First report of Phytophthora insolita and Phytophthora inflata in Rhododendron

In the summer of 2003 we conducted a state-wide survey to determine if Phytophthora ramorum, the causal agent of Sudden Oak Death, was present in Ohio ornamental nurseries. The survey was concentrated on rhododendrons because this genus is considered one of the likely hosts for spread of the pathogen, due to the high volume of interstate commercial distribution and its wide natural presence in many eastern hardwood areas, where it could serve as a conduit to infection of highly susceptible eastern red oaks. 240 samples were randomly collected in 12 nurseries from rhododendrons showing foliar necrotic lesions and twig dieback symptoms. The samples yielded 51 Phytophthora spp. isolates on PARP-V8 agar. The ITS region of all isolates was amplified using the universal primers ITS1 and ITS4 and sequenced. Consensus sequences from the two directions were then blasted against the GenBank database, allowing for the identification to species of ~80% of all isolates. These identifications, and the ~20% unknowns were confirmed using morphological characterization according to (1). No P. ramorum was detected among the isolates, while we identified P. cactorum, citricola , citrophthora and nicotianae, as expected. However, we also detected two occurrences of P. inflata Caros & Tucker and one of P. insolita Ann & Ko. (P. inflata: e-value ≤ e-179, identities ≥ 95%; P. insolita: e-value = 0.0; identities = 95%.) To the best of our knowledge these species are not known to occur on rhododendron, and P. insolita has never been reported outside of South East Asia, where it has been recovered from soil only. P. inflata was isolated from two independent locations, in one cases from dead twigs, and in one case from a necrotic leaf tip. P. insolita was isolated from a leaf tip necrotic lesion with a well defined margin. Koch’s postulates were satisfied by inoculating two rhododendron plants (one cv. PJM and one cv. Nova Zembla) with the putative pathogens. Three leaves were inoculated using a plug PARP-V8 agar of 0.5 cm containing a fresh, fully grout colony, the inoculation point was covered with transparent tape. Same procedure involved three one year twigs. The appropriate controls consisted of inoculation with pure PARP-V8 agar medium. We were able to reproduce the lesions both on leaf and on twigs. The presence of the parasite was confirmed by re-isolation on sterile medium.