Harry A. J. Hoitink and Steven T. Nameth, Professors, Department of Plant Pathology, Ohio Agricultural Research and Development Center, The Ohio State University; and James C. Locke, Research Plant Pathologist, U.S. Department of Agriculture, Agricultural Research Service, Application Technology Research Unit, Wooster, Ohio.
Phytophthora diseases affect many nursery crops. They can be divided into dieback diseases that involve blights of leaves and shoots, and root and crown rots. Many nursery crops are susceptible. Plants in the family Ericaceae (rhododendrons, azaleas, etc.) are among the most susceptible.
Phytophthora diseases, such as root rot of Fraser fir and of Chamaecyparis lawsonii, apparently are caused by a single species of the Phytophthora fungus. Others, such as Phytophthora root rot of rhododendron, may be caused by any of eight Phytophthora species.
Three different species of Phytophthora have been isolated from the same infected rhododendron plant, demonstrating rhododendron's high level of susceptibility to this root-rot disease. Azalea and Pieris, like rhododendron, are susceptible to both root rot and dieback.
Lilac is another crop highly susceptible to Phytophthora root rot and dieback. Potentilla is susceptible to dieback only, and Taxus and junipers to root rot only. Crabapples and several other rosaceous plants are also susceptible to collar rots.
In the 1960s and early 1970s, root rots were the most common diseases of nursery plants. They were especially severe on plants in poorly drained soils or in peat mixes. Bark-amended media, now used widely for disease control in the industry, helped suppress root rots.
After reducing the threat of root rot through the use of bark mixes, nurserymen were able to increase fertility levels and shorten production cycles. After increasing fertility, Phytophthora dieback (foliar and stem diseases) became more important than root rots on plants grown in containers.
Subsequent research showed that the nitrogen concentration in the new growth of rhododendron plants is directly related to Phytophthora dieback susceptibility. This relationship probably applies to other crops as well. For example, plants of Rhododendron cv. Roseum Elegans with a nitrogen concentration greater than 2.0% in young foliage (last flush) are very susceptible. Lesions on new growth of high-nitrogen plants may extend down to the crown in 10 days or less. The same infection on a low-nitrogen plant (1.2% N in new growth) remains small (pin head in size) and may not be detected.
Native rhododendrons growing in unfertilized natural settings have such low nitrogen levels that they produce only one flush of growth per year. The disease has not been observed on wild plants, even though the pathogen is very widespread in distribution.
The best way to control Phytophthora dieback in the landscape is to maintain moderate nitrogen fertility levels. For nursery production in containers, this practice is not practical because young plants maintained under lower fertility levels grow too slowly to permit economic production. In field-grown rhododendrons, dieback is less severe. In part, this is due to lower fertility levels maintained in the field.