Phytophthora Crown and Root Rot

Species of the soilborne “water mold” Phytophthora can invade walnut trees, killing them by destroying essential  water and nutrient conduction tissues in the tree’s roots, root crown, and trunk tissues.    


Phytopthora cinnamomi and P. citricola are the most aggressive and economically important species of Phytophthora that affect walnut and can be devastating.


Phytophthora species can be isolated in culture from affected plant tissues.


Pear fruits can be used to detect Phytophthora in soil and water samples infested with the pathogen. 


Phytophthora species can be spread in surface sources of irrigation water as well as by infested plant material, soil, and activities of man.


Evaluating walnut rootstock germplasm for resistance to Phytophthora in a greenhouse trial.


Development of a stem canker caused by Phytophthora in one of the evaluations of resistance.


Orchard evaluation of rootstock resistance to P. cinnamomi in an orchard infested with the pathogen.  Left, walnut tree growth on the susceptible Paradox seedling rootstock; right, tree growth on RX1 rootstock, which was determined to be resistant to the pathogen. 



Symptoms of Paradox Canker Disease (PCD), including crown and root rot and ultimate tree death, appear similar to those caused by Phytophthora.  However, the cause of PCD is unknown.  See Emerging Threats for more details on PCD.