First Report of Root Rot Caused by Phytopythium helicoides on Pistachio Rootstock in California

Abstract

HomePlant DiseaseVol. 100, No. 11First Report of Root Rot Caused by Phytopythium helicoides on Pistachio Rootstock in California PreviousNext DISEASE NOTES OPENOpen Access licenseFirst Report of Root Rot Caused by Phytopythium helicoides on Pistachio Rootstock in CaliforniaE. J. Fichtner, G. T. Browne, M. Mortaz, L. Ferguson, and C. L. BlomquistE. J. FichtnerSearch for more papers by this author, G. T. BrowneSearch for more papers by this author, M. MortazSearch for more papers by this author, L. FergusonSearch for more papers by this author, and C. L. BlomquistSearch for more papers by this authorAffiliationsAuthors and Affiliations E. J. Fichtner , University of California Cooperative Extension Tulare Co., Tulare, CA 93274 G. T. Browne , USDA-ARS, CPGRU, Department of Plant Pathology, University of California Davis, CA 95616 M. Mortaz L. Ferguson , Department of Plant Sciences, University of California Davis, CA 95616 C. L. Blomquist , California Department of Food and Agriculture, Plant Pest Diagnostics Laboratory (CDFA-PPDC), Sacramento, CA 95832. Published Online:16 Aug 2016https://doi.org/10.1094/PDIS-12-15-1424-PDNAboutSections ToolsAdd to favoritesDownload CitationsTrack Citations ShareShare onFacebookTwitterLinked InRedditEmailWechat Approximately 25 potted 2-year-old pistachio rootstock trees in a Kern County, CA, research plot maintained outdoors and irrigated to container capacity exhibited wilting in June 2013. Symptomatic plants gradually declined and died through November 2013. Extensive root necrosis was observed on symptomatic plants. Necrotic roots of seedling UCB-1 rootstock (Pistacia atlantica × Pistacia integerrima) submerged in CMA-PARP selective medium (Jeffers and Martin 1986) yielded a Pythium-like organism. On V8 juice agar, a representative isolate produced rapidly growing colonies (188 mm/day, radial) that produced subglobose, papillate sporangia measuring 25 to 42.5 × 25.0 to 37.5 µm (33.4 × 30.4 µm average). Sexual structures were not observed. DNA was extracted from the isolate and amplified by PCR with primers ITS1 and ITS4 (White et al. 1990). BLAST query of the sequence, GenBank accession no. KT901799 matched HQ643382, Phytopythium helicoides, with 98% identity. Diseases caused by P. helicoides have been reported on several plants and include root and stem rot on begonia (e.g., Yang et al. 2013) and rhizome rot on lotus (Yin et al. 2016). Phytopythium is a recently established genus containing organisms formerly classified as Pythium Clade K species (de Cock et al. 2015). For completion of Koch’s postulates, two isolates of P. helicoides were grown for 14 days on V8-oat-vermiculite substrate. UCB-1 clonal rootstock plants were purchased from a wholesale nursery, acclimatized in the greenhouse, grown to approximately 12 cm in height, and then transplanted into potting medium mixed with either uninfested substrate (the control) at a rate of 5% (v/v), or P. helicoides-infested substrate (2.5% v/v of each isolate). To induce zoospore release, pots were submerged in water for 4 h with the water surface maintained 1 cm above the soil. Control plants were treated identically to those in infested soil. The soil was then allowed to drain, and plants were watered daily. Two experimental runs included eight and 12 replicate plants of each treatment, respectively. Plants challenged with P. helicoides wilted within 1 week of inoculation. By 2 weeks after inoculation, 100% and 43% of inoculated plants died in the first and second experimental runs conducted in summer and winter, respectively. Control plants remained asymptomatic. By 6 weeks after inoculation, root mass of inoculated plants was only 28% and 65% of that of control plants in respective experimental runs. P. helicoides was consistently isolated from symptomatic plants and never from control plants. To our knowledge, this is the first report of root rot caused by P. helicoides on pistachio. The results indicate that the pathogen can be aggressive on UCB-1 rootstock, which is used for approximately half the commercial pistachio acreage in California. The greater mortality observed in summer than in winter suggests that variation in host or environmental factors may modulate disease development. The source of primary inoculum in the original symptomatic plants is unknown. Future work is needed to assess the importance and epidemiology of P. helicoides in commercial pistachio orchards.