Euphorbia susannae (common name Suzane's spurge) and E. inermis (Euphorbiaceae family) are grown in pots and commercialized in northern Italy. In March 2012, plants of these species grown on the same commercial farm showed signs of powdery mildew. On E. susannae, apexes of stems were covered with white mycelia and conidia, with the ultimate development of necrosis on symptomatic tissues more interested by the disease. Of 5,000 plants, 5% were affected. Conidia were hyaline, elliptical, borne in short chains (two to three conidia per chain), and 27.7 (24.4 to 30.6) μm long and 16.1 (13.6 to 19.1) μm wide. Conidiophores were erect, with a foot cell straight or slightly flexuous, 82.8 (52.7 to 117.1) μm long and 10.0 (9.2 to 11.2) μm wide, followed by two to three shorter cells 19.8 (14.7 to 28.9) μm long and 10.7 (8.7 to 13.0) μm wide. On E. inermis, a white mycelium covered the stems starting from the apexes, causing yellowing and necrosis on leaves finally killing infected tissues. Of 500 plants, 80% were affected. Conidia were 33.2 (25.7 to 42.0) μm long and 17.3 (12.6 to 21.6) μm wide. Conidiophores were erect, with a foot cell straight or slightly flexuous, 96.7 (67.0 to 138.6) μm long and 9.5 (7.7 to 11.7) μm wide, followed by two to three shorter cells 26.3 (17.5 to 42.2) μm long and 11.8 (8.3 to 16.2) μm wide. Fibrosin bodies were present on both hosts, while chasmothecia were not observed in samples from either host. The internal transcribed spacer (ITS) region of rDNA of samples of mycelium and conidia collected from the two hosts was amplified using the primers ITS1-ITS4 (3) and directly sequenced (GenBank Accession Nos. JX179221 and JX179219 for E. susannae and E. inermis, respectively). By performing BLAST analysis, the 692-bp amplicon from E. susannae and the 541-bp amplicon from E. inermis showed 100% homology with several sequences of Podosphaera spp. On the basis of morphological characteristics of the imperfect state and the ITS analysis, the causal agent of powdery mildew on E. susannae and E. inermis was identified as Podosphaera sp. Pathogenicity was confirmed by gently brushing healthy 20-month-old potted plants of E. susannae and E. inermis with mycelia and conidia from diseased plants. Four plants/treatment/species were used. Non-inoculated plants belonging to the two species served as control. Plants were maintained in a glasshouse at temperatures ranging from 18 to 25°C and relative humidity from 65 to 80%. About 20 days after inoculation, typical symptoms of powdery mildew developed on inoculated plants. The fungus observed on inoculated plants was morphologically identical to the original isolate. Non-inoculated plants did not exhibit symptoms. The pathogenicity test was performed in duplicate. Several agents of powdery mildew have been reported on Euphorbia spp. Among others, Podosphaera euphorbiae was reported on numerous Euphorbiaceae (1), P. euphorbiae-hirtae was observed on E. hirta (1) and P. euphorbiae-helioscopiae on E. pekinensis (2). To our knowledge, this is the first report of Podosphaera sp. on E. susannae and E. inermis in Italy. This disease is not presently of economic importance. Specimens are available at the Agroinnova Collection.
References: (1) U. Braun. A Monograph of the Erysiphales (Powdery Mildews). J. Cramer, Berlin-Stuttgard, German Democratic Republic, 1987. (2) S. Y. Liu. Plant Dis. 95:1314, 2011. (3) T. J. White et al. PCR Protocols: A Guide to Methods and Applications. M. A. Innis et al., eds. Academic Press, San Diego, 1990.