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.
Control of Root Rots
Cultural practices are very important for control of root rots. The most important factors are the container medium, the irrigation system (irrigation and pond systems, container base, etc.), and fungicide drenches.
Cultural Practices
The most widely used Phytophthora-suppressive container media are those amended with tree barks. Both composted hardwood and pine barks suppress root rot caused by Phytophthora cinnamomi. Part of this effect is due to the improved aeration and drainage properties as compared to peat-sand container media. The percent air-filled pore space after saturation and drainage must be 25% or higher. The percolation rate must be greater than 0.5 inch per minute. Puddling on the surface of media must be avoided because it clearly enhances root rot. Containers should never be placed on plastic film because this allows Phytophthora to spread from pot to pot in puddles on the plastic.
Composted rice hulls, sewage sludges, cow manures, etc., also can be added to mixes. Most of these materials contain fine particles and therefore only small amounts can be added to mixes to avoid drainage problems. Composted sewage sludges and manures also may contain high levels of nutrients, and this is an advantage if the right amounts are used. Generally, it is best to incorporate 5 to 10% by volume of these materials in mixes containing 60% or more aged pine bark. These mixes supply adequate amounts of trace elements for one year after potting.
The container medium formulations presented here are examples of those that naturally suppress Phytophthora root rots. The quantity of lime added to the three media listed here varies with the quality of the irrigation water available at nurseries. It typically ranges from 3 to 8 lbs. per cubic yard of mix to maintain the pH within the range of 5.3 to 6.3. Micronutrients do not have to be added to mixes if composted biosolids or manures are included in the mix.
None of the control procedures described here are fully effective without the use of strict sanitation procedures during propagation and a proper layout in the container area. The base on which containers are placed should consist of gravel, stone, or well-drained sand. Covering such a base with the "plastic screen" mulches that control weeds but do not allow puddles to form is ideal. This is because propagules of pathogens present in abscised leaves, as well as pruning residues and other forms of crop infested residues, can be removed easily from this base before the next crop is introduced.
Water draining away from crops should be recycled to a primary settling pond. From there it should drain to a second pond from which irrigation water is pumped. Ponds should be cleaned out occasionally so that ample depth for settling remains. This approach is not totally effective, however. Phytophthora inoculum is disseminated in most irrigation systems used at nurseries. To control Phytophthora inoculum that is disseminated in these systems, nurseries can install a slow-sand filtration system for water that is used in the greenhouse production area of nurseries.
Some nurseries store water in two separate pond systems, each used for a particular container area. The second pond contains runoff from the first production area. The greatest potential for pathogen re-circulation, therefore, is associated with the second system. Crops that are more resistant to Phytophthora root rots and salinity should be irrigated with water draining from the production area where susceptible plants are produced. It is important to monitor chlorides and salinity in water. They specifically increase root rot and can break down resistance in plants to these diseases.