In the genus Acer there exist many varieties all of which produce a range of colour unsurpassed by most other tree species, be they the small to medium Japanese maples including the ‘trident’ maple Acer buergerianum, the ‘broad palmate leaf’ maple Acer palmatum shirasawanum; (‘Shirasawa’s’ maple or ‘full moon’ maple) and the serrated ‘red leaf’ maple Acer palmatum dissectum amoenum. To larger varieties including the ‘sugar maple’ Acer saccharum, the American ‘sycamore’ Acer pseudoplatanus and the ‘Norway’ maple Acer platanoides. The family Acer is extensive especially with Japanese varieties therefore, this discussion will be restricted to two – these being the ‘Norway’ maple and the serrated ‘red leaf’ maple.
The Norway maple A. platanoides can be found in eastern and central Europe, Scandinavia, Asia and Russia its range extends well above the arctic circle for example, TromsØ in Norway. It is a deciduous tree that when mature can reach heights from 20–30m, its trunk shallow grooved and grey-brown in colour attains a diameter of approximately 1.5m. The dark green palmate 5 lobed leaves are opposite to each other and are between 7–14cm long by 8–20cm across with the petiole 8–20cm long, which secretes a milky fluid when damaged or cut.
The flowers 15–30 yellow to green have 5 sepals and 5 petals 3–4mm and are arranged in a cluster with stalks that are proportionally longer, allowing them to form a flat or slightly convex head. Flowering begins in early spring before the new leaves emerge. The seeds are flat and disc shaped contained in a two-winged casing arrangement that when shed, rotate as they fall to the ground. The autumn colour is usually yellow to orange to red.
Under ideal conditions Norway maples can live up to 250 years, but as they are often planted in urban areas for decorative and shade purposes this reduces their life expectancy, because if the root system is restricted they will intertwine eventually girdling and killing the tree. A. platanoides has become a popular species for bonsai in Europe and can be seen as medium to large bonsai sizes in a multitude of styles.
Natural enemies – A. platanoides is a hardy species although in some areas it is under threat from the Asian long-horn beetle Anoplophora glabripennis a large insect approximately 1.7 to 3.9cm with antennae twice its body length. Originally native to Japan, Korea and China it is now found in North America, Canada and in Europe; the UK, France, Austria, Italy and Germany. It is said that this pest arrived in the western hemisphere in wooden crates in egg form, which later hatched and multiplied.
This insect kills trees by eating its way through the trunk and to date has not only infected the maple, but other species including Aesculus, (Soapberry, Buckeye and Horse chestnut) Alder, Birch, Hornbeam, Beech, Ash, Prunus sp., Willow, Planes, Sorbus and some species of Poplar. Signs of attack are exit holes 1–2cm diameter, sawdust or ‘frass’, (insect excreta) sap oozing from exit holes, dead or dying branches and yellowing leaves. Insecticides such as imidacloprid, clothianidin and dinotefuran have been used to eradicate adult beetles and as trunk injections to target larvae.
Other unwanted guests include a variety of lepidopterans that feed on its foliage. A. platanoides like most fauna does have its fair share of diseases, although they are more unsightly opposed to life threatening. Such diseases include the ‘powdery mildew’ Uncinula bicornis, and verticillium wilt caused caused by 6 species of this genus V. dahliae, V. albo-atrum, V. longisporum, V. nubilum, V. theobromae and V. tricorpus and White spot caused by the fungus phyllosticta. Shown below.
The serrated ‘red leaf’ maple Acer palmatum dissectum amoenum commonly referred to as the palmate maple is native to Japan, China, Korea, south eastern Russia and eastern Mongolia and many cultivars have been produced for their colouration and various leaf forms. This small tree or shrub when mature can attain heights of between 6 to 10m in single or multiple trunk form. Its leaves are palmate but serrated resembling a delicate ‘lace-like’ pattern with 7 to 9 pointed lobes, approximately 6 to12cm long with single serrated margins. The flowers cluster around a central stem, which bears a single flower that is first to develop, with five red or purple sepals and five whitish petals. The fruit consists of a pair of winged samaras (a winged nut or achene containing one seed) each being 20 to 25mm in length containing a 6 to 8mm seed.
