Reprinted from the Great Lakes Chapter newsletter, March 1994
In the 1960s and early 1970s root rot devastated nursery stock - particularly highly fertilized liners in containers. Bark-amended media, now widely used in the industry, suppress root rots. With the threat of root rot reduced, nurserymen increased plant fertility levels. Subsequently, dieback superseded root rot as the main Phytophthora problem in the nursery.
Heavier fertilization created a favorable environment for Phytophthora dieback diseases. Research indicates that nitrogen concentrations in new growth of rhododendron plants is directly related to Phytophthora dieback disease susceptibility. Indeed, maintaining moderate to low nitrogen levels is a key to controlling Phytophthora die-back in landscape plants.
Phytophthora species that affect woody plants may over winter in infected plants. They can also survive in decaying infested tissues on (or in) the soil or container medium. They generally do not survive after infested crop residues have decomposed. Extremely cold winters will kill most Phytophthora species in crop residue.
Phytophthora species produce several types of spores. The spores germinate in water, and splashing droplets may carry them to plant foliage. Water running over the soil or plant bed surface can also move the spores about. Some spores known as "zoopores" can move through still water. Proper management of irrigation water is the key to managing Phytophthora dieback and root rot. To prevent the spread of Phytophthora dieback, avoid using irrigation water draining away from susceptible crops. Instead, recycle it to resistant crops. Irrigate early in the day so that foliage dries out quickly. Trickle irrigation is preferable because it keeps the foliage dry and avoids splash dispersal of dieback pathogens. Do not use flood irrigation for susceptible crops; it has severely enhanced the spread of Phytophthora diseases in the past.
Phytophthora dieback epidemics, which are most common with container-grown stock, generally occur from early summer to early fall. Typical temperatures range from 75°F to 95°F. In some parts of the world, low temperature Phytophthora diseases are active during mild winters as well. When temperatures favor dieback outbreaks, apply preventive fungicides. Once symptoms appear, fungicides may be unable to control the spread of disease.