Root & Crown Rot: Phytophthora spp.
Impact in Walnut Production
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.
Fig. 1, 2 & 3. Isolates of Phytophthora from diseased walnut trees are used to screen walnut rootstock germplasm in the greenhouse for resistance.
Fig. 4. Aerial mapping indicates the extent of tree loss due to Phytophthora disease.
Genetic resistance offers the most sustainable approach to managing Phytophthora in walnut.
Fig. 5. Phytophthora species can be isolated in culture from affected plant tissues.
Fig. 6. Walnut rootstock germplasm indicating Phytophthora RESISTANCE (left) and SUSEPTIBILITY (right).
Fig. 7. Phytophthora species can be spread in surface sources of irrigation water as well as by infested plant material, soil, and activities of man.
Fig. 8. Evaluating walnut rootstock germplasm for resistance to Phytophthora in a greenhouse trial.
Fig. 9. 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.
Fig. 10. Walnut tree growth on the susceptible Paradox seedling rootstock; right, tree growth on RX1 rootstock, which was determined to be resistant to the pathogen.