First report of Sclerotinia sclerotiorum causing rots at the shoot base of Actinidia chinensis

The cultivation of yellow- and red-fleshed kiwifruit (Actinidia chinensis) is rapidly spreading. In May 2023, canker lesions on shoots were observed on approximately 20% of the shoots in an orchard of A. chinensis cv. Jintao in southern Italy (38°21'44.69"N, 15°55'23.92"E). Symptoms consisted of dark brown lesions, rots of the bark and internal tissues, and the abundant production of a white mycelium (Figure 1). Infected shoots died within a few days after the onset of symptoms. Following incubation in a moist chamber, several round or oval black sclerotia varying from 3 to 7 mm in diameter grew outside (Figure 2a) and inside (Figure 2b) infected tissues. To isolate the potential pathogen, infected shoots were surface sterilised by dipping for 30 seconds in 70% ethanol and 5% sodium hypochlorite, consecutively. The outer bark was removed, and tiny pieces of internal tissues were dissected and placed on potato dextrose agar (PDA). A white mycelium arose from most dissected tissues after four days' incubation at 20 ±2°C. Fifteen representative isolates had identical morphological features and were tentatively identified as Sclerotinia sclerotiorium (Figure 3a). The identity of the isolates was confirmed using the internal transcribed spacer (ITS), the RNA polymerase II largest subunit (RPB2), and the translation elongation factor 1-α (EF1-α) as barcoding genes (GenBank Accession Nos. PP544438, PP554484 and PP554485, respectively). Sequences were compared to those deposited in GenBank using BLAST and all sequences of our isolates had 100% identity with the corresponding sequences of reference strains of S. sclerotiorum (e.g. CP017816.1, CP017820.1 and MK089777.1). The identity of the isolates was confirmed by constructing a phylogenetic tree using representative ITS reference sequences of S. sclerotiorum and closely related species (Figure 3b). We conducted pathogenicity tests to fulfill Koch's postulates using unripe fruit and healthy stem cuttings of A. chinensis. A mycelial plug of S. sclerotiorum was used to inoculated wounded fruit and 10–15 cm long stem cuttings with a thin layer of bark removed. After six days at 20 ±2°C and high relative humidity, inoculated fruit (Figure 4a) and shoots (Figure 5a) developed extended tan-brown lesions and typical cottony-white mycelium. After eight and twelve days, abundant black sclerotia were produced on both fruits (Figure 4b) and stems (Figure 5b), respectively, and we were able to re-isolate S. sclerotiorum from all samples. Fruit and stem cuttings of A. chinensis which were mock inoculated with sterile agar plugs as controls, remained asymptomatic. Sclerotinia sclerotiorum is a destructive and cosmopolitan plant pathogen affecting many of the world's most important crops (Derbyshire et al., 2022). It has been reported as a postharvest pathogen of kiwifruit (Boland & Hall, 1994, Lee et al., 2015) but to our knowledge, this is the first report of rots caused by S. sclerotiorum on shoots of A. chinensis. The spread of this pathogen might severely impact kiwifruit production as infected shoots die within a few days, reducing the number of fruiting shoots. Additional investigations are needed to assess the incidence of the disease, its distribution across the cultivated area and its potential role in causing rots on fruits in the field. This study was carried out within the Agritech National Research Center and received funding from the European Union Next-GenerationEU (Piano Nazionale di Ripresa e Resilienza (PNRR) – Missione 4 Componente 2, Investimento 1.4 – D.D. 1032 17/06/2022, CN00000022). This manuscript reflects only the authors' views and opinions, neither the European Union nor the European Commission can be considered responsible for them.