A. palmatum amoenum is usually found at higher altitudes throughout Japan and South Korea and can be considered hardy, but is prone to perish in periods of drought, hence watering regularly is required. However, A. palmatum amoenum will not survive in soggy or wet conditions therefore the medium should be well drained in addition, fertilization is not necessary, but if applied should preferably be in a slow release form and used sparingly.
Natural enemies – As with other maple varieties it is susceptible to attack from pathogens and verticillium wilt as described above, it is also prone to attack from the common Gypsy moth Lymantria dispar. This insect is classified as a pest and is listed as one of the 100 most destructive invasive species known worldwide, because its voracious larvae can consume the leaves of over 500 species of trees, shrubs and plants.
Another unwanted pest briefly mentioned in the article ‘Selecting material for bonsai part III’ (Looking for bargains) is the ‘red spider’ mite Tetranychidae urticae, which includes approximately 1,200 species. These mini beasts live on the undersides of plant leaves spinning fine protective silk webs and causing damage by puncturing the plant cells to feed resulting in wilting, discoloured crumbling leaves. The image below shows an A. palmatum amoenum prior to ‘red spider’ attack, the whole left hand side of the plant and the mid to upper section of the right side was saturated with these pests.
To eradicate these mini beasts a number of approaches were used including tepid water and various insecticides all of which had little effect. Another solution was to use an aerosol spray called ‘Raid’ from SC Johnson that did kill the insects, but had an adverse affect on the plant insomuch that the residue from the spray penetrated the leaf pores causing them to wilt and die. A more serious problem was that it was causing die-back in the petioles and seeping back into the branches. The only solution was to cut out the infected wood, thus 75% of the tree was removed. (Red dotted areas)
The remaining part of the trunk and root system was thoroughly cleaned and repotted in fresh soil then placed in a shady position for the duration of the summer 2016. In October the same year the tree was brought inside where a constant temperature was maintained with no humidity and placed under a full spectrum lighting system. This A. palmatum amoenum showed no signs of life until late June 2017 when a few buds appeared, in late July it is starting to produce growth as depicted below, but will take some considerable time to fully recover.
Arguably, the moral of the story is to check any plant you might acquire for signs of infection and if it is attacked by unwanted pests, try using horticulture soap or a diluted pyrol bug and larvae killer; it is a safer option as opposed to using aerosol insecticide sprays. However, when disease becomes a problem with the Acer genus there is not much we can do, because there is no remedy for fungi that cause canker disease for example.
Root Rot – is caused by the pathogen phytophthora, which attacks the root system and crown of many species of Japanese maple. A tree that is infected will show signs of discolouration, wilt, branch die-back, leaves falling prematurely and black or red sap oozing from the trunk. To prevent canker disease it is important to ensure that the medium in which the tree is planted has good irrigation.
Anthracnose – is a fungal disease that causes foliage distortion, stem die-back, cankers and girdling. The signs that a Acer is infected are black spots appearing on the leaves, which is caused by humidity and wet conditions. Any leaves infected by anthracnose must be removed and destroyed immediately, because there is no chemical treatment available. Although it is said that anthracnose can be prevented by applying a preventative fungal spray when new buds are about to break.
Leaf Spot – disease can be caused by various strains of fungi commonly known to attack plants and signs of infection are coloured spots on the leaves, as a result leaves may be discoloured and fall prematurely from the tree. Another form of spotting is ‘White-spot’ caused the fungus phyllosticta it is prevalent in humid and wet conditions. A preventative measure to ward of leaf spot is to make sure any and all unwanted dead or decaying debris is removed from the soil surface and to make sure that the soil medium has good drainage. Although leaf spot is not considered detrimental to the tree’s health it is annoying because its presence is ugly.
Powdery Mildew – is a powder-like fungal growth found on stems, buds and leaves, which can spread quite rapidly and does not need any moisture to grow and is prevalent in shaded areas having moderate temperatures. Signs of powdery mildew disease is easily visible as it causes leaf distortion. Some practitioners use Neem and horticultural oil to prevent powdery mildew attacking maples whereas others state that good care, maintenance and well-being are sufficient to keep this disease at bay.
Until next time BW, N.