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Ministry of Agriculture, Food and Rural Affairs

Phytophthora root rot

Severe crown and collar rot on apple tree cause trees to be blown over in high winds Severe crown and collar rot on apple tree – note the dark cracked canker To diagnose crown and collar rot, remove soil from the base of the tree An orange-red canker limited by a dark margin separating it from the white healthy tissue
Click to enlarge.

Beginner

Scientific Name
Phytophthora spp.

Identification

  • Disease symptoms vary and early symptoms are very difficult to detect.
  • Collar rot affects the tree above the soil line usually around the scion.
  • Crown rot affects the tree below the soil, in the area where the roots join the stem.
  • Root rot affects the roots.
  • Severely infected buds swell, break dormancy and appear to emerge normally in spring but then wilt and the whole tree suddenly collapses.
  • Trees weakened by root rots are easily pushed over by heavy winds.
  • A purplish canker is often observed at the base of infected trees.
  • Yellow, chlorotic leaves that look similar to iron deficiency symptoms.
  • To diagnose, remove soil around the crown and roots of declining or dead trees and scrape the bark away along the trunk at the base of the tree and roots:
    • Orange to dark reddish brown canker or streaks along the cambium of the collar or crown at ground level or just under the epidermis of the roots; the canker is often limited by a dark or black margin separating it from the white healthy tissue.
  • For positive identification, send a sample of the diseased roots or crown tissue to a qualified pest diagnostic laboratory for accurate identification.

Often Confused With

  • Fire Blight collar and rootstock blight - Infected suckers and shoots from Fire blight exhibiting bacterial ooze may accompany the canker. For accurate identification, send a sample of the diseased roots or crown tissue to a qualified pest diagnostic laboratory for accurate identification
  • Iron deficiency - Iron deficiency leaves are yellow between the veins, but the veins will remain green except in extreme cases which may be due to mechanical injury or cankers of trunk but could be result of high pH soils.

Period of Activity
The first symptoms on severely infected trees are often observed in spring.

Infected trees can decline slowly over several years or die within weeks of the first symptoms, depending on the size and health of the tree

Scouting Notes
Regular scouting is the best way to assess disease levels. Monitor trees for reduced shoot growth and small fruit size. Look for signs of purplish canker at the crown or collar area.

Thresholds
None established.  

Advanced

Scientific Name
Phytophthora spp.

Identification
Disease symptoms vary:

  • Collar rot affects the tree above the soil line usually around the scion
  • Crown rot affects the tree below the soil, in the area where the roots join the stem
  • Root rot affects the roots

Early symptoms of collar, crown and root rot are very difficult to detect. The buds of severely infected trees swell, break dormancy and appear to emerge normally in spring and then buds wilt and the whole tree suddenly collapses. Trees weakened by root rots are easily pushed over by heavy winds due to lack of support from rotten roots. A purplish canker is often observed at the base of infected trees. To diagnose crown collar and root rot, remove soil around the crown and roots of declining or dead trees and scrape the bark away along the trunk at the base of the tree and roots. Orange to dark reddish brown canker or streaks along the cambium of the collar or crown at ground level or just under the epidermis of the roots. The reddish orange canker is often limited by a dark or black margin separating it from the white healthy tissue. For positive identification, send a sample of the diseased roots or crown tissue to a qualified pest diagnostic laboratory for accurate identification. Infected trees have that look similar to iron deficiency symptoms.

Most actively growing trees tolerate a certain amount of root and crown rot, and may limit the advancement of the disease for a short while. However, dormant young trees or trees growing slowly due to other stresses are most vulnerable particularly when the pathogen is still active.

Often Confused With

  • Fire Blight collar and rootstock blight - Infected suckers and shoots from Fire blight exhibiting bacterial ooze may accompany the canker. For accurate identification, send a sample of the diseased roots or crown tissue to a qualified pest diagnostic laboratory for accurate identification
  • Iron deficiency - Iron deficiency leaves are yellow between the veins, but the veins will remain green except in extreme cases which may be due to mechanical injury or cankers of trunk but could be result of high pH soils.

Biology
Several pathogens are known to cause root rot in apples. The most common root rot pathogens of apples in Ontario are species of Phytophthora. These soil-borne pathogens are sometimes referred to as “water molds” although they are not technically molds. Phytophthora spp. are closely related to yellow-brown algae and although not considered true fungi, are fungal-like organisms that prefer very wet conditions. Some species of these pathogens attack specific crops while others species such as those infecting apple roots, have a much broader host range.

Period of Activity
The first symptoms on severely infected trees are often observed in spring. Infected trees can decline slowly over several years or die within weeks of the first symptoms, depending on the size and health of the tree.

Scouting Notes
Regular scouting is the best way to assess disease levels. Monitor trees for reduced shoot growth and small fruit size. Look for signs of purplish canker at the crown or collar area

Thresholds
None established.

Management Notes

  • There is no cure once Phytophthora has infected and become established in apples trees. Some fungicides are registered for control in apples but only on non-bearing apple trees as preventative treatments.
  • When planting a new orchard, select fields with good drainage and light soils if possible. If the soil is heavy or retains water for prolonged periods of time, consider installing sub-surface drainage tiles.
  • Since the pathogen infects when soils are saturated for long periods of time, managing irrigation to avoid over watering reduces potential infections and the spread of these pathogens.
  • Select rootstocks with some resistance to Phytophthora particularly when planting into heavy poorly drained soils. Rootstocks developed with resistance to Phytophthora include CG.30, CG.6210 and G.16.  M.9 and seedling rootstocks have some resistance to Phytophthora root rot but M.26, M.7 and MM.106 are considered moderately to very susceptible.
  • Carefully inspect roots, crown and scion of young trees to ensure only trees with healthy crowns and roots are planted in the orchard.
  • Inspect mesh used around the collars of young trees to protect them from rodent damage in the spring and remove debris that is stuck in the collar. Leaving debris caught in between the mesh and the collar retains moisture around the crown and collar and is conducive for Phytophthora development and advancement.
  • See OMAFRA Publication 360, Guide to Fruit Production - Chapter 3 Apples (PDF) or Apple Calendar only (PDF) :