First Report of Phytophthora palmivora Causing Stem and Root Rot of Salix babylonica (Babylon Willow) in Fujian Province, China

Salix babylonica (L.) (Babylon willow), a popular ornamental tree species of willow native to dry areas of northern China, is planted widely in China for use in bioremediation of contaminated water or soil environments (Li et al. 2015). During July 2015, 2-year-old plants exhibiting leaf discoloration, wilting, stunted growth, and basal stem and root rot with soft water-soaked to dark lesions were found in several commercial nurseries in Quanzhou, Fujian, China (118.37°E, 24.54°N). A Phytophthora-like species was consistently isolated from symptomatic tissues of roots on potato dextrose agar (PDA) medium containing 3-hydroxy-5-methylisoxazole (HMI), benomyl, nystatin (mycostatin), pentachloronitrobenzene (PCNB), and other antimicrobial agents (rifampicin and ampicillin) at 25, 10, 25, 25, 10, and 500 µg/ml, respectively (Masago et al. 1977). Resultant isolates formed mycelial colonies in which terminal and intercalary globose to subglobose chlamydospores were observed. Chlamydospore diameters averaged 33 µm with a range of 32 to 42 µm (n = 30). Sporangia were papillate, ovoid with a broad base, and caduceus (deciduous) with short (5-µm) pedicels (n = 30). Sporangia were variable in size with an average length of 48 µm and breadth of 32 µm (n = 30). Such morphological characteristics match the descriptions for Phytophthora palmivora (Erwin and Ribeiro 2005). To confirm the identification, the nuclear rDNA ITS region, cytochrome oxidase subunit 1 (COI) gene, and ras-related protein (Ypt1) gene of a representative isolate, named QZ-3, were amplified and sequenced with primers ITS1/ITS4 (Naher et al. 2011), FM85/FM80 (Martin and Tooley 2003), and Yph1F/Yph2R (Schena et al. 2008), respectively. A BLAST search of the ITS sequence of isolate QZ-3 (KU645763) revealed a 100% identity with a reference isolate of P. palmivora (KT175509). The other two consensus sequences (GenBank accessions KU645764 for COI and KU645765 for Ypt1) also showed 100% identity with the sequences of P. palmivora available in GenBank (HQ261380 and KJ755166, respectively). Based on morphological characteristics and molecular analysis, isolate QZ-3 was identified as P. palmivora. Pathogenicity tests were conducted on 1-year-old basal stems of S. babylonica growing in pots. A 5-mm-diameter bark plug was removed aseptically and a same-size PDA disc of QZ-3 was placed on the wound. Control plants were treated in the same manner with sterile PDA discs. Inoculation points were sealed with sterile moist cotton and Parafilm. Treated plants were incubated at 25°C in a moist chamber with a 12-h photoperiod. The entire experiment was repeated 2 weeks later. Within 3 to 4 weeks after inoculation, all inoculated plants showed clearly noticeable necrotic lesions around the inoculation points in basal stems, symptoms of root and stem rot developed rapidly thereafter, and the plants wilted with chlorotic foliage. All symptoms were identical to those observed on the naturally diseased hosts. Control plants remained symptomless. P. palmivora was reisolated only from the inoculated stems, thus fulfilling Koch’s postulates. Therefore, to our knowledge, this is the first report of P. palmivora causing stem and root rot of S. babylonica in China. Such information would be helpful for growers to develop management strategies for control this new disease. © 2016, American Phytopathological Society. All rights reserved.