Main content area

First Report of Fusarium Stem Canker on Pyrularia pubera, a Rare Native Parasitic Shrub in Forests of Southwestern Pennsylvania

Wickert, K. L., Metheny, A. M., Davis, D. D., Geiser, D. M., Wenzel, J. W., Planinsek, D., Kasson, M. T.
Plant disease 2018 v.102 no.9 pp. 1852
Carya cordiformis, Fusarium solani, Robinia, agar, conidia, conservation areas, databases, dendrochronology, dieback, forests, fungi, glucose, growth rings, hosts, leaves, mycelium, parasitic plants, pathogenicity, peptide elongation factors, phylogeny, rare species, shrubs, sodium hypochlorite, sporodochia, sprouting, statistical analysis, stem cankers, stems, surveys, wood, yeast extract, Pennsylvania
Pyrularia pubera Michx. (buffalo nut; Cervantesiaceae/Santales) is a parasitic shrub native to the eastern United States (Nickrent et al. 2010). More than 60 plant species in 31 families have been recorded as hosts (Leopold and Muller 1983). At its northern range limit in Pennsylvania, P. pubera is classified as rare (Randle and Wenzel 2014). In June 2015, extensive dieback of dozens of Pyrularia was reported from the Powdermill Nature Reserve (PNR) in Westmoreland Co., PA. Symptoms included green leaf wilt and branch dieback, sunken stem lesions, and epicormic sprouting. Lenticular sporodochia were present on dead stems. Dendrochronology of growth rings from declining shrubs revealed prolonged radial growth decline from 1999 to 2015. Surveys during 2016 to 2017 revealed additional dieback on the Forbes State Forest (FSF). From the necrotic margin of 16 stem cankers from PNR, 128 wood plugs were extracted, surface disinfested in 5% sodium hypochlorite, and plated onto glucose yeast extract agar. After 7 to 10 days, a fungus with dense floccose aerial mycelium and hyaline, oval to fusiform, 0 to 1 septate, microconidia characteristic of Fusarium (Leslie and Summerell 2008) emerged from 55% of plugs and 100% of cankers. Sporodochia that subsequently formed had 3 to 5 septate, cylindrical to slightly curved macroconidia. Translation elongation factor 1-α (EF1) sequences were generated for 20 isolates (deposited as MG975094 to MG975096) and analyzed using BLASTN against GenBank and FUSARIUM-ID databases. Results revealed that EF1 sequences from the tested isolates were 99 to 100% identical to Fusarium sp. NRRL 22586 (FSSC 13-b; AF178353, from Robinia sp.) and Fusarium solani isolate FS0723 (HQ647286; from Carya cordiformis, Park 2011). Maximum-likelihood phylogenetic analysis of these sequences along with representatives of all known species in the F. solani species complex (FSSC) nested them within FSSC undescribed species 13 (FSSC 13). To confirm pathogenicity, inoculations using isolates P1-C1, P1-C3, and P2-C4 were conducted. Healthy Pyrularia stems (approximately 2 cm in diameter) were harvested from FSF in June 2017. In the lab, ends of 20-cm stem sections were waxed and were submerged in 10% sodium hypochlorite for 15 min prior to inoculation. Stem sections were wounded with a sterile 9-mm-diameter cork borer. Blank agar plugs (control) or plugs colonized by one of three fungal treatments were aseptically placed inside wounds and sealed with tape. Ten stems were inoculated per treatment and incubated at 23°C in the dark for 6 weeks. At 6 weeks postinoculation, all inoculated stems had sunken cankers, whereas controls remained canker free. Lenticular sporodochia were abundant on fungus-treated stems. Mean canker linear growth, (length + width)/2, for all isolates was significantly larger (P < 0.001; 8.0, 8.0, and 9.5 cm) than the control (1.3 cm, wound only), but not from each other. Consistent isolation of FSSC 13 from all fungus-treated stems, coupled with molecular confirmation of 10 isolates across these same treatments, fulfilled Koch’s postulates, thus confirming pathogenicity of FSSC 13 on Pyrularia. To our knowledge, this is the first report of Fusarium stem canker caused by members of the FSSC on P. pubera or any member of the Cervantesiaceae.