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.
Disease Prevention During Propagation
Propagation benches must be raised and must provide excellent drainage. If a second crop is to be grown in the same medium, it should be a cultivar that roots quickly and is not susceptible to the same diseases as the first crop. The floor and walks between beds should be free of puddles and weeds. Proper management of irrigation water is a key to management of these diseases. Puddling and flooding should be avoided at all cost to reduce sporulation and spreading of the pathogen.
Before new propagation media are placed in the greenhouse, the headhouse, greenhouse, and benches should be brushed free of all organic debris and old propagation media. Infected residues need to be removed from container production areas between crops, if at all possible.
All cleaned surfaces should be washed with sanitizing agents such as Physan 20 or Greenshield (PT-2000) or other similar materials. Propagating knives and other tools should be dipped from time to time when harvesting cuttings. Watering hoses must be kept off the floor to reduce introduction of pathogen inoculum onto the bench with the hose.
A common propagation medium for Ericaceae (particularly azalea) is a 1:1 mixture of fibrous light Sphagnum peat and coarse Perlite. Mixtures of aged pine bark and fibrous peat (6:4) are used widely for rhododendron cultivars. The percent air-filled pore space in rooting media should be as high as possible (25 to 35%). In a poorly aerated propagation medium (15 to 20%), large calluses are formed on rhododendron cuttings and root initiation is delayed.
If fine, rather than fibrous, peat is used, the air-filled pore space often is below 20%. This results in less rooting and more disease caused by stress pathogens. This is a very common problem encountered in propagation. Always use fibrous peat in propagation media!
Cuttings should be taken from plant parts free of soil, and stock plants must be treated with fungicides as described later.