First Report of Cobweb Disease Caused by Cladobotryum dendroides on Shiitake Mushroom (Lentinula edodes) in Spain

From the paper

Abstract

Lentinula edodes, known as shiitake, is the most cultivated edible mushroom worldwide, representing about 22% of the world’s output, with China being the major producer. In 2017, symptoms of cobweb were observed on cultivated shiitake mushrooms in Castilla-La Mancha (Spain), affecting more than 30% of the blocks of pasteurized substrate. Cobweb appeared at the end of the crop cycle as circular white patches over the surface of the blocks of substrate and fruitbodies. The fine gray-white mycelium rapidly covered pinheads, stalks, pileus, and gills, eventually resulting in the decomposition of the entire fruitbody. The mycelium can colonize the surface of the substrate completely and may turn reddish brown. To isolate the parasitic fungus, infected tissues of L. edodes were plated onto potato dextrose agar (PDA). Fungal colonies consisted of white, cottony, aerial mycelium that spread rapidly on the PDA, changing from white to yellow and finally pink, as the red pigment spread into the agar. The conidiogenous cells were 30 to 55 µm long, 5 to 6 µm wide at the base, and tapered slightly toward the tip, with evident rachis on the phialides. Conidia were hyaline, cylindrical to ellipsoidal, 16 to 28 × 8 to 11 µm, 0 to 3 septate, with a predominance of 2-celled (50%) and 4-celled (31%) conidia. Brown chlamydospores were present. These morphological characteristics were consistent with previous reports (Rogerson and Samuels 1993). Genomic DNA was extracted from seven isolates and the internal transcribed spacer (ITS) region was amplified using ITS1F/ITS4 primers and sequenced (Gea et al. 2017). The obtained ITS sequences were identical (GenBank accession no. MF992212). A BLAST search in GenBank showed the highest similarity with a sequence of Cladobotryum dendroides (teleomorph Hypomyces rosellus) and 99% similarity with two sequences of the same species corresponding to the accession numbers Y17092, Y17091, and Y17090 (McKay et al. 1999), respectively. A pathogenicity trial was performed in an experimental mushroom-growing room using 15 blocks containing pasteurized, spawned, and incubated L. edodes substrate (18 kg, 0.64 m² of useful surface). Eight days after starting the cropping cycle, a conidial suspension (10⁶ conidia ml⁻¹) of one isolate of C. dendroides was sprayed (10 ml per block) onto the surface of 10 blocks. Five blocks were sprayed with sterile distilled water as a control. A temperature of 15 to 17°C and relative humidity of 90 to 95% were maintained throughout cropping. The first cobweb symptoms developed 5 days after inoculation. The number of shiitake fruitbodies affected by cobweb was 50.1% in the blocks inoculated with C. dendroides, and the yield of diseased shiitake mushrooms was 47.6% of the total yield, whereas the noninoculated blocks remained symptomless. C. dendroides was consistently reisolated from the inoculated blocks (100%). Control methods include measures to prevent conidia dispersal within culture facilities and the application of fungicides, mainly metrafenone. Cobweb disease has been reported as infecting different species of cultivated edible mushroom. Recently, C. mycophilum was reported as the causal agent of cobweb on cultivated king oyster (Pleurotus eryngii) and white button mushroom (Agaricus bisporus) in Spain (Gea et al. 2011, 2012) but, to the best of our knowledge, this is the first report of C. dendroides causing cobweb disease in shiitake mushroom.