Location: Foreign Disease-Weed Science ResearchTitle: Phytophthora stricta isolated from Rhododendron maximum in Pennsylvania
Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/5/2017
Publication Date: 2/9/2018
Citation: Widmer, T.L., Mcmahon, M.B., Frederick, R.D. 2018. Phytophthora stricta isolated from Rhododendron maximum in Pennsylvania. Plant Disease. 102:827.
Interpretive Summary: Phytophthora species can cause devastating plant diseases. However, the natural hosts of many recently described Phytophthora species are unknown. It is important to identify new pathogen/host associations so that a risk assessment can be accurately reported. In 2013, a leaf disease was noticed on some native rhododendron plants in Pennsylvania, near a small stream. Samples were collected and based upon molecular techniques and morphological characteristics, the cause of the disease was identified as P. stricta. This particular species has only been identified previously from water samples collected in Mississippi and Virginia. No natural plant hosts have been reported before. Rhododendrons are an important understory plant in many forests. The impact of P. stricta on rhododendrons and other plants is unknown and should be studied further.
Technical Abstract: During a survey in October 2013, in the Michaux State Forest in Pennsylvania , necrotic Rhododendron maximum leaves were noticed on mature plants alongside a stream. Symptoms were nondescript necrotic lesions at the tips of mature leaves. Colonies resembling a Phytophthora sp. were observed from collected leaf samples plated on a Phytophthora selective medium. Mycelial tips were transferred to 20% V8 agar (isolates MSF-C and MSF-F). The isolates were identified as P. stricta based on sequences from the internal transcribed spacer (ITS) region and cytochrome oxidase I (coxI) and their morphological features produced on V8 agar. ITS alignments of the 813-bp amplicon of MSF-C (Genbank accession MF138883) and MSF-F (accession MF138884) shared 100% identity to each other and exhibited 99% identity to P. stricta (accession KF192694). The coxI alignments of the 867-bp amplicon of MSF-C (accession MF138885) and MSF-F (accession 138886) were 100 and 99% identical to accession KF192694. Sporangia averaged 60.8 µm in length and 33.0 µm in width, fitting within the range for P. stricta. No chlamydospores were observed and the isolates were not tested for their mating type. Pathogenicity was confirmed by transferring mycelial plugs to R. maximum leaves that were either wounded by pricking with a sterile needle or not. Controls using V8 agar only were set up in the same manner. After 2 d at 20oC, necrosis was observed on the leaves regardless if wounded nor not. No necrosis was observed on the controls. The pathogen was re-isolated from necrotic lesions, while no colonies were recovered from the control leaves. Phytophthora stricta has been isolated previously only from irrigation water in Mississippi and stream water from Virginia. To the authors' knowledge, this is the first report of P. stricta being isolated from host material collected in a natural environment resulting from a natural infection. Further research will need to be conducted on this species and the impact it has on the environment.