Cause Phytophthora rubi, a fungal-like organism, causes a destructive root rot on red raspberry cultivars throughout the region. Damage is most severe in fields with poor drainage that may be the result of heavy soils with greater clay content, hardpans, excessive irrigation, or low-lying areas where water collects. This fungal-like microorganism survives as mycelia and as thick-walled oospores that are produced in dying and dead raspberry plant tissues. As the dead plant tissues decompose, oospores are released into soil. Under saturated conditions in winter or spring, swimming zoospores are produced that swim to nearby roots and initiate infection. Small roots rot quickly, and infection moves up into larger structural roots and then into the lower canes.
Root-lesion nematodes and dagger nematodes also also contribute to raspberry root decline and rot, particularly in northern Washington.
'Anne', 'Canby', 'Comox', 'Qualicum', 'Malahat' and 'Skeena' are very susceptible; 'Amity', 'Chilcotin', 'Nootka', and 'Willamette' are susceptible; and 'Chilliwack', 'Meeker', 'Sumner', and 'Summit' are moderately resistant. Young 'Meeker' plants are very susceptible but mature plants seem to have some field tolerance. 'Cascade Bounty' and 'Cascade Delight' are very resistant. In Idaho trials, 'Killarney' and 'Summit' have performed well on heavily infested soils.
Symptoms The most noticeable symptom is wilting and death of canes from early spring to late summer. Wilting often coincides with the onset of warmer, drier conditions. Leaves on infected canes may turn yellow or bronze, wilt, and develop scorch symptoms before dying. Dead leaves usually remain on the stem. Floricanes often start to die just as the fruit begins to ripen in June, causing the fruit to be undersized or to wither before ripening. Primocanes usually begin to wilt from the tip down. Primocane tips bend over, exposing the silvery undersurfaces of the leaves forming a shepherd's crook. New primocanes may also be killed before they break the soil surface. In many cases, one or a few canes of a plant may die while the rest remain healthy. Often, multiple plants in low lying areas of the field die, leaving large open patches known as disease pockets.
Lesions are formed on the roots and may extend up into the canes. If the epidermis of the infected area is scraped off, a sharp transition zone is visible between the healthy root and stem tissues (white or bright-green) and the diseased, rotted tissues (reddish-brown). Excavation of compromised root systems reveal that most larger-diameter roots are dead and that there are few, if any, smaller-diameter feeder roots. New roots may form above the decayed root system during the summer and the plants may appear to recover, but infection resumes during the cold and wet weather of fall and winter.
May be confused with crown borer damage, Verticillium wilt, Armillaria root rot, or cane blight.
Cultural control Best results occur when several cultural and chemical practices are integrated together.
- Use certified planting stock and set out in fertile, deep, well-drained soil (3- to 4-ft water table in winter) that has not grown small fruit (strawberry, raspberry, brambles) for several years.
- Keep affected plantings economically productive as long as possible by good cultural and fertilizing practices.
- Plant resistant cultivars if available.
- Subsoil in alleyways to promote drainage away from plants in winter.
- Install drain tiles in field to improve drainage.
- Plant in beds raised so that the top of the bed is at least 12 inches above the surrounding soil. Amend soil with gypsum (6 tons/A) before making raised beds and planting.
- Slope soil away from plants to the center of the alleyway between rows.
- Preplant soil solarization has been helpful in western Washington but not Idaho. Place clear plastic (preferably anti-condensation film) directly on smooth, rototilled ground, which has been irrigated to field capacity and then allowed to drain for 1-2 days. Bury the edges of the plastic to trap the heat. Solarize for 4-6 weeks (or longer) during the hottest part of the summer, beginning in early- to mid-July. Use in combination with other techniques.
Chemical control Results are best when several cultural and chemical practices are integrated. Limit the use of any one group during crop production.
- Preplant soil fumigation may be effective for severe root rot, but it delays disease onset only a few years.
- Spring and/or fall applications to the foliage or soil. Group 4-based products tend to be more effective than Group P7-based products when resistance is not an issue. Rotate applications between these materials to help prevent building up resistant populations.
- Agri-Fos at 2.5 quarts/A. Group P7 fungicide. 4-hr reentry.
- Aliette WDG at 5 lb/A. Do not apply within 60 days of harvest. Four (4) applications are necessary. Apply to foliage twice in fall: in early September before fall rains, then 3 to 4 weeks later. Apply twice in spring, starting after bud break when new growth is 1 to 3 inches long, then 3 to 4 weeks later. Also registered in British Columbia. Group P7 fungicide. 12-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 1 pint/1000 linear ft of row in a three-ft band over the row. Do not use within 45 days of harvest. Group 4 fungicide. 48-hr reentry.
- Orondis Gold 200 at 4.8 to 9.6 fl oz/A as a banded, soil-directed spray. Irrigate with 0.25 to 0.5 in water after application. Do not use within 1 day of harvest. Group 49 fungicide. 4-hr reentry.
- OxiPhos at 2.5 to 5 quarts/A as a foliar spray. Group P7 fungicide. 4-hr reentry.
- Phostrol at 4.5 pints/A. Group P7 fungicide. 4-hr reentry.
- Rampart at 1 to 3 quarts/100 gal water/A. Group P7 fungicide. 4-hr reentry.
- Ridomil Gold SL at 0.25 pint/1000 feet of row on the soil surface in a 3-ft band over the row. Apply in fall just before the first rains. It is critical to wash this material into the soil as soon after application as possible. Apply during rain or irrigate immediately after application. Repeat in spring if disease is severe. Do not apply within 45 days of harvest. Only one (1) application a year is allowed in British Columbia between harvest and November 30. Some reduced efficacy has been observed in western Washington. Group 4 fungicide. 48-hr reentry.
References Stewart, J.E., Kroese, D., Tabima, J.F., Larsen, M.M., Fieland,V.J., Press, C.M., Zasada, I.A., and Grünwald, N.J. 2014. Pathogenicity, fungicide resistance, and genetic variability of Phytophthora rubi isolates from raspberry (Rubus idaeus) in the Western United States. Plant Dis. 98:1702-1708.
Weiland, J.E., Benedict, C., Zasada, I.A., Scagel, C.R., Beck, B.R., Davis, A., Graham, K., Peetz, A., Martin, R.R., Dung, J.K., and Gaige, A.R. 2018. Late-summer Disease Symptoms in Western Washington Red Raspberry Fields Associated with Co-Occurrence of Phytophthora rubi, Verticillium dahliae, and Pratylenchus penetrans, but not Raspberry bushy dwarf virus. Plant Disease 102:938-947.
Wilcox, W.F., Pritts, M.P., Kelly, M.J. 1999. Integrated control of Phytophthora root rot of red raspberry. Plant Disease 83:1149-1154.