Phytophthora root rots
By Dr Glynn Percival & Miss Emma Schaffert
2018-10-22
Bartlett Tree Research Laboratory, Shinfield Research Site, University of Reading
The genus Phytophthora are composed of 60 species of pathogens that cause considerable damage to trees and shrubs world-wide. Phytophthora translated means “plant destroyer”. Within the UK several species of Phytophthora exist, including P. ramorum that has had a devastating impact on Japanese Larch forests after “host jumping” from rhododendrons. Free water is generally necessary for Phytophthora to successfully infect a tree. Consequently most Phytophthora outbreaks are commonly associated with soils that are poorly drained or receive excessive irrigation. Phytophthora can be especially prevalent on newly developed sites where soil is severely disturbed or compacted from construction and building activities. Phytophthora is also a common disease in commercial ornamental tree and shrub nurseries. Infection usually progresses from the root collar or fine roots, into the larger ones and eventually the tree trunk. Loss of water and nutrient absorbing capacity and stored carbohydrate reserves in the root cause a eventual decline of the tree canopy.
Phytophthora is common on azalea, rhododendron, Japanese holly, boxwood, hemlock, laurel, fir, yew. Significant mortality has also been recorded across a wide range of genus such as Aesculus, Tilia, Prunus, Taxus, Chamaecyparis, Abies and Erica, with the main causal agents usually being P. cambivora, P. cinnamomi, or P. cactorum.
Symptoms of Phytophthora vary depending on the susceptibility of the tree species, the virulence of the specific Phytophthora and site/environmental conditions. Symptoms include a reduction in shoot growth, small leaves, thinning of the crown, chlorosis, twig and branch dieback and eventually death. Diseased roots are reddish brown and brittle. These symptoms are often confined to fine roots and lateral roots less than 5 mm in diameter. Decline occurs over a period of months or years.
Management
Where soils are poorly drained and prone to flooding, use species that are tolerant to waterlogged/flooded soil conditions and/or apply drainage. Use soil moisture probes to monitor soil moisture conditions. If the soil is compacted, prepare the planting area by cultivating and incorporating organic matter. Likewise, excavation of the root collar is recommended using air-excavation technology. The systemic fungicide Subdue (active ingredient metalaxyl-M) is approved for Phytophthora control in amenity landscapes and is also recommend as a soil drench. Drenches should be applied twice annually, or three times if the tree is heavily infected, across spring, summer, and autumn. Avoid excessive mulch on susceptible tree species. A 5 cm maximum mulch depth is recommended. Favour coarse mulches such as bark nuggets or wood chips. Avoid shredded bark products that tend to compact and hold water. Research trials world-wide show phosphites highly beneficial in the suppression of Phytophthora diseases. Phosphites work by enhancing the trees own immune/defense system. Phosphites can be applied as bark or foliage sprays and/or soil drenches. Treating surrounding susceptible plants on a preventative basis is highly recommended
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