Nothophoma populi Y. Zhang

Guo, Lin-Na, Wang, Jia-Wen, Zhao, Li-Li, Zhang, Lin & Zhang, Ying, 2025, Fungal canker disease on Populus spp. in China caused by three species of Nothophoma including N. populi sp. nov., Phytotaxa 681 (2), pp. 199-210 : 204-205

Nothophoma populi Y. Zhang ter & L.N. Guo, sp. nov. ( Fig. 3 )

Mycobank:— 856310

Etymology:— in reference to the host genus, Populus .

Asexual morph: Conidiomata pycnidial, 290–355 × 290–420 μm, semi-immersed or superficial on the agar surface, aggregated, solitary or confluent, darkening with age, appearing as small black spots, mostly globose to irregular, pale brown, dark brown near the ostioles, glabrous, thick-walled, with hyphae around the ostioles, on a distinctly elongated neck, up to 70 μm long, wall pseudoparenchymatous, composed of isodiametric or globose cells, 3 to 5 layers, outer two cell layers slightly pigmented. Conidiogenous cells phialidic, hyaline, smooth, ampulliform or lageniform, 4.7–7.3 × 2.7–3.9 μm. Conidia hyaline but incidentally olivaceous buff, ovoid, oblong to ellipsoidal, smooth and thin-walled, aseptate, 5.5–7.2× 3.8–5.2 μm (av. = 4.7 ± 0.3 × 6.2 ± 0.5 μm, n = 30), sometimes with some very small guttules. Sexual morph: not observed.

Material examined:— CHINA. Beijing City: Changping District, Xiaoshahe Industrial Park, from stems of Populus spp. , 10 May 2023, Y. Zhang & L. N. Guo (holotype BJ 231.1; ex-type living culture XXX). CHINA. Beijing City: Changping District, left embankment of Wenyu River, from stems of Populus spp. , 20 June 2023, Y. Zhang & L. N. Guo (paratype BJ 232.1, BJ 232.4, living culture XXX, XXX).

Culture characteristics:— Colonies on MEA, 45–51 mm diam. after 7 d at 25 ° C, margin regular, aerial mycelium flat, dark brown to black, reverse pale brown to black, darkening gradually after 7 to 10 days. Colonies dense and fluffy, with abundant pycnidia and irregular distribution on the surface of the medium, producing wet conidial mass in a drop.

According to the phylogenetic analysis of combined ITS, RPB 2 and TUB 2 sequences, Nothophoma populi nests within Nothophoma and formed a moderately supported subclade with N. spiraeae , N. quercina and N. brennandiae . Morphologically, the hyaline but incidentally olivaceous buff conidia of N. populi could be readily distinguished from the brown conidia of N. brennandiae and N. quercina ( Martino et al. 2024, Aveskamp et al. 2010). In particular, the conidia of N. populi are longer than those of N. spiraeae ( Zhang et al. 2020) . All reported species of Nothophoma were included in the phylogenetic analysis, except for N. multilocularis , which lacks available DNA sequences (http://www. indexfungorum.org.asp, 5 April 2024). Nevertheless, N. multilocularis and N. populi can be readily distinguished based on conidial dimensions (5.5–7.2 × 3.8–5.2 μm vs. 9–20 × 3–4 µm) ( Abdel-Wahab et al. 2017). Morphological characteristics and multi-locus phylogenetic analysis ( ITS, RPB 2, and TUB 2) confirmed N. populi as a new species of Nothophoma .

Pathogenicity test

These tests showed that Nothophoma populi , N. platycladus , N. juglandis were pathogens of poplar ( Fig. 4 ). Sunken cankers became evident on the stems four weeks post-inoculation. The lesion lengths caused by N. juglandis were significantly longer than those of N. populi and N. platycladus , while there was no significant difference between N. populi and N. platycladus . ( P <0.01) ( Fig. 5 ). All three inoculated Nothophoma species were re-isolated from the necrotic stems, thus fulfilling Koch’s postulates.

Figures

FIGURE 3. Morphology of Nothophoma populi. A, B. Colony surface and reverse side after 7 days incubation on MEA. C, D. Colony surface and reverse side after 10 days incubation on MEA. E, F. Pycnidia produced on colony surface. G, H. Conidiogenous cells. I, J. Released conidia. Scale bars: E, F = 250 µm, G–J = 10 µm.

FIGURE 4. Symptoms of three Nothophoma species inoculated on poplar at four weeks. Control on detached stems (A1, A2). Disease symptoms inoculated with N. populi (B1, B2); N. platycladus (C1, C2); N. juglandis (D1, D2).

FIGURE 5. Lesion length caused by the three isolated taxa inoculated on detached poplar stems. Columns represent the mean values of three replicate stems. Vertical bars with different letters indicate a significant difference (P <0.05) according to Tukey’s honest significant difference test.