Phytophthora protection

Help keep propagative and finished materials safe from this pathogen. One of the most common Phytophthora pathogens is P. cinnamomi, which attacks several important nursery crops. Some hosts include rhododendrons, azaleas, Camelia japonica, dogwood, mountain laurel, heather, juniper, Fraser fir, yew and Pieris.

Symptoms include above-ground leaf chlorosis, necrosis, wilt, leaf curl and death. Stem necrosis may not occur for many weeks after the development of wilting symptoms. Below-ground symptoms are most severe in poorly drained soils and include necrosis of young feeder roots and the lower vascular tissues around the crown and just below the soil line. Cankers may become visible at the base of 1-2 year old plants. The roots of older plants may recover from disease and may not develop a canker of the base of the stem. Older plants may remain symptomless, or display only mild dieback despite severe root rot.

Phytophthora root rot is favored by high soil moisture and warm soil temperatures, according to D. Michael Benson, plant pathologist at North Carolina State University. The disease does not occur as frequently and may not be as severe on well-drained sandy soils as in heavy clays or poorly drained soils, etc. The disease is common and severe in areas where run-off water, rain water from roofs, etc. collects around plant roots. Shallow soils with underlying rock or compacted hard pans, setting woody plants deeper than the soil level in the nursery or container, overwatering plants, or long periods of heavy rain also favor disease development.

Phytophthora on rhododendron.

To help prevent Phytophthora spp., avoid plants that lack normal green color, appear wilted in the morning, evergreen plants that have excessive winter defoliation or dark discolored roots.  Plant root-rot susceptible plants in well-drained areas. If excess water from any source collects in the planting site, avoid planting root-rot susceptible plants. If soil is heavy clay or does not have good internal drainage, set root-rot susceptible plants in raised beds and thoroughly mix a porous material such as bark (not sawdust or peat) into the bed. The material should be incorporated to a depth of 8-12 inches. In some areas drain tile and gravel placed 6-12 inches below the surface may also help reduce excess soil moisture.

P. parasitica is a similar fungus that attacks boxwood. According to the Virginia Cooperative Extension, above-ground symptoms include poor growth and off-color foliage. Leaves are at first light green and may turn yellow, bronze, or straw-colored. Leaves turn upward and lateral leaf margins roll inward. Leaf symptoms may appear on just a few branches or on the entire plant, depending on the extent of infection of the roots.

Usually, the bark at the base of the infected plant dies and can be easily separated from the wood. By the time foliar symptoms are observed, roots are few in number and many are brown in color. The lack of functioning roots precedes the yellowing and death of the top of the plant. Plants growing in soils that have become water-logged following overwatering or heavy rains in summer are predisposed to infection by P. parasitica.

 P. parasitica on boxwood.

Photos courtesy of NCSU and Oregon State University