Identifying Phytophthora Root, Collar and Crown Rot in Fruit Trees

Root, crown and collar rots of fruit trees caused by soil borne Phytophthora spp. appear to be prevalent in fruit trees this spring, probably due in part to the long cool wet fall in 2006. Tender fruit, apple and pear growers should become familiar with symptoms. On severely infected trees, buds will swell break dormancy and appear to flush out in the spring; however, these severely infected trees often wilt, and collapse suddenly after bud break. Severe winter injury caused by a sudden drop in temperatures similar to what was experienced in some orchards this past January can also cause similar symptoms. Often the bark splits on cold injured trees. If the bark is not split or there is no evidence of cold injury, growers should remove soil around the crown and roots of declining or dead trees and scrape away the bark 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 is a good indication of Phytophthora infection. Often the reddish orange canker is limited by a dark or black margin separating it from the white healthy tissue. Growers who find these symptoms in their orchards should consider sending a sample of the diseased roots or crown tissue to a qualified pest diagnostic laboratory for accurate identification.

Most actively growing trees can 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 that are growing slowly are most vulnerable, particularly when the pathogen is still active. This appeared to be the situation last fall and early winter. Infected trees may decline slowly over several years or they may die within weeks of the first symptoms depending on the size and health of the tree. Growers should monitor the growth of trees over the next few seasons, since reduced shoot growth and small fruit size are symptom of Phytophthora crown and root rot. As the disease advances, infected trees often produce yellow, chlorotic leaves that look similar to iron deficiency symptoms. In fact, foliar symptoms as a result of crown and root rot are often confused with other disorders such as nutrient deficiency.

Several Phytophthora spp. cause crown, collar and root rot. Phytophthora spp. are fungal-like organisms that are favoured by wet conditions and therefore tend to cause more problems on trees growing in heavier wet soils or soils that retain water for a long period of time. Unfortunately, last fall and early winter many orchard soils were saturated and some had standing water in low areas over long periods of time. These prolonged wet soil conditions are ideal for pathogens such as Phytophthora to continuously infect roots and crowns allowing the disease they cause to progress for an extended period of time. There is no cure once the disease has become established. Aliette and Ridomil fungicides are registered for collar rot control in apples but only on non-bearing trees as preventative treatments. When planting a new orchard, select fields that have good drainage and light soils if possible. If the soil is heavy or retains water for prolong periods of time, consider installing sub-surface drainage pipes. Since the pathogen infects when soil are saturated for long periods of time, managing irrigation to avoid over-watering, whether it is through drip or overhead irrigation, will reduce the further spread.

Scraping away the bark of a Phytophthora infected tree crowns reveals a reddish -orange canker just under the bark near the soil line.