Apple Proliferation Phytoplasma - Fact Sheet

Figure 1: Witches' broom symptoms on apple trees infected with 'Candidatus Phytoplasma mali'
Figure 1: Witches' broom symptoms on apple trees infected with 'Candidatus Phytoplasma mali'

Apple proliferation phytoplasma (APP) (Candidatus Phytoplasma mali ['Ca. P. mali']) is a plant pest that is considered to be a quarantine pest by Canada and the United States (U.S.) .

Apple trees are the main host of APP. In Europe, however, it has been reported in other economically important species such as oaks, hazelnuts, hawthorns, plums, magnolias, dahlias, roses and European and Asian pears, but the impact of the disease on these secondary hosts remains unclear.

APP is considered one of the most economically important apple diseases in Europe, causing economic losses of 10-80%. Economic damage is due to reduced fruit size, weight, and quality, as well as reduced tree vigour.

Distribution

In Europe, it has been detected in the following countries: Albania, Austria, Balkans, Belgium, Bosnia-Herzegovina, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, France, Germany, Greece, Hungary, Italy, Moldova, Netherlands, Norway, Poland, Romania, Russia, Serbia, Slovakia, Slovenia, Spain, Switzerland, and Ukraine.

The disease has also been detected in Turkey and Syria.

Biology

APP is found in the phloem (food-conducting tissue in vascular plants) of infected plants. It spreads primarily via infected planting material. It can also be transmitted between plants by insects, and through natural root grafting of adjacent plants. APP 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, it re-colonizes in the shoots and the stem of the tree.

APP can be spread through propagation practices, including budding and grafting. Therefore, 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.

After acquiring the pathogen, some insects may transmit APP for the rest of their lives.

Symptoms

The symptoms of APP may 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 other branches 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 21-24°C.

Symptoms include the following:

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 (see Figure 1).

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 (see Figure 2).

Leaf stipules: Leaves, particularly on witches’ brooms and leaf rosettes, often have enlarged leaf stipules and shortened leaf petioles (see Figure 3).

There are also other symptoms that are 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.

Management and control

There are currently no known treatments for APP. Once a plant is infected, it will remain infected for the remainder of its life. Removal of infected trees and the control of insect vectors may limit the spread of APP. Different apple and rootstock varieties may vary in their sensitivity to APP and in the severity of symptoms. In Europe, APP-resistant rootstocks are commonly used.

Health concerns

APP is a plant disease that does not affect the health of humans or animals.

Figure 2: Leaf rosettes on apple tree infected with 'Ca. P. mali'
Figure 2: Leaf rosettes on apple tree infected with 'Ca. P. mali'
USDA - New Pest Response Guidelines - PDF (2.62 mb)

Figure 3: Leaf (bottom) with enlarged stipules and shortened petiole from a tree infected with 'Ca. P. mali', compared to a healthy leaf (top) (eppo.int)
Figure 3: Leaf (bottom) with enlarged stipules and shortened petiole
from a tree infected with 'Ca. P. mali', compared to a healthy leaf (top) (eppo.int)