First Report of Fusarium Wilt of Blackberry (<i>Rubus</i> subgenus <i>Rubus</i>) Caused by <i>Fusarium oxysporum</i> f. sp. <i>mori</i> in Arkansas

In September of 2023 and 2024, several blackberry plants in a commercial blackberry farm in White Co., AR were observed with chlorotic leaves and cane dieback. Some canes had a distinct reddish-black streak running up one side of the cane. Signs included sporodochia of Fusarium on the surface of some dead canes. Colonies of Fusarium were isolated from most symptomatic stems. Based on colony morphology, macro- and microconidia, and conidiophores, 21 isolates were putatively identified as F. oxysporum. Four single-spored isolates (RF2, RF4, RF7, and RF12) were randomly selected for pathogenicity tests on blackberry (cv. Prime-Ark 45, Twilight, and Navaho), vegetative compatibility analysis, and whole genome sequencing (Illumina, 60x coverage). Sequences of the translation elongation factor 1- (TEF), -tubulin (TUB), and intergenic spacer (IGS) regions were extracted from each assembled genome. BLAST analyses in FUSARIOID-ID placed the isolates in the F. oxysporum species complex. A maximum likelihood phylogeny was generated in MEGA12 (Kumar et al. 2024) with concatenated alignments of TEF, TUB, and IGS from the four AR isolates, three F. oxysporum reference strains (CBS 144134, CBS 144135, and CBS 221.49), and 34 strains of F. oxysporum f. sp. mori (Gordon et al. 2016; Pastrana et al. 2017). The phylogenetic tree grouped RF2, RF4, RF7, and RF12 with F. oxysporum f. sp. mori and indicated high sequence similarity among the AR isolates (Fig 1). The AR isolates grouped closely with isolates GL1312 and GL1313 from California. The IGS sequences of GL1312 and GL1313 (GenBank: KY515229 and KY515230) were referenced in the first report of F. oxysporum f. sp. mori causing Fusarium wilt of blackberry in North Carolina (Pastrana et al. 2020). All sequences used to identify the AR isolates as F. oxysporum f. sp. mori were submitted to GenBank (TEF: PV176845-PV176848; TUB: PV176841-PV176844; IGS: PV176837-PV176840). Pathogenicity tests were conducted by placing 30-day-old plants into a spore suspension (1 x 106 spores/ml) for 12 hrs. Plants were then transplanted to 10 cm diameter pots containing Sunshine Mix #4 (https://www.sungro.com/) and evaluated for symptom development and mortality for 120 days. Control plants were treated with sterile H2O. Ten replications were used for each isolate for each cultivar. Symptoms, including stunting and yellowing of foliage, developed on all inoculated cultivars after 21-28 days and progressed over a 96-day period. Mortality of plants was scored between 80-117 days post-inoculation and varied among the three cultivars. Mortality and distinct reddish-black stem lesions were observed on 30-90% of the plants of cv. Prime-Ark 45 after 117 days, with isolates RF4 and RF7 causing 90% mortality. Mortality occurred on 0-20% and 0% of the plants of cv. Twilight and Navaho, respectively. No symptoms developed on mock-inoculated controls. The pathogenicity test was repeated twice with similar results in all three tests. F. oxysporum was recovered from the stem tissue of inoculated and symptomatic plants, but not from any of the control plants. All 21 field isolates, and those recovered from inoculated plants, belonged to a single vegetative compatibility group. This is the first report of Fusarium wilt of blackberry caused by F. oxysporum f. sp. mori in Arkansas. This report, along with previous reports from California and North Carolina, signals the spread of an emerging pathogen that could threaten blackberry production in the U.S.