Draft Import Risk Analysis - Biosecurity New Zealand

Malus florentina (Zuccagni) C. K. Schneid. Malus sieversii (Ledeb.) M. Roem.
Malus floribunda Siebold ex Van Houtte Malus sikkimensis (Wenz.) Koehne ex C. K. Schneid.
Malus fusca (Raf.) C. K. Schneid. Malus spectabilis (Aiton) Borkh.
Malus halliana Koehne Malus spontanea (Makino) Makino
Malus honanensis Rehder Malus sylvestris (L.) Mill.
Malus hupehensis (Pamp.) Rehder Malus toringo (Siebold) Siebold ex de Vriese
Malus ioensis (Alph. Wood) Britton Malus transitoria (Batalin) C. K. Schneid.
Malus kansuensis (Batalin) C. K. Schneid. Malus trilobata (Poir.) C. K. Schneid.
Malus komarovii (Sarg.) Rehder Malus tschonoskii (Maxim.) C. K. Schneid.
Malus leiocalyca S. Z. Huang Malus yunnanensis (Franch.) C. K. Schneid.
Malus maerkangensis M. H. Cheng et al. Malus zhaojiaoensis N. G. Jiang
Malus mandshurica (Maxim.) Kom. ex Skvortsov Malus zumi (Matsum.) Rehder
2.3. Biology and ecology of Malus domestica
Malus domestica is a small to medium-sized tree, 5-10 m tall, freely branching with long shoots
and various types of short shoots (spurs) with a single trunk. When growing unattended in the
tropics the plants revert to a stiff upright bush, 2-4 m tall, through reiteration of axes near the
ground. Young stems and twigs are tomentose with the leaves being elliptic-ovate, 4-13 cm by
3-7 cm in size, rounded at the base, the margins irregularly saw-toothed, and usually densely
tomentose beneath (CPC 2007).
At high latitudes Malus domestica requires a mild growing season (no extremes of sunshine,
temperature or humidity), a sufficiently cold winter to break dormancy and excellent soil
conditions to limit stress, as this affects fruit quality and, if more severe, fruit size and floral
development for the next crop. Windbreaks are needed for exposed sites. In the tropics a short
growth cycle requires favourable (mild) growing conditions throughout the year, as may be
found close to the equator: altitude 800-1200 m (temperature 16-27°C), sunshine more than 50%
of potential sunshine duration, rainfall 1600-3200 mm, relative humidity 75-85%, good soils
with irrigation facilities. For an annual growth cycle in the tropics there should be a prominent
change of seasons, the growing season meeting the above requirements, whereas the off-season
should preferably be overcast as well as cool, since low light levels as well as low temperature
appear to have a dormancy-breaking effect. Such conditions are usually found further from the
equator at elevations of 1200-1800 metres (CPC 2007).
Growth flushes are terminated by the formation of inflorescences (consisting of 2-5 flowers with
a terminal “king” flower), or vegetative spurs. Flowers are insect-pollinated, and the fruit ripens
3.5-5 months after flowering, depending on the cultivar. During fruit development, some lateral,
undeveloped buds develop floral structures within the protective budscales. After a period of
dormancy, these buds will grow out to form the next season’s inflorescences. Buds that did not
develop floral structures will grow as a vegetative spur, or will not grow at all.
In temperate climates such as New Zealand, there is usually a single major flush in spring. Fruit
ripens in the autumn after which the leaves are shed. Low winter temperatures are instrumental
in breaking bud dormancy in time for the spring flush.
In the tropics growth is very different: shoots all grow more or less vertically, leaves are retained
much longer so that the plant becomes evergreen, there is little shoot growth, scattered over the
entire year and largely limited to shoot extension, few laterals being formed (CABI, 2006).
DRAFT Import Risk Analysis: Viruses, Viroids, Phytoplasma, Bacteria and Diseases of
Unknown Aetiology on Malus Nursery Stock
August 2009 Page 9 of 203