I believe two of my large vine maples (planted two years ago) are dying of root rot. They both have collar lesions. There are three others in the vicinity. I have read up on Phytophthora and know the basics. Is it likely to spread to the other three? Is it important to remove the dying ones immediately? Is there an effective treatment for the remaining three?

Collar lesions on maples are often the result of Phytophthora infections, particularly those with soil or heavy mulch above the root collar. However, lesions alone are not diagnostic for this disease and may be caused by other factors. If a Phytophthora spp. is the cause, it may have directly infected the symptomatic area, or it could be the progression of a longer term root infection. The likelihood of infection in the healthy maples depends on several factors. The pathogen is soil borne and will move with water (downhill). If the healthy trees are uphill and planted in well drained soil, there is less chance of spread. Removal of infected trees may nominally reduce the presence of inoculum, but the soil is infested and the pathogen can survive in soil without a host for extended periods. In the Phytophthora species that are commonly found infecting maple, the infection usually takes place after rain or irrigation splashes spores from the soil on to the trunk. A proper layer of organic mulch will help to prevent this spore dispersal and has been shown to suppress the pathogen in the soil. There are materials registered for use in the landscape that can be used to treat and/or prevent Phytophthora lesions, but long term management should be based on managing drainage and soil moisture, and properly mulching the critical root zone. Vine maple is also a very common host for Verticilliium wilt and may produce similar symptoms. Phytophthora infection should be confirmed by a plant diagnostic laboratory.

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