Phytophthora Root and Crown Rot & Paradox Canker Disease in Walnuts

Mohamed T. Nouri, UCCE Orchard Systems Advisor, San Joaquin County, Stockton, CA
Natalia Ott, PhD student, University of California, Davis, CA
Greg T. Browne, Research Plant Pathologist, USDA-ARS, Department of Plant Pathology, University of California, Davis, CA

1- Diagnosis of Phytophthora and Paradox Canker Diseases

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 few years before succumbing to the disease.

Phytophthora disease can first be managed with an accurate diagnosis. Many type of cultural practices and chemical controls can be implemented for Phytophthora problems, but collecting all the facts/patterns related to this problem for an accurate diagnosis will help long-term management efforts. Knowing the appropriate conditions that favor the pathogen development also helps the diagnosis. However, the most important factor for an accurate diagnosis is the presence of the organism on the affected plant.

Phytophthora can be detected by plating diseased tissue onto selective agar medium, or baiting the organism by placing the rotted material or soil in a container of water and floating a healthy piece of plant material (e.g. pear) on top could be another way of detection. If the desired pathogen is present, it may grow out from the tissue and then identified based on morphological and molecular means. The problem with these methods is that the chance of recovering the pathogen alive is low in some cases, which we are facing during this time of the year. The less frequent recovery of the organism during the summer time has raised the question whether this is due to differences in soil composition or different climatic conditions.

On the other hand, trees declining from Phytophthora root and crown rot are frequently misdiagnosed as suffering from “wet feet” (root asphyxiation), or are sometimes confused with paradox canker disease. Paradox canker disease still under investigation and no recognized pathogens were isolated to be identified as the causal agent.

2- Phytophthora

Symptoms

Usually, disease symptoms appear as dark, water-soaked lesions with irregular margins and are similar among Phytophthora species. 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). This year we visited several walnut orchards in San Joaquin County with trees showing typical symptoms of Phytophthora crown and root rot disease (Fig. 1). Most orchards are between 10 – 15 years- old. The increased number of symptomatic trees that shows up this summer make us thinking that infections on these orchards started few years ago (~2-3 years), especially with environmental conditions that may favor tree stress and disease, including late winter and early spring rains and the hot and dry summer we faced recently: Conditions become more conducive for infection, allowing the pathogen to infect plant roots/crown causing tree decline and eventual death.

Causal Organism and Disease Cycle

Species of Phytophthora, fungus-like microorganisms, is a member of the Oomycetes or “water molds”. Present in virtually all California tree crop orchards it can cause significant disease problems. In walnut, Phytophthora rots can girdle the rootstock just below the soil surface (crown rot), and cause necrosis and death of roots (root rot). Multiple species of Phytophthora have been implicated in trees damage with P. cinnamomi and P. citricola two of the more significant species.

Phytophthora species are present in most orchard soil and can cause root and crown rots. They mainly persist as thick-walled spores called chlamydospores, which are extremely resilient and can remain viable in the soil for long periods of time. In the presence of water, such as during wet weather, Phytophthora chlamydospores or oospores germinate to form sporangiophores bearing sporangia. These sporangia are full of swimming zoospores, which in saturated soils they can move/swim to find new roots to attack.

Cultural Controls

In the beginning to mid-summer check your orchards for trees with weak aerial growth or declining symptoms, especially in poor soil drainage. When Phytophthora rots are suspected, check the base of the tree at and below the soil line for any crown and root rot symptoms.

Monitor soil moisture and limit periods of soil saturation: Rapid buildup of pathogen inoculum and a high rate of infection are more likely influenced by the presence of water and excessive moisture. Plant trees on berms/ridges to help raising their crowns above the primary zone of zoospore activity and provide an important protection especially for young trees. Prevent soil compaction and do not allow water to splash on trees trunks (If using sprinklers). Duration of irrigation should not exceed 24 hours in soils with low infiltration rates or high holding capacities. In this case, shorter irrigation sets could be applied.

Rootstock selection: Resistant rootstocks are the best solution for the control of phytophthora diseases. Clonal Paradox RX1 rootstock, which has high resistance to P. cinnamomi and moderate to high resistance to P. citricola, could be the best rootstock choice in soils with phytophthora history or when conditions are conducive for phytophthora development.

Chemical Controls

Focus on cultural control methods first. These methods can be very effective. Chemicals are used to help supplement these methods. Phosphonate fungicides may be used to manage the disease in an integrated management program. Applications are typically made one to two times a growing season, such as in spring and early fall as preventative treatments.

3- Paradox canker disease

Symptoms

Crown symptoms of this disease superficially resemble those of Phytophthora crown rot: bleeding bark that originates below the soil surface and spreads up and around the root crown and tree trunk. 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).

Control

The cause of Paradox canker still unknown. Identification of the causal agent are of utmost importance for controlling plant diseases and mitigating the economic losses they can cause. Best thing to do for now is to remove dead trees without spreading any of the dead tissue in the orchard.

Fig. 1. Symptoms of Phytophthora disease in walnut trees: dark, water-soaked lesions with irregular margins. — **Fig. 1.** Symptoms of Phytophthora disease in walnut trees: dark, water-soaked lesions with irregular margins.

Fig. 2. Symptoms of Paradox Canker disease in walnut trees: cankers tend to be more rounded or lobed at their margins. — **Fig. 2.** Symptoms of Paradox Canker disease in walnut trees: cankers tend to be more rounded or lobed at their margins.