Monitor for phytophthora root, collar and crown rot diseases in fruit trees
The deluge of rain in early May this year saturated soils and created ideal conditions for the infection and spread of soil-borne Phytophthora pathogens (several species). Symptoms of collar, crown and root rots caused by Phytophthora spp. may become prevalent in fruit trees this spring and summer due the cool wet weather experienced in April and May.
Phytophthora spp. are fungal-like organisms that are favoured by wet conditions and are often referred to as 'water molds'. They produce spores that swim in saturated soils to roots where they can infect. Phytophthora tend to cause more problems on trees growing in heavier wet soils, in soils that retain water for a long period of time, or when prolonged wet weather results in persistent wet soil conditions in orchards. These conditions are ideal for pathogens such as Phytophthora to continuously infect roots and crowns, allowing the disease they cause to spread and progress for an extended period of time.
To diagnose Phytophtora collar, crown and root rot, fruit tree growers should look for purplish cankers on the lower trunk or base of trees (Figure 1). It is best to remove soil around the crown and roots of infected or declining trees, and scrape away the bark at the base of the tree and along the roots. Underneath the purplish canker, a reddish orange tissue that is limited by a dark or black margin separating it from the white healthy tissue can be observed (Figure 2). Growers who find these symptoms in their orchards should consider sending a sample of the diseased roots, collar or crown tissue to a qualified pest diagnostic laboratory for accurate identification and confirmation.
Figure 1. Sunken canker at the base of fruit trees is an indication of Phytophthora crown or collar rot.
Figure 2. Orange to dark reddish brown canker or streaks along the inside bark of the collar or crown at ground level or just under the epidermis of the roots is a good indication of Phytophthora infection.
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, young, compromised trees, or trees that are growing slowly, are most vulnerable, particularly when the pathogen is active (which appears to be the situation due to all of the wet weather experienced in Ontario this past spring). 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 also symptoms 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 caused by Phytophthora are often confused with other disorders such as nutrient deficiency.
There is no cure once the disease has become established. Aliette WG and Ridomil 480SL fungicides are registered for root crown and collar rot prevention and management in some non-bearing fruit tree crops. Consult and follow the label of these fungicides before application.
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. Since the pathogen infects when soil are saturated for long periods of time, proper timing and management of irrigation to avoid over-watering, whether it is through drip or overhead irrigation, will reduce further spread.
For more information:
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|Author:||Michael Celetti - Plant Pathologist Horticulture Crops Program Lead/OMAFRA|
|Creation Date:||17 May 2017|
|Last Reviewed:||17 May 2017|