First Report of Dieback on Hybrid Rhododendrons Caused by Neofusicoccum luteum and N. parvum in Spain
Authors: Pintos Varela C. , Redondo Fernández V. , Mansilla Vázquez J. P. , Aguín Casal O. 
Published: November 2010

During the conducting of Phytophthora ramorum surveys at Galician public parks (northwestern Spain) in 2010, established Rhododendron spp. plants were observed to be exhibiting leaf spots and necrosis, shoot blight, and cankers and dieback of shoots and branches. Branches and leaves of affected rhododendrons contained pseudothecia with bitunicate asci and hyaline pseudoparaphyses, and pycnidia were observed within the same stromatic masses. Symptomatic samples were disinfested in 0.5% sodium hypochlorite for 3 min. Tissues were cut from the margin of lesions, placed onto malt extract agar amended with streptomycin (25 μg ml–1), and incubated at 25°C in the dark. Cultures displaying morphological characteristics associated with Botryosphaeriaceae species were subcultured on 2% water agar with sterilized Pinus pinaster needles as a substrate and incubated at 25°C under near-UV light to encourage pycnidial production (1). Single conidial cultures gave rise to two distinct colonies on potato dextrose agar (PDA) at 25°C. In type 1, isolates produced a sparse, aerial mycelium and a characteristic yellow pigment that was more intense after 3 days, thereafter becoming violaceous and gradually turning dark gray. Growth occurred in the range of 4 to 38°C with an optimum at 29°C. Conidia were hyaline, fusiform, aseptate, thin walled, and averaged 21.1 (14.3 to 25.0) × 5.7 (4.3 to 6.8) μm with a length/width (L/W) ratio of 3.7 ± 0.4 (n = 100). On the basis of these characteristics, isolates were identified as Neofusicoccum luteum (1,3). Colonies of type 2 produced a dense, white-to-yellowish mycelium that rapidly became gray followed by marked diurnal zonation. Mycelial growth occurred in the range of 6 to 38°C with an optimum at 29 to 30°C. Conidia were hyaline, elliptical or fusiform, aseptate, thin walled, and averaging 18.3 (14.1 to 20.7) × 5.8 (4.6 to 7.0) μm with a L/W ratio of 3.2 ± 0.4 (n = 100). These isolates were identified as N. parvum (1,2). Identity was confirmed by DNA sequences analysis of internal transcribed spacer (ITS) regions. Comparison of the sequences of type 1 and 2 showed 100% homology with N. luteum and N. parvum (GenBank Accession Nos. EU673311 and GU251146, respectively). Representative sequences were deposited at GenBank (Accession Nos. HQ197352 and HQ197351). Pathogenicity of each isolate of N. luteum and N. parvum was confirmed by inoculating four 3-year-old Rhododendron spp. seedlings grown in pots. Shallow cuts were made in three branches of each plant. A colonized 6-mm agar plug, removed from the margin of an actively growing colony, was inserted beneath the flap and sealed with Parafilm. Four control seedlings received only sterile PDA agar plugs. Plants were maintained at 26°C and 70% humidity for 21 days. Inoculated plants began showing symptoms after 3 days. Necrosis progressed quickly and bidirectionally from the wound, resulting in death of leaves and wilting of shoots. N. luteum and N. parvum were reisolated from all inoculated plants but not from the controls. To our knowledge, this is the first report of N. luteum and N. parvum on Rhododendron spp. in Spain.

References: (1) P. W. Crous et al. Stud. Mycol. 55:235, 2006. (2) S. R. Pennycook et al. Mycotaxon 24:445, 1985. (3) A .J. L. Phillips et al. Sydowia 54:59, 2002.