“Witches' Broom symptoms on apple trees infected with Candidatus Phytoplasma mali. – CFIA photo
A Pacific Gala tree in an orchard near Kentville, Nova Scotia has tested positive for apple proliferation phytoplasma (APP) — one of the most critical diseases of the apple tree.
This is the first time APP has been detected in North America, the Canadian Food Inspection Agency said May 17. It has quarantined the orchard.
The grower imported the trees in 2008 from the United States through Scotian Gold Co-operative Limited.
The US Department of Agriculture was notified in April about the APP detection and its Animal and Plant Health Inspection Service is conducting tests at the source nurseries.
While not harmful to humans or animals, APP, or Witches’ Broom, is the apple disease with the greatest economic impact, with losses of 10 to 80 per cent in Europe, CFIA said. It reduces fruit size, weight, quality and the tree’s vigour.
APP has been found in 28 European countries and Syria. In Europe, the disease also affects oaks, hazelnuts, hawthorns, plums, magnolias, dahlias, roses and European and Asian pears.
The CFIA said APP is found in the phloem - food-conducting tissue in vascular plants - of infected plants. It spreads primarily via infected planting material, but can also be transmitted between plants by insects and through natural root grafting of adjacent plants. It is not spread through seed, fruit or pruning.
APP redistributes itself seasonally within an infected tree. During the winter months, the phytoplasma survives below the ground in the roots of the tree. In the spring, the CFIA said, it re-colonizes in the shoots and the stem of the tree.
The diseas can be spread through propagation practices, including budding and grafting, so long-distance dispersal of APP can occur through the trade of infected rootstock, scionwood or budwood.
In Europe, psyllids in the genus Cacopsylla are the primary insects that can spread APP. Some species of leafhoppers and froghoppers have also been reported to spread the disease less efficiently.
A widespread froghopper, Philaenus spumarius, and a leafhopper, Fieberiella florii — both reported to exist in British Columbia and Ontario — are capable of spreading APP. After acquiring the pathogen, the agency said, some insects may transmit APP for the rest of their lives, but the insects’ efficiency in spreading the disease in the natural environment is unclear.
The agency said APP symptoms can vary depending on the plant and how long the phytoplasma has been present. Some branches on an infected tree may appear normal and produce normal fruit, while others may show symptoms.
Additionally, symptoms may disappear for one or more years, and reappear after heavy pruning or grafting.
Temperature also appears to have a significant impact on symptom development, with optimum symptom development occurring between 21 to 24 C.
CBCreported that dozens of Nova Scotia apple growers imported Pacific Gala trees in 2008
Branches: Witches' broom – the development of axillary buds produces a proliferation of secondary shoots, which creates a broom-like appearance at the end of the affected branch.
Leaves: Leaf rosettes – a rosette of terminal leaves may develop late in the season at the end of shoots in place of normal terminal buds, or shoot tips may die back.
Leaf stipules: Leaves, particularly on witches’ brooms and leaf rosettes, often have enlarged leaf stipules and shortened leaf petioles.
The CFIA said there are also other symptoms sometimes associated with APP, including excessive suckering near the base of the plant and a reduction in fruit size, fruit quality and root weight. Leaves may roll downwards and become brittle, are finely and irregularly serrated, are smaller than normal and may appear more yellow than healthy leaves during the summer.