Vol.:(0123456789) 1 3 Australasian Plant Disease Notes (2022) 17:8 https://doi.org/10.1007/s13314-022-00456-0 Pathogenicity of Fusarium euwallaceae towards apple (Malus domestica) and grapevine (Vitis vinifera) Mignon de Jager 1  · Francois Roets 1 Received: 15 March 2022 / Accepted: 13 April 2022 © The Author(s) under exclusive licence to Australasian Plant Pathology Society Inc. 2022 Abstract Pathogenicity of Fusarium euwallaceae, a mutualist of the Euwallacea fornicatus beetle, was assessed on apple and grapevine following its frst detection on these hosts in South Africa. Fusarium euwallaceae caused lesions in xylem tissues of apple (Pink Lady), but not grapevine (Merlot and Cabernet Sauvignon). Koch's postulates were satisfed through re-isolation from artifcially inoculated tissues. Keywords Euwallacea fornicatus · Pathogen · Polyphagous shot hole borer Euwallacea fornicatus [Polyphagous Shot hole Borer (PSHB)], native to Southeast Asia (Li et al. 2016), is a recent invader in South Africa (Paap et al. 2018). It vec- tors a fungal mutualist, Fusarium euwallaceae, that enables beetle colony establishment in hosts. Fusarium euwallaceae colonizes xylem tissues surrounding PSHB galleries. PSHB feeds on the fungus that grow within galleries (Eskalen et al. 2013). Highly susceptible hosts support beetle reproduc- tion, but F. euwallaceae may infect hosts without beetle colony establishment through indiscriminate host probing by PSHB (Eskalen et al. 2013). PSHB colonization results in Fusarium dieback, reported from numerous tree species in South Africa, the U.S.A., Israel, Europe and Australia (Eskalen et al. 2013; Mendel et al. 2017; IPPC, 2021; Lynch et al., 2021; Van Rooyen et al. 2021). Despite common use of the term ‘Fusarium dieback’, experiments demonstrating F. euwallaceae pathogenicity are scant and, in commercial crops, are restricted to Avocado (Jones and Paine 2017), Macadamia (Twiddy et al. 2021), and Almond (Moreno et al. 2018). Apple production in South Africa is valued at ca. $700 million (Hortgro 2020) while the South African grapevine industry is worth nearly $3,5 billion (www.sawis.co.za). PSHB infestations on grapevine (Vitis vinifera) and apple (Malus domestica) was recently observed in gardens in the Western Cape province (Van Rooyen et al. 2021) (Figs. 1–2). Host lists from the U.S.A. include grapevine and Malus spp. as susceptible to F. euwallaceae (Eskalen et al. 2013), but Koch’s postulates have not been satisfed in any previous reports. This study determines the host status of apple and grapevine to F. euwallaceae. Isolates were obtained from grapevine (CMW-IA2, George, August 2019), apple (CMW-IA3, George, Septem- ber 2018) and Cherry (CMW-IA4, George, August 2019) by exposing PSHB galleries in wood, excising pieces (3 mm 3 ) of fungus-stained wood (Fig. 2) and placing these onto Potato Dextrose Agar plates (PDA, Biolab, South Africa). Plates were incubated for 7 d at 24 °C. Pure cultures were obtained by transferal of single hyphal tips to fresh PDA plates. Isolate identity was confrmed by DNA sequence comparisons of the elongation factor 1-α region to that of the ex-holotype strain of F. euwallaceae (GenBank JQ038007). Genomic DNA was extracted using a Sigma-Aldrich™ plant extraction kit, and the target region was amplifed using the primer pair EF1 and EF2 and PCR thermocycling condi- tions following Paap et al. (2018). Sequences (GenBank OM963006—OM963008) were identical to the F. euwal- laceae type. Reference isolates are kept at the Culture Col- lection of Innovation Africa, South Africa. Apple cv. ‘Pink lady’ and grapevine (cvs. ‘Merlot’ and ‘Cabernet Sauvignon’) on commercial farms [Lourens- ford estate (-33°35′ 36″S, 18°55′ 53″E) and Nietvoorbij (33°54′05″S 18°52′34″E), respectively] were inoculated with the three F. euwallaceae isolates in early Autumn (March 2021). Inoculum was prepared by saturating sterile * Francois Roets fr@sun.ac.za 1 Department of Conservation Ecology and Entomology, Stellenbosch University, Stellenbosch 7600, South Africa