First report of root rot caused by Phytopythium helicoides on pistachio rootstock in California

Authors: FITCHTNER, ELIZABETH - University Of California - Cooperative Extension Service; Browne, Greg; MORTAZ, MORAD - University Of California; FERGUSON, LOUISE - University Of California; BLOMQUIST, CHERYL - California Department Of Food And Agriculture

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/5/2016
Publication Date: 8/16/2016
Citation: Fitchtner, E.J., Browne, G.T., Mortaz, M., Ferguson, L., Blomquist, C.L. 2016. First report of root rot caused by Phytopythium helicoides on pistachio rootstock in California. Plant Disease. http://dx.doi.org/10.1094/PDIS-12-15-1424-PDN.

Interpretive Summary: We examined pathogenicity of Phytopythium helicoides, a water mold fungus, on UCB-1 rootstock to investigate its role in root disease and collapse observed on potted pistachio plants. 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 levels of root rot were observed on the symptomatic plants. An oomycete was consistently isolated from the roots and identified as Phytopythium helicoides, based on its production of subglobose, papillate sporangia measuring 25 to 42.5 × 25.0 to 37.5 µm (33.4 × 30.4 µm average), a lack of sexual structures, and 98% ITS 1 and 2 sequence homology with voucher sequences of the pathogen. To test pathogenicity of P. helicoides isolates, UCB-1 clonal rootstock plants grown to approximately 12 cm in height were 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 from the inoculants, pots were submerged in water for 4 h, with the water surface maintained at 1 cm above the soil surface. Control plants were treated identically to those in infested soil. The inoculated plants, but not the control plants, consistently developed root rot and suffered high mortality rates. P. helicoides was consistently isolated from roots of the symptomatic plants and never from control plants. 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. To our knowledge, this is the first report of root rot caused by P. helicoides on pistachio. Future work is needed to assess the importance and epidemiology of P. helicoides in commercial pistachio orchards.

Technical Abstract: We examined pathogenicity of Phytopythium helicoides on UCB-1 rootstock to investigate its role in root disease and collapse observed on potted pistachio plants. 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. An oomycete was isolated from the roots and identified as Phytopythium helicoides, based on its production of subglobose, papillate sporangia measuring 25 to 42.5 × 25.0 to 37.5 µm (33.4 × 30.4 µm average), a lack of sexual structures, and 98% ITS 1 and 2 sequence homology with voucher sequences of the pathogen. UCB-1 clonal rootstock plants grown to approximately 12 cm in height were 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 inoculated plants, but not the control plants, consistently developed root rot and suffered high mortality rates. P. helicoides was consistently isolated from symptomatic plants and never from control plants. 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. To our knowledge, this is the first report of root rot caused by P. helicoides on pistachio. Future work is needed to assess the importance and epidemiology of P. helicoides in commercial pistachio orchards.