Michael A. Ellis,
Sally A. Miller,
A. F. Schmitthenner,
Department of Plant Pathology,
Ohio Agricultural Research and Development Center,
The Ohio State University,
Wooster, Ohio.
Kenneth D. Cochran,
Secrest Arboretum,
Ohio Agricultural Research and Development Center,
The Ohio State University,
and The Ohio State University Agricultural Technical Institute,
Wooster, Ohio.
Originally published in Plant Disease. 1992. Vol. 77, 537. Reprinted with permission.
Phytophthora cinnamomi Rands has been reported to cause a serious die-back and root rot disease of Taxus spp. in Indiana (4). The disease occurred on cuttings in rooting benches and rooted cuttings and older plants in lath houses and in the field. Symptoms on infected plants included a gradual loss of normal green color (chlorosis) and die-back of the aerial portion of affected plants. The root system of affected plants showed extensive discoloration (reddish-brown to black) and a dry root rot. The disease was most severe in plantings on poorly drained sites or under conditions of high soil moisture. Growers frequently attributed losses to poor soil drainage, unfavorable weather, or poor quality stock (4).
During the summer of 1991, several accessions in the Taxus collection at the Secrest Arboretum on the Wooster campus of The Ohio State University's Ohio Agricultural Research and Development Center (OARDC) developed symptoms of foliar chlorosis, reduced growth, and eventual dieback of the aerial portions of affected plants. The purpose of this paper is to report the results of studies to determine the cause of these symptoms.
In October 1991, the species and cultivars of Taxus listed here were dug and the roots examined - T. cuspidata 'TV spreading,' T. x media 'F&F compacta,' T. x media 'Mitiska Upright,' and T. x media 'Wilsonii.' All plants showed above-ground symptoms similar to those described earlier. Plants were dug to include major lateral roots and the crown. Root samples were taken from each plant, placed in individual plastic bags, and stored at 4°C until they were assayed for the presence of Phytophthora spp. No sample was stored more than 24 hours prior to conducting isolations.
Root samples were washed in running tap water then examined for lesions. A lesion was considered to be an area of discoloration (reddish brown to black) with a sharp line of demarcation between healthy (white) and diseased tissue. The epidermis was removed by scraping with a scalpel, and discolored tissue samples were taken from near the edge (line of demarcation) of the lesion. A minimum of 25 tissue pieces (five pieces per plate) were placed on penta-chloronitrobenzene-benomyl-neomycin-chloramphenicol (PBNC) medium (3), a selective medium for Phytophthora spp. Plates were incubated at 24°C for five to seven days. The recovery of fungi was recorded from day five through seven. Representative isolates of the different fungi recovered were made for identification purposes.
One species of Phytophthora was consistently recovered from lesions on affected roots. Pathogenicity tests using this fungus were conducted by inoculating two-year-old transplants of T. x media 'Densiformis.' Bare-rooted plants were planted in a peat-sand-soil mix (1-1-2) containing infested oat kernels. Oat kernels were autoclaved, allowed to cool, then inoculated with plugs of the test fungus taken from the margin of five-day-old cultures on PBNC medium. Kernels were incubated at room temperature for eight weeks. Fifteen grams (approximately 150 kernels) of infected oat kernels were thoroughly mixed with each liter of potting mix. Plants were planted in two liters of potting mix in 2.5 liter ceramic pots. Plants planted in potting mix containing noninfected oat kernels served as controls. Pots were saturated with water each day, but were not maintained under flooded conditions. The test consisted of 10 inoculated and 10 noninoculated plants in individual pots. After 12 weeks, all plants were removed from pots and examined for disease symptoms. Isolations were conducted to recover the test fungus from diseased roots.
The Phytophthora species that was consistently isolated from lesions on affected plants was P. cinnamomi. Identification of the fungus was confirmed by A. F. Schmitthenner, OARDC. This is the first report of this pathogen on Taxus in Ohio. No other Phytophthora spp. or other fungi were recovered during the course of this study. After 12 weeks, all inoculated plants were dead, and Phytophthora cinnamomi was recovered from lesions on roots of all inoculated plants. Plants in noninfested soil developed no above-ground symptoms, and their roots appeared normal.
The results demonstrated that P. cinnamomi does cause a root rot of Taxus spp. in Ohio. Another species of Phytophthora (P. citrophthora) has also been reported to cause a root rot of Taxus in Ohio (2). Phytophthora root rot is usually associated with poorly drained areas or other situations that result in periods of high and sustained soil moisture (1). The years 1990 and 1992 have seen some of the wettest growing seasons in recorded history. Conditions such as this could result in an increased incidence of this disease in many regions of Ohio.
The extent to which this disease occurs in Ohio is not known. Several other disorders can cause above-ground symptoms similar to those observed with this disease. Crown and root feeding by the black vine weevil, Otiorhynchos sulcatos F., results in above-ground symptoms on Taxus that are almost identical to those caused by Phytophthora root rot; however, the two disorders are easily distinguished upon visual observation of exposed crowns and roots. Physical damage (tunnels) caused by feeding of the black vine weevil larvae was not observed on any of the plants used in this study. Further research is needed to determine the extent to which this disease is distributed in Ohio.
1. Erwin, D. C., S. Bartnicki-Garcia, and P. H. Tsoo, Editors. 1983. Phytophthora: Its Biology, Taxonomy, Ecology, and Pathology. APS Press. St. Paul, Minn. 392 pp.
2. Gerlach, W. W. P., H. A. J. Hoitink, and A. F. Schmithenner. 1975. Survival and host range of Phytophthora citrophthora in Ohio nurseries. Phytopathology. 66:309-311.
3. Schmitthenner, A. F. 1973. Isolation and identification methods for Phytophthora and Pythium. Proceedings of the First Woody Ornamental Disease Workshop. University of Missouri, Columbia. 128 pp.
4. Schreiber, L. R. and R. J. Green. 1959. Die-back and root rot disease of Taxus spp. in Indiana. Plant Disease Reporter. 43:814-817.