Apple Proliferation Disease

The Canadian Food Inspection Agency (CFIA) recently announced the discovery of a regulated and quarantine disease called apple proliferation (AP), in an apple orchard in Nova Scotia. This is the first detection of apple proliferation in North America and it is not known to be present in Ontario or any other apple growing region of North America at this time. The United States Department of Agriculture has been informed and an investigation is ongoing to determine the source of this disease introduction into Nova Scotia. AP is considered a critical disease of apples and has been found in many European countries where it causes significant apple tree decline and yield loss.

AP is caused by a phytoplasma (Candidatus Phytoplasma mali) which is a bacteria-like organism that does not have a cell wall. Without a cell wall the phytoplasmas can only survive inside the plant host or insect vector. Most phytoplasmas only invade and live in the nutrient and sugar conducting tubes called the phloem of susceptible host plants. Other examples of plant diseases caused by phytoplasmas in Ontario include X-disease in peach and Aster yellows that infects many vegetable crops each year. Unfortunately plant diseases caused by phytoplasmas are difficult to manage.

Apple trees infected with the AP phytoplasma often produce multiple auxiliary buds very close together at the end of one or several branches. These buds develop into dense secondary shoots at the end of a branch resulting in a "witches broom" appearance. The symptom of multiple shoots spaced close together growing at the end of the same branch is called 'proliferation' from which this disease gets its name. Similarly, shoots and spurs on infected trees may develop multiple leaves close together in a rosette. These rosettes of terminal leaves will often break dormancy late in the growing season and are often covered with powdery mildew. Leaves on infected trees are often smaller with large stipules, lighter green or sometimes yellowish in colour. The leaves will turn red instead of the normal yellow colour often observed in the fall and defoliate prematurely. Some or many flower buds on infected trees may break dormancy in late summer or early fall and the resulting blossoms are often larger with a longer than usual stock or peduncle. Sometimes the flower petals develop into green leaf-like structures. Fruit produced by severely infected trees are smaller, with less sugar, less acid, and appear flat and pale green. This disease can cause 30-100% marketable yield loss.

There are many reports of alternative hosts of the AP phytoplasma including European pear (Pyrus communis), pink periwinkle (Catharanthus roseus), hazel (Corylus avellana), grapevine (Vitis vinifera), many Prunus spp., Ribes (Ribes rubrum), hawthorne (Crataegus monogyna), lily cv. Siberia (Lilium spp.), dahlia (Dahlia × cultorum), native tobacco (Nicotiana occidentalis) and cultivated tobacco (Nicotiana tabacum) however, these hosts act as reservoirs of the AP phytoplasma and not much is known about the effect of AP phytoplasma on theses hosts. Field bindweed (Convolvulus arvensis) and Bermuda grass (Cynodon dactylon) have also been reported to be hosts but not confirmed.

The AP phytoplasma is spread when contaminated bud wood or rootstocks are grafted to make new trees. Long distance spread occurs when infected trees are transported to another region. Once it is introduced into a region, the phytoplasma can be spread from tree to tree within an orchard and to other nearby orchards by specific piercing and sucking insect vectors. The main insect vector of AP in Europe is a tiny psyllids (Cacopsylla melanoneura) that is not known to occur in North America. The other two vectors capable of spreading this pathogen are the privet leafhopper (Philaenus spumarius) and meadow spittle bug (Fieberiella florii) that are known to occur in both Ontario and British Columbia. Research has also shown that both the privet leaf hopper and meadow spittle bug are not very efficient vectors of the AP phytoplasma and do not occur on apple trees very often. Regardless they do show up from time to time in orchards and there is the potential for this disease to spread if introduced into Ontario.

Insect vectors acquire the phytoplasma during feeding activities on infected plants. Once acquired, the phytoplasma moves throughout and multiplies within the insect vector's body for a period of time before it can be transmitted in a persistent manner to other hosts plants. This latent period is temperature dependent and ranges from a few days to several weeks. After the insect vector picks up the phytoplasma and incubates it, the vector insect remains invective for the rest of its life. Studies have shown that the AP phytoplasma can overwinter in some adult vectors and their offspring.

The AP phytoplasma are often not evenly distributed within infected trees for several years. During the summer months AP phytoplasma can be found in both the above ground plant parts and roots of infected trees, however when temperatures decrease during the fall and winter months, the AP phytoplasmas moves into the roots where they over winter and cannot be detected in the above ground plant parts until they move back during the following spring probably with sap flow.

Once the phytoplasma is introduced into an orchard, management relies on controlling the vectors within and outside the orchard. The immediate removal of alternative hosts in and around the orchard as well as infected trees helps reduce the source of the phytoplasma. New orchards should be planted with non-infected trees produced from budwood obtained from disease-free, phytoplasma-tested mother trees.

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