Adapted from the article “Managing Phytophthora in Walnuts” by Bill Krueger, UCCE Farm Advisor Emeritus, Glenn County and Greg Browne, USDA Research Plant Pathologist, UC Davis in the March 2010 Sacramento Valley Walnut News.
Phytophthora crown and root rot is a serious cause of tree loss for most tree crops, including walnut. The disease is caused by species of Phytophthora, a soil-inhabiting fungus-like organism. More than 10 species of Phytophthora attack walnut, some mainly invading the root crown or trunk base and some mainly invading the roots. The root system or crown can be compromised or destroyed, resulting in tree decline and death.
Prolonged periods of soil water saturation favor reproduction, spread, and infection by Phytophthora. All species of Phytophthora produce zoospores, microscopic swimming spores that can swim short distances through water-saturated soil pores to infect roots. Phytophthora can be spread long distances by movement of infested soil on farm equipment, infested surface irrigation water or infected nursery stock.
Symptoms. Symptoms include small leaves, sparse foliage and lack of terminal growth. Trees appear drought stricken early in the growing season. By mid-summer the leaves turn yellow and drop. Infected trees may decline for several years or die within the same season that symptoms first appear. Dark-brown cankers on and/or under the bark measuring inches to feet across may be evident on the tree trunk or the root crown near the soil line. Active cankers tend to have a “zonate,” banded appearance at their edges. Unlike Armillaria rot (oak root fungus), Phytophthora does not produce whitish mycelial plaques between the bark and woody tissue.
Management to Avoid Infection.
- Plant walnuts on well drained soils not prone to flooding, long periods of saturation, pooling or high water tables.
- Encourage good soil drainage with proper pre-planting preparation – deep ripping, slip plowing, or backhoeing; grading and leveling, etc. – and avoid soil compaction
- Plant trees on berms or mounds to avoid water saturation around the root crown
- Avoid standing water for more than 18 hours because spore production increases after this. Avoid irrigation water hitting the limbs or trunk.
- Choose an appropriate rootstock. Paradox hybrid is more tolerant of several Phytophthora species than Northern California black. The clonal Paradox rootstock RX 1, has moderate to high resistance to cinnamomi and P. citricola.
Treatment. Phosphonate treatments have been shown to reduce the severity of canker development in walnut trees inoculated with P. citricola. Whether inorganic (i.e. mono- and di-potassium phosphonates ) or alkyl (i.e. fosetyl-Al- Alliette) formulations of phosphonate are used, the active ingredient is phosphonic acid. Evidence suggests that phosphonic acid disrupts growth of Phytophthora and intensifies host defenses. Phosphonate treatments are not known to affect other walnut diseases.
Phosphonate treatments for walnut have been most effective when applied as a foliar spray. When appropriately applied, phosphonates become systemic in the tree, providing weeks to months of protection. Almond research suggests that an early fall foliar spray is optimal, before the leaves begin to senesce and while the trees are actively transporting to the roots. If a foliar treatment with phosphonate is applied in the spring, it is advisable to wait until the leaves have fully emerged and expanded. Check the product label for rates and restrictions.
Phosphite residues have come under increasing scrutiny recently in the European Union. Check with you handler before applying phosphonate treatments.