Ribes lobbii (Gray) is a native, deciduous shrub from British Columbia, California, Oregon, and Washington that is grown for its pendulous red and white flowers, bristly fruit, and attractive, aromatic foliage. It is uncommon in nature and has only recently begun being propagated for sale in ornamental and native nurseries. In April 2013, 2-year-old containerized plants (15/50 plants) were found with severe wilt and chlorosis symptoms at a nursery production facility in western Oregon. Most fine roots were completely rotted and larger roots exhibited numerous black lesions. By the end of August, 50% of the plants were affected and most died within a few weeks of initial symptom development. At least three initially asymptomatic plants developed symptoms after being sold and planted. Isolation was attempted from 10 plants by plating pieces of necrotic root tissue (3 mm2) that had been surface disinfested for 1 min in 10% bleach and 1 min in 70% ethanol onto PARP medium (2). Eight Phytophthora isolates were recovered and identified as P. cactorum (seven isolates, GenBank Accession No. KM085441) and P. citrophthora (one isolate, KM085442) on the basis of morphology and 99 to 100% internal transcribed spacer (ITS) sequence similarity to published sequence data (1). One isolate of each species was used to prepare vermiculite inoculum (3) and subsequently to infest potting media (Sunshine Professional Growing Mix) at 100 propagules per gram. Due to the scarcity of R. lobbii in nature and the nursery trade, only a limited number of plants were available for pathogenicity tests. Three 1-year-old rooted cuttings of R. lobbii (~15 to 30 cm tall) were transplanted into individual 10-cm2 pots containing 175 g of the respective infested media for each pathogen. Three plants transplanted into individual pots of noninfested media served as negative controls. Plants were then watered to pot capacity and incubated in a greenhouse at 28/24°C day/night. The entire experiment was repeated 2 weeks later. Within 1 week after inoculation, all inoculated plants in both trials wilted and died, regardless of the Phytophthora species used; negative control plants remained healthy. Each pathogen (with identity confirmed by ITS sequence analysis) was re-isolated from the roots and stem of each respective inoculated plant, but was not isolated from negative controls, thus fulfilling Koch's postulates. Therefore, to my knowledge, this is the first report of P. cactorum and P. citrophthora as root rot pathogens of R. lobbii. The use of infected, but asymptomatic native plants in habitat restoration efforts will likely compromise success and there is considerable risk to nearby plant species given the broad host range of both pathogens.
References: (1) D. E. L. Cooke et al. Fungal Genet. Biol. 30:17, 2000. (2) M. E. Kannwischer and D. J. Mitchell. Phytopathology 68:1760, 1978. (3) J. E. Weiland et al. Plant Dis. 97:744, 2013.