1 alpha (TEF1-α), and RNA polymerase II subunit (RPB1, RPB2) regions as well as morphological comparisons revealed that two novel fungal associates Fusarium kuroshium sp. nov. and Graphium kuroshium sp. nov. obtained from KSHB were related to, but distinct from the fungal symbionts F. euwallaceae and G. euwallaceae associated with PSHB in California. Pathogenicity tests on healthy, young avocado plants revealed F. kuroshium and G. kuroshium to be pathogenic. Lesion lengths from inoculation of F. kuroshium were found to be significantly shorter compared with those caused by F. euwallaceae, while no difference in symptom severity was detected between Graphium spp. associated with KSHB and PSHB. These findings highlight the pest disease complexes of KSHB-FD and PSHB-FD as distinct, but collective threats adversely impacting woody hosts throughout California.

Plant rhizobacteria have been successfully used as biocontrol agents against fungal phytopathogens. However, their potential to control two important avocado diseases, namely Fusarium dieback (FD) and Phytophthora root rot (PRR), has been poorly studied. FD is an emerging disease triggered by fungi associated with two ambrosia beetle species (Euwallacea fornicatus species complex), while PRR is caused by Phytophthora cinnamomi, a soil-borne oomycete. In the present work, the antifungal activity of bacteria isolated from avocado rhizosphere was tested in dual culture assays against Fusarium euwallaceae, Graphium euwallaceae and Graphium sp., causal agents of FD, and against P. cinnamomi. In 2015, rhizosphere soil samples of FD infested and non-infested avocado trees were collected from a commercial avocado orchard in Escondido, California. In an initial screening, 72 of the 168 assessed bacterial isolates reduced mycelial growth of F. euwallaceae by up to 46%. Eight bacterial isolates showing inhibition percentages larger than 40% were then selected for further antagonism assays against the other fungal pathogens. Five bacterial isolates, determined by 16S rDNA sequencing to belong to the Bacillus subtilis/Bacillus amyloliquefaciens species complex, successfully inhibited the mycelial growth of both Graphium species by up to 30%. The same isolates and an additional isolate identified as Bacillus mycoides, inhibited the growth of P. cinnamomi by up to 25%. This is the first report of avocado rhizobacteria with antifungal activity against pathogens responsible for FD and PRR in avocado.

No abstract

Fusarium dieback (FD) is a new vascular disease of hardwood trees caused by Fusarium spp. and other associated fungal species which are vectored by two recently introduced and highly invasive species of ambrosia beetle (Euwallacea spp. nr. fornicatus). One of these ambrosia beetles is known as the polyphagous shot hole borer (PSHB) and the other as the Kuroshio shot hole borer (KSHB). Together with the fungi that they vector, this pest–disease complex is known as the shot hole borer–Fusarium dieback (SHB-FD) complex. Mitigation of this pest–disease complex currently relies on tree removal; however, this practice is expensive and impractical given the wide host range and rapid advancement of the beetles throughout hardwoods in southern California. This study reports on the assessment of various pesticides for use in the management of SHB-FD. In vitro screening of 13 fungicides revealed that pyraclostrobin, trifloxystrobin, and azoxystrobin generally have lower effective concentration that reduces 50% of mycelial growth (EC50) values across all fungal symbionts of PSHB and KSHB; metconazole was found to have lower EC50 values for Fusarium spp. and Paracremonium pembeum. Triadimefon and fluxapyroxad were not capable of inhibiting any fungal symbiont at the concentrations tested. A 1-year field study showed that two insecticides, emamectin benzoate alone and in combination with propiconazole, and bifenthrin, could significantly reduce SHB attacks. Two injected fungicides (tebuconazole and a combination of carbendazim and debacarb) and one spray fungicide (metconazole) could also significantly reduce SHB attacks. Bioassays designed to assess fungicide retention 1 year postapplication revealed that six of the seven fungicides exhibited some level of inhibition in vitro and all thiabendazole-treated trees sampled exhibiting inhibition. This study has identified several pesticides which can be implemented as part of an integrated pest management strategy to reduce SHB infestation in low to moderately infested landscape California sycamore trees and potentially other landscape trees currently affected by SHB-FD.

No abstract

California has been invaded by two distinct Euwallacea spp. that vector unique plant pathogenic symbiotic fungi on multiple hosts and cause Fusarium dieback. The objective of this study was to develop multiplex real-time quantitative PCR assays using hydrolysis probes targeting the β-tubulin gene to detect, distinguish, and quantify fungi associated with the polyphagous shot hole borer (PSHB; Euwallacea whitfordiodendrus, Fusarium euwallaceae, Graphium euwallaceae, and Paracremonium pembeum) as well as the Kuroshio shot hole borer (KSHB; Euwallacea kuroshio, Fusarium kuroshium, and Graphium kuroshium) from various sample types. Absolute quantification reaction efficiencies ranged from 88.2 to 104.3%, with a coefficient of determination >0.992 and a limit of detection of 100 copies µl−1 for all targets across both assays. Qualitative detection using the real-time assays on artificially inoculated avocado shoot extracts showed more sensitivity compared with conventional fungal isolation from wood. All symbiotic fungi, except P. pembeum, from PSHB and KSHB female heads were detectable and quantified. Field samples from symptomatic Platanus racemosa, Populus spp., and Salix spp. across 17 of 26 city parks were positively identified as PSHB and KSHB through detection of their symbiotic fungi, and both were found occurring together on five trees from three different park locations. The molecular assays presented here can be utilized to accurately identify fungi associated with these invasive pests in California.

A recently discovered ambrosia beetle with the proposed common name of polyphagous shot hole borer (Euwallacea sp., Coleoptera: Curculionidae: Scolytinae), is reported to attack >200 host tree species in southern California, including many important native and urban landscape trees. This invasive beetle, along with its associated fungi, causes branch dieback and tree mortality in a large variety of tree species including sycamore (Platanus racemosa Nutt.). Due to the severity of the impact of this Euwallacea sp., short-term management tools must include chemical control options for the arboriculture industry and private landowners to protect trees. We examined the effectiveness of insecticides, fungicides, and insecticide-fungicide combinations for controlling continued Euwallacea sp. attacks on previously infested sycamore trees which were monitored for 6 mo after treatment. Pesticide combinations were generally more effective than single pesticide treatments. The combination of a systemic insecticide (emamectin benzoate), a contact insecticide (bifenthrin), and a fungicide (metconazole) provided some level of control when applied on moderate and heavily infested trees. The biological fungicide Bacillus subtilis provided short-term control. There was no difference in the performance of the three triazole fungicides (propiconazole, tebuconazole, and metconazole) included in this study. Although no pesticide combination provided substantial control over time, pesticide treatments may be more effective when trees are treated during early stages of attack by this ambrosia beetle.