In January 2024, symptoms of a stem rot were observed on about 15% of 5,000 potted plants of Polaskia chichipe grown in a farm located in Ventimiglia (Imperia province, northern Italy). A black rot appeared at the top of the stems that showed soft and blackish internal tissues (Picture 1A). Finally, the affected plants died. Affected tissues contained pale brown septate conidia, ellipsoidal to fusiform to obclavate in shape, measuring 23.5 to 73.2 × 9.8 to 14.8 (average 45.8 × 12.1) µm (Picture 1B). Soft, dark green fungal colonies (Picture 1C) were isolated from the margins of affected and healthy tissues on potato dextrose agar (PDA) amended with streptomycin sulphate (25 mg/L). Nineteen morphologically similar isolates (about 68%) were obtained from five symptomatic samples. The DNA was extracted from monosporic cultures of two representative isolates (24-05-OA2, CBS 154573; and 24-05-OA3, CBS 154575). Translation elongation factor 1-alpha (tef1) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) regions were examined using the primers EF-983F/ EF-2218R (Schoch et al. 2009) and gpd1/gpd2 (Manamgoda et al. 2012). Isolate 24-05-OA2 yielded sequences of 909 (tef1) and 554 (GAPDH) bp (GenBank accessions: PV423382.1, PV423383.1); isolate 24-05-OA3 yielded sequences of 948 (tef1) and 550 bp (GAPDH) (GenBank accessions: PV753714.1 and PV696458.1). For tef1, BLASTn analysis showed 99.77% (24-05-OA2) and 99.78% (24-05-OA3) identity when compared to Curvularia cactivora (accession number MN688857.1). For GAPDH, BLASTn analysis showed for both the isolates 100% identity when compared to C. cactivora (accession number LT715853.1). For pathogenicity tests, a conidial suspension was made from a monosporic culture of the isolate 24-05-OA3 and used at the concentration of 6.7 × 105 conidia/ml. The inoculum (45 ml) was sprayed on 18 (9 wounded and 9 non-wounded) plants of P. chichipe. Stems were wounded with a sterile needle (9 wounds per plant). Eighteen (9 wounded and 9 non-wounded) control plants were sprayed with sterile water. All plants were enclosed in moistened bags and maintained at 22 ± 1°C (12 h fluorescent light, 12 h dark). Initial symptoms appeared on wounded stems about 4 days post-inoculation, whereas on non-wounded inoculated plants symptoms appeared after 6 days. Finally, rotten tissues (size from 1 to 10 cm approximately) were about 60% and 10% of wounded and non-wounded stems respectively. No symptoms appeared on controls. The pathogenicity test was conducted two times with the same result. Tef1 and GAPDH regions of isolate 24-05-OA2-REIS (CBS 154574) reisolated from inoculated plants were examined and two sequences of 948 (tef1) and 571 bp (GAPDH) were acquired (GenBank accession numbers PV753713.1 and PV696457.1). BLASTn analysis confirmed 99.78% and 100% identity for tef1 and GAPDH respectively when compared to C. cactivora (accession numbers MN688857.1 and LT715853.1). Multi-locus phylogenetic analysis confirmed the identification. The stem rot caused by C. cactivora (Syn.: Drechslera cactivora and Bipolaris cactivora; Marin-Felix et al. 2020) has been reported on several succulent plants in Italy (Polizzi 1996; Garibaldi et al. 2019). However, this is the first report of this pathogen on P. chichipe in Italy, as well as worldwide. This report provides information for diagnosis of C. cactivora that is helpful to avoid the spread of the disease on P. chichipe and on numerous succulent plants that are potential hosts of this pathogen.
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