Plectosphaerella species associated with root and collar rots of horticultural crops in southern Italy

Abstract

Plectosphaerella cucumerina, most frequently encountered in its Plectosporium state, is well known as a pathogen of several plant species causing fruit, root and collar rot, and collapse. It is considered to pose a serious threat to melon (Cucumis melo) production in Italy. In the present study, an intensive sampling of diseased cucurbits as well as tomato and bell pepper was done and the fungal pathogens present on them were isolated. Phylogenetic relationships of the isolates were determined through a study of ribosomal RNA gene sequences (ITS cluster and D1/D2 domain of the 28S rRNA gene). Combining morphological, culture and molecular data, six species were distinguished. One of these (Pa. cucumerina) is already known. Four new species are described as Plectosphaerella citrullae, Pa. pauciseptata, Pa. plurivora and Pa. ramiseptata. Acremonium cucurbitacearum is shown to be a synonym of Nodulisporium melonis and is transferred to Plectosphaerella as Plectosphaerella melonis comb. nov. A further three known species of Plectosporium are recombined in Plectosphaerella.

Keywords: D1/D2; ITS; LSU; Plectosporium; phylogeny; rDNA; systematics; taxonomy.

Fig. 1

Maximum Likelihood tree obtained from LSU sequence data with bootstrap support values in Maximum Likelihood/Bayesian Posterior Probability scores.

Fig. 2

Maximum Likelihood tree obtained from ITS sequence data with bootstrap support values from Maximum Parsimony/Bayesian Posterior Probability/Maximum Likelihood. Ex-type isolates are in bold face red.

Fig. 3

Plectosphaerella citrullae. a, b. Colonies on PDA after 14 d at 23 ± 2 °C; c–e. conidiophores and phialides; f–h. phialides; i, j. hyphal coils; k. conidia. — Scale bars: c–e, i = 10 μm; f–h, j, k = 5 μm.

Fig. 4

Plectosphaerella cucumerina. a–e. Colonies on PDA after 14 d at 23 ± 2 °C; f–h. asci and ascospores; i. hyphal coil with phialides; j–p. phialides; q. aseptate conidia; r. 1-septate conidia; s. conidiophore. — Scale bars: f–i = 10 μm; j–s = 5 μm.

Fig. 5

Variation in culture morphology in Plectosphaerella cucumerina. All cultures were grown at 23 °C on PDA for 14 d.

Fig. 6

Plectosphaerella melonis. a. Colony on PDA after 14 d at 23 ± 2 °C; b–d. conidiophores with phialides; e–g. phialides; h, i. conidia; j. hyphal coil; k–m. chlamydospores. — Scale bars: b–e = 10 μm; f–m = 5 μm.

Fig. 7

Plectosphaerella pauciseptata. a, b. Colonies on PDA after 14 d at 23 ± 2 °C; c, d. hyphal coils; e–l. phialides; g, h, j. polyphialides; m, n. conidia. — Scale bars: c = 10 μm; d–n = 5 μm.

Fig. 8

Plectosphaerella plurivora. a–d. Colonies on PDA after 14 d at 23 ± 2 °C; e–i. phialides; j, k. conidia; l, m. hyphal coils; n, o. swollen conidia becoming chlamydospore-like. — Scale bars: e–k, n, o = 5 μm; m, n = 10 μm.

Fig. 9

Plectosphaerella ramiseptata. a, b. Colonies on PDA after 14 d at 23 ± 2 °C; c–k, phialides; d. hyphal coil with phialides; l, m. conidia. — Scale bars: c–m = 5 μm.