Northwest berry Foundation

Management Detail

Phytophthora Root Rot

in Raspberries

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Latin name: Phytophthora spp.

Description:

Reason for Concern:
1) Causes more economic damage to raspberries than any other disease.
2) Especially severe in the southern growing region (Oregon and Southwest Washington), which has heavier soils.
3) Damage can be compounded when susceptible varieties are grown, nematode populations are high, and/or where soils are wet for long periods.

Identification/Symptoms:
Low vigor, stunted growth, low yield and sparse plant stand. Canes and leaves on mature plants suddenly wilt and yellow or bronze, and scorch and die in early spring. Fruit stems are shortened and berries, if formed, are small and wither before ripening. Plants may appear to recover, but new roots are often weak and lack lateral development. Plant productivity declines; cold, wet weather the next fall and winter infects new roots. Infected roots rot and turn brown with sloughed outer tissue and few fibrous roots. On new canes, black or purplish lesions develop at base level. Scraped crown or main root surfaces of recently wilted plants are reddish brown with a distinct line where infected and healthy tissues meet. Infected tissue darkens as tissue decays. Frequently, new shoots develop from healthier crown portions. Plants frequently occur in patches, which may spread along rows.

Disease Cycle:
Fungi and fungi-like organisms act alone or as a complex; some only infect previously stressed (weakened) plants. Fungi survive in soil for years, by infecting roots or going dormant as spores. Spores germinate in moist soil and spores expel, infecting fine roots and growing through root tissue into the plant crown. As water remains standing and oxygen depletes from the root zone, the plant is less capable of resisting the fungus. Each new infection site is a potential source of additional spores. Sites subjected to repeated periods of standing water, allow for epidemic disease development. The optimum infection season is not certain, likely spring and fall, but it is assumed that infection can occur throughout the growing season if soil moisture is favorable.

Links:

Scouting:

  • Wilting and collapsing plants may be caused by many factors (winter injury, cane borers, etc.) to diagnose PFR it is necessary to examine the root system of infected plants.
  • Suspect plants should be dug up and the epidermis (outer surface) scraped off the main roots and crown. On healthy plants, the tissue just beneath the epidermis will be white; on plants with Phytophthora root rot; this tissue will be a characteristic brick red (eventually turning dark brown as the tissue decays).
  • Sometimes a distinct line can be seen between infected and healthy tissue, especially on the below-ground portion of the crown.
  • In many fields, plants that are dying and declining because of Phytophthora root rot had previously been diagnosed as suffering from winter injury or "wet feet." One major difference in distinguishing between root rot and winter injury is that plants infected with Phytophthora root rot will continue to decline as time goes on and will not produce healthy primocanes, whereas winter-injured plants will usually send up healthy primocanes the year following the damaging winter.
  • During the winter, note poorly drained areas. Watch these areas for symptoms of root rot when the weather starts to warm up. After hot, dry periods, watch for wilting of fruiting canes.
  • Canes and leaves on mature plants wilt, turn yellow and die.
  • Symptoms may vary. In some fields floricanes collapse with the first hot weather in the growing season. In other fields, primocanes show extensive wilting and collapse.
  • Roots become rotted and lack fibrous roots. Higher rate of incidence on heavier soils.
  • Plants may appear to recover, but new roots are often weak and lack lateral development.
  • Target yellow plants with sudden wilting especially after periods of hot weather.
  • Pull samples as soon as symptoms are seen. Keep sample refrigerated or cool. On healthy plants, the tissue just beneath the epidermis will be white; on plants with Phytophthora root rot, this tissue will be a characteristic brick red (eventually for ELISA testing to determine phytophthora; positives go for PCR testing turning dark brown as the tissue decays).
  • Sometimes a distinct line can be seen between infected and healthy tissue, especially on the below-ground portion of the crown.
  • Collect drain water from field, bait and analyze for phytophthora load in the field.
  • Monitor propagules. Sort through plants before putting in field. Discard plants with poor root systems.

 
Cultural Controls:

  • Plant only clean, certified planting stock.
  • Plant on a clean site with no history of caneberry root rot problems.
  • Any practice that improves drainage will impact this disease since the spores depend on saturated soil to spread from plant to plant. Useful practices include: Planting on well-drained soil; Tiling; Planting on raised beds; and Subsoiling.
  • Amending the soil with high rates of gypsum have also shown promise. Calcium ions appear to inhibit Phytophthora fungal growth.
  • Avoid overwatering.
  • Preplant soil solarization has looked promising in suppressing the disease.
  • Minimize the movement of soil and/ or plant material from infected sites to clean sites.

 

For information about chemical controls, check with our Pesticide Guide.

images


photo by T. Peerbolt


photo by T. Peerbolt


photo by T. Peerbolt


photo by T. Peerbolt


photo by T. Peerbolt


photo by T. Peerbolt


First stage of disease - photo by T. Peerbolt


photo by T. Peerbolt


photo by T. Peerbolt

 

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