Phytophthora Crown and Root Rot
Hosts: all fruit trees
Biology: Phytophthora crown and collar rot of fruit trees is a fungal disease that affects all species of pome and stone fruit. The disease frequently kills trees 5-7 years in age. It is caused by various species of Phytophthora, including P. cactorum, P. cambivora, P. megasperma, P. dreschsleri, and P. syringae that all require saturated soils to cause infection.
The fungus can be introduced into an orchard through infected planting stock, contaminated farm implements, or through contaminated irrigation water. Spores are released in water and infect tree roots and crown tissues. Initial infections result in cankers on the trunk between the soil line and the crown roots. The pathogen can also spread via root to root contact.
Symptoms: The most evident symptoms are dead areas (cankers) on the base of the tree. These begin on the bark between the soil line and crown roots. The infected bark tissue darkens and becomes increasingly sunken and the canker expands slowly. Detection requires removal of the outer bark. The inner bark of affected area will be a cinnamon brown color.
Affected trees will have early fall color and leaf out late in spring. Fruit will be stunted and leaves will be abnormally small. Eventually, the canker will girdle the tree and kill it, seemingly overnight. Leaves remain attached to Phytophthora-killed trees.
Monitoring: Check orchards in mid-summer for trees with weak aerial growth, especially in orchard areas prone to poor soil drainage or low spots that may experience ponding. In late summer to early fall, check for trees that have early fall coloration. On suspect trees, check the trunk at and below the soil line for any canker development or presence. Tissue samples at the edge of the canker can be collected for an attempt to isolate the fungus and confirm the presence of Phytophthora.
Management: Best control is obtained through preventive cultural management practices.
- Select orchard sites with good drainage. Keep water away from tree trunks (no basins around the trunk, space trickle irrigation outlets away from the trunk). Plant trees on raised beds. Keep irrigation runs to 8 hours or less.
- Select resistant varieties and rootstocks or do not plant susceptible rootstocks where soil is poorly drained.
- For apple: East Malling (M) rootstocks M-9, M-26, M-7, and M-111 have intermediate resistance while Malling-Merton (MM) rootstocks MM-104 and MM-106 are susceptible.
- For cherry: Mahaleb rootstock is susceptible, while Mazzard, Stockton Morello, and Colt are less susceptible.
- For peach, nectarine, apricot, and plum trees, Nemaguard, Myrobalan plum, or Marianna 2624 are moderately resistant.
- Avoid deep planting; plant trees with the graft union several inches above the soil line. Scion varieties often are more susceptible to collar rot infection than are the rootstocks, and trees planted with graft unions at or below the soil line have increased potential for the scion to self-root and provide an entry for collar rot.
Other options: Several approaches can help provide control after infection.
- One can bridge-graft over damaged tissue if less than 30-50% of the trunk circumference is affected, in-arch graft 1 yr-old whips of a resistant variety into the trunk well above the diseased area.
- Remove soil from around the base of infected trees and allow the infected area to dry out and stop further progression of the disease.
- Spray the lower trunk with a fixed copper fungicide (50% metallic copper), using 2-3 Tbsp of fungicide/gal. of water. Refill the soil depression around the trunk with fresh soil in late autumn in order to prevent winter injury to the tree collar.
- Irrigation practices that keep the soil saturated for 36 or more hours should be avoided, especially when temperatures are 60-70°F.
- Soil drench of metalaxyl (Ridomil Gold EC) or a foliar sprays of phosetyl-Al (Aliette) can be helpful.