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The crown colour of trees dying from root infection changes uniformly from healthy green to red and brown in one or two years (Trione, 1959). P. lateralis colonizes and kills the inner bark (phloem) tissues of roots and stems of infected trees. Healthy inner bark of C. lawsoniana is white whereas necrotic tissues are red-brown, and there is usually a distinct demarcation between healthy and diseased tissues (Betlejewski et al., 2011; Hansen, 2011). Outer bark is red-brown on both healthy and diseased trees, and wood remains white regardless of infection. On some seedlings and saplings healthy outer bark may still be green, and diseased outer bark may turn red-brown. Necrotic inner bark is not a specific symptom for P. lateralis, however, and P. cinnamomi and P. cambivora cause similar symptoms in Port Orford cedar (C. lawsoniana).
As with most invasive pathogens, the key to disease control is to prevent establishment in the first place (Greenup, 1998; Hansen, 2008). For soil/root borne Phytophthoras, this means identifying sources of inoculum and blocking the transport of potentially infested materials from those sources. With P. lateralis in landscape situations, this requires inspection and regulation of nursery stock before sale and transport. This tree is readily propagated locally from vegetative material or seeds and so there is no need for its international trade, which risks further introductions of P. lateralis.
In regions where the disease is already locally established, efforts must be concentrated on containment of the pathogen, blocking transport, via soil or on contaminated equipment, to uninfested areas. Early detection of new infestations is important for containment success. Once established, there is little chance for eradication; eradication efforts also risk aggravating the situation by moving infested soil. With time in host-free conditions (about ten years), the pathogen will die out (Hansen and Hamm, 1996).
In vulnerable areas of forests in western United States where P. lateralis is already broadly established, management of Port Orford cedar (POC) root disease is focussed on stopping further spread and providing special protection for the remaining uninfested stands of POC. This has led to permanent or seasonal closure of many roads which might provide pathways for vehicular transport of the pathogen, as well as systematic efforts to reduce the population of P. lateralis in infested areas as a means to lower the probability of its further transport (USDA, 2004). Young cedars which have naturally regenerated next to roads are at great risk. Their systematic removal, locally termed ‘sanitation’, reduces the risk of further disease increase should the pathogen be introduced to the area (Goheen et al., 2012). P. lateralis is a poor saprophytic competitor and dies quickly in the surface soil in the absence of host roots.
In addition to road closures and roadside sanitation, equipment washing requirements are invoked as appropriate to protect healthy areas. High pressure hoses effectively remove mud from vehicles. Water alone is sufficient so long as soil is removed; steam cleaning or the use of biocides is not necessary. Care must be taken to insure that contaminated wash water flows off the road and back into already diseased areas or into host-free areas (Goheen et al., 2012).
When disease is established in an area, the challenge is then to protect nearby cedars from further spread of the pathogen. The main strategy is to identify trees that can be saved and prevent the transport of P. lateralis to them via either soil or water. Trees growing with roots intertwined with an infected tree, or growing downslope along a waterway, may already be infected and probably cannot be saved. Cutting the most vulnerable cedars to create a gap between infected trees and those to be saved may be effective if care is taken to prevent soil disturbance. P. lateralis is a poor saprophytic competitor and will not spread through a dead root system. Mulching cut trees and using the chips in landscaping risks spreading POC root disease; unless the chips are properly composted at elevated temperatures, P. lateralis will survive in contaminated mulch and spread.
In windy areas where wind spread and foliar infection by P. lateralis are evident, the challenge is still greater. In some situations it may be possible to cut trees with foliar infection, removing the above ground source of aerial inoculum. Pruning low hanging branches may prevent initial foliar infection from soil splash.
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources: