Cause Many Phytophthora species, fungus-like microorganisms, cause a root rot of stone fruits. P. cactorum is the only species reported from the Pacific Northwest and then only on apricot and peach in Washington. OSU Plant Clinic records indicate P. cactorum on cherry in the Willamette Valley of Oregon. This is not a common disease problem in Oregon because orchards are planted on well-drained hillsides. Cool, wet, or waterlogged soils favor the disease.
Symptoms Aboveground symptoms are generally slow or poor growth, which can be mistaken for other problems such as nutritional deficiencies. Root systems; however, are rotted with few if any feeder roots. In advanced cases, many roots are in various stages of decay. Cambium near the root crown shows a brownish discoloration that has a distinct border with healthy tissue.
- Plant orchards on well-drained soil.
- Avoid excessive irrigation that saturates the soil or frequently wets the root crown.
- Mazzard and Colt are more resistant than Mahaleb rootstock. Results for trees on Gisela rootstocks have been variable.
- Plant in raised beds so that the top of the bed is at least 12 inches above the surrounding soil may be useful in certain situations.
Chemical control Focus on cultural control methods first.
- Agri-Fos at 2.5 quarts/A. Do not combine with a copper-spray program for control of other diseases. WA only. Group P7 fungicide. 4-hr reentry.
- Aliette WDG at 5 lb/A for nonbearing trees only. Do not use with adjuvants. Group P7 fungicide. 24-hr reentry.
- Fosphite at 1 to 3 quarts/A. Do not use copper products within 20 days of treatment and do not use spray adjuvants. Group P7 fungicide. 4-hr reentry.
- MetaStar 2E at 2 gal/A or 6 fl oz/1000 sq ft. Treat the entire root zone; do not concentrate near the base of the tree. Group 4 fungicide. 48-hr reentry.
- Organocide Plant Doctor at 1 oz/3 gal water as a foliar spray or at 16 fl oz/16 fl oz water plus Pentra-Bark as a basal trunk spray. Group P7 fungicide. H
- OxiPhos at 2.5 to 5 quarts/A as a foliar spray. Group P7 fungicide. 4-hr reentry.
- Phospho-Jet is registered as a basal bark spray at 62.4 fl oz in 62.4 fl oz water plus 3 fl oz of Pentra-Bark. Spray first 5 feet of trunk including scaffold limbs until runoff. Group P7fungicide. 4-hr reentry.
- Phostrol at 4.5 pints/A as a foliar spray. Group P7 fungicide. 4-hr reentry.
- Rampart at 1 to 3 quarts/100 gal water/A. Do not use copper products within 20 days of treatment. Can also be trunk injected. Group P7 fungicide. 4-hr reentry.
- ReLoad at 4.5 pints/A. Do not use with copper products. Group P7 fungicide. 12-hr reentry.
- Ridomil Gold SL at 2 quarts/A in sufficient water to move it into the root zone. Treat the entire root zone; do not concentrate near the base of the tree. Will not revitalize trees with moderate to severe symptoms. Group 4 fungicide. 48-hr reentry.
References Thomidis, T. 2001. Testing variability in pathogenicity of Phytophthora cactorum, P. citrophthora and P. syringae to apple, pear, peach, cherry and plum rootstocks. Phytoparasitica 29:47-49.
Wilcox, W.F., and Mircetich, S.M. 1985. Effects of flooding duration on the development of phytophthora root and crown rots of cherry. Phytopathology 75:1451-1455.