Phytophthora cathayensis sp. nov., a new species pathogenic to Chinese Hickory (Carya cathayensis) in southeast China

Abstract

Crown decline and mortality associated with collar lesions were observed on Carya cathayensis (Chinese hickory) trees in a plantation in Zhejiang province, China. Examination of active lesions resulted in the isolation of a homothallic, papillate Phytophthora sp. Detailed morphological and physiological studies and phylogenetic analysis, using ITS, beta-tubulin, cytochrome oxidase I, and heat shock protein 90 gene regions, revealed that all isolates belonged to an undescribed species residing in phylogenetic Clade 4, which is described here as Phytophthora cathayensis sp. nov. Inoculation trials were conducted under greenhouse conditions on C. cathayensis and C. illinoensis (pecan) plants to fulfill Koch postulates and hypothesize a possible pathway of the incursion. An existing report of a Phytophthora species with the same ITS sequence was reported on C. illinoensis from the USA in 2009. The difference in susceptibility of the two inoculated Carya species, and the report from the USA, suggest a possible introduction with plant material from the USA to China. Citation: Morales-Rodríguez C, Wang Y, Martignoni D, Vannini A (2020). Phytophthora cathayensis sp. nov., a new species pathogenic to Chinese Hickory (Carya cathayensis) in southeast China. Fungal Systematics and Evolution 7: 99-111. doi: 10.3114/fuse.2021.07.05.

Keywords: alien; global trade; invasive; new taxon; oomycetes; pecan; systematics.

Fig. 1.

A–C. Severe dieback and mortality in Carya cathayensis orchard in Zhejiang province. D. Necrosis descending to the root. E. Edge of a collar rot lesion. F. Collar rot, tongue-shaped, brown-dark orange necrosis of the inner bark.

Fig. 2.

Bayesian tree for Clade 4 Phytophthora species produced from concatenated sequences of the ITS, beta-tubulin, cytochrome oxidase I and heat shock protein 90 gene regions using GTR + G model. Maximum likelihood was conducted on the same dataset with MEGA v. 7 and resulted in the same topology. Numbers above the branches reflect support obtained from the analysis of the same dataset (Bayesian posterior probabilities/Bootstrap values estimated by MEGA v. 7). Phytophthora plurivora (clade 2) and P. pseudosyringae (clade 3) were used as outgroup. The scale bar corresponds to substitutions per nucleotide site.

Fig. 3.

A. Ovoid papillate, laterally inserted sporangia. B. Laterally inserted sporangium with short hyphal appendice. C. Conspicuous basal plugs on empty sporangium. D. Laterally inserted semipapillate sporangium with markedly curved apex and swelling before sporangial base. E. Sporangia produced on the tips of hyphae radiating from a hyphal swelling. F. Globose chlamydospore with thin walls. G. Limoniform sporangium. H. Elongated ovoid semipapillate sporangium. I. Hyphal swelling. J. Paragynous antheridium on an immature oogonium. K. Mature oogonia with thick-walled oospore and two pellucid bodies. L. Oospore germination. M. Mature aplerotic oogonia with think walled oospore and ooplast. N. Aborted oospore. O. Aplerotic and plerotic oospores. Scale bars = 5 μm.

Fig. 4.

A–D. Colony morphologies of Phytophthora cathayensis sp. nov. Cultures were grown at 20 °C on A (upper line). V8A. B. PDA. C. MEA. D. PARPNH. Photographed 7 d after inoculation.

Fig. 5.

Radial growth rates (mean ± SE) of three isolates of Phytophthora cathayensis on V8 juice agar at different temperatures.

Fig. 6.

Necrotic lesions caused by Phytophthora cathayensis (isolate CP30) in the under-bark inoculation trial after 10 d 25 °C: on A. Carya illinoensis and B. Carya cathayensis. Scale bars = 1 cm.

Fig. 7.

Mean of length of necrosis (left) and area of the necrosis (right) caused by Phytophthora cathayensis isolates on Carya illinoensis and C. cathayensis 7 d post inoculation. Different letters indicate significant differences at P < 0.05, according to Tukey’s post-hoc test. Vertical bars indicate standard deviation.