Excerpt from the article “Phytophthora Root and Crown Rot & Paradox Canker Disease in Walnuts” by Mohamed T. Nouri, UCCE Orchard Systems Advisor, San Joaquin County, Stockton, CA, Natalia Ott, PhD student, University of California, Davis, CA, and Greg T. Browne, Research Plant Pathologist, USDA-ARS, Department of Plant Pathology, University of California, Davis, CA on the San Joaquin Valley Trees and Vines website.
Phytophthora crown and root rots and Paradox canker disease are difficult to detect until the above- ground parts of the tree show symptoms, which usually appear at later stage of the infection especially in the summer time. These trees may have decreased yield and vigor for a few years before succumbing to the disease. Phytophthora disease can first be managed with an accurate diagnosis, and many types of cultural practices and chemical controls can be implemented for Phytophthora problems. Declining trees are characterized by chlorotic foliage as well as wilting, defoliation and eventual tree death. Trunks often expressed gumming together with crown rot symptoms (Fig. 1).
Identification of the Paradox canker disease causal agent is of utmost importance for controlling plant diseases and mitigating the economic losses they can cause. Upon peeling back the outer bark of the trunk, Paradox canker disease cankers tend to be more rounded or lobed at their margins compared to those caused by Phytophthora (relatively irregular margins) (Fig. 2).
Read the full article on the San Joaquin Valley Trees and Vines website for a more detailed discussion of identification and control of Phythphthora crown and root rots and Paradox canker disease in walnuts.