First Report of <i>Phaeoacremonium minimum</i> Causing Decline of American Sycamore in West Virginia

Phaeoacremonium minimum (formerly P. aleophilum) is one of the causal agents of Petri and Esca disease, which causes disease in numerous woody plants, including stem and branch rot of grape species worldwide (Gramaje et al. 2015; Ye et al. 2020), but has yet to be isolated on American sycamore (Platanus occidentalis). In September of 2022, a large (38 cm diameter at breast height), declining P. occidentalis was reported in Monongalia County, WV. The tree displayed numerous disease symptoms including crown dieback and light brown bark cankers (25-40 cm) down the length of the trunk. Wood plugs were excised from canker margins, surface-disinfected in 10% bleach, plated onto Potato Dextrose Agar (PDA) and incubated at 25°C under dark conditions. After 10-12 days, a pale brown mycelium grew characteristic of Phaeoacremonium (Mostert et al. 2006). DNA sequencing of the ITS1+5.8S+ITS2 of the ribosomal RNA gene repeat (rDNA) and of a fragment of the beta-tubulin gene using primers T1/Bt2b (Glass and Donaldson 1995) matched P. minimum Accession numbers AY179938.1 and MN634159.1. The sequences obtained were deposited in GenBank under the accession numbers PQ135959 and PQ623190. Pathogenicity was confirmed by placing a 6-mm diameter PDA plug from 10-day-old P. minimum cultures into scalpel wounds of twelve sycamore seedlings (mean diam 1.03 cm) at approximately 50 mm from the base of the seedling. Five seedlings were treated with sterile PDA as a control. For all seedlings, the bark was pressed back to cover the wound and closed with parafilm. Inoculated seedings were kept in a growth room at 25°C for 16 weeks and checked regularly for symptom development. By 16 weeks, all P. minimum seedlings displayed necrotic cankers on the stem while controls remained canker-free. A nonparametric Kruskal-Wallis test was used to compare canker ratings between treated and control seedlings. Mean vascular necrosis of P. minimum inoculated isolates were significantly greater (16.18 mm, P = 0.003) than the controls (0.0 mm). Control plants did not show any symptoms. Mean length of streaking for P. minimum inoculated isolates was also significantly longer (47.4 mm, P = 0.001) than controls (0.0 mm). The fungus was reisolated from discolored tissue of all inoculated stems, completing Koch’s postulates and confirming pathogenicity of P. minimum on healthy P. occidentalis. To our knowledge, this is the first documented report of P. minimum infections on American sycamore.