Draft Import Risk Analysis - Biosecurity New Zealand

phytoplasma titre, seasonal fluctuations in phytoplasma infection, or uneven distribution of some
phytoplasmas within their host plants (Postman pers. com. 2008).
4.3.4. Diseases of unknown aetiology
The causal agent of these diseases is, by definition, unknown, so detection and identification
must rely on visual inspection of the mother plants or through grafting onto plants that are
known to be susceptible. Options for specific requirements for inspection are included in the
following chapter related to these diseases, but in general inspections should occur at all stages
of growth. Freedom from common graft- or mechanically-transmitted pathogens might indicate
a reduced risk of certain diseases of unknown aetiology (Postman pers. com. 2008).
To enhance the likelihood of disease expression during woody indexing, plant material being
prepared for quarantine should be allowed to go through a natural winter dormancy period with
adequate chilling (BA 2002). Testing should involve a period of heat stress to improve symptom
expression, and should occur over a minimum of two growing seasons.
4.3.5. Bacteria and fungi
Symptom expression of diseases caused by many hazard bacteria is enhanced in higher
temperature and humidity levels. At a minimum over the summer and autumn periods,
temperatures in quarantine should be kept above 18.5ºC and the growing area misted or watered
from overhead to maintain a relatively high humidity. Temperatures may need to be lowered
during winter to enhance viral symptom expression in spring. Plants should not be routinely
sprayed with fungicides or bactericides as these may suppress or mask disease expression and
are unlikely to decontaminate the plant material. Any fungicides or bactericides used to maintain
plant health should have relatively short persistence periods in plant or spoil material e.g. a few
weeks only.
4.3.6. General plant health and containment requirements
There are a number of general plant health and containment requirements that may be taken into
account when considering the handling, growing and inspection of imported Malus nursery stock
in the country of origin or while in post entry quarantine in New Zealand. These requirements
include:
o As a general rule nursery stock and/or indexed material should be inspected over two
growing seasons unless specific tests warrant reduced inspection periods.
o Unless stated otherwise, all imported nursery stock (each plant) should be regularly
inspected each growing season for disease symptoms.
o All parts (leaves, stems and roots) of each import plant should be inspected for disease
symptoms, even when bud stocks have been grafted onto local rootstocks.
o Environmental conditions should, where possible, be manipulated to enhance disease
symptom expression, including exposing the imported nursery stock to heat and water
stress, and variable temperature and day length conditions.
o Containment conditions should be sufficient to both limit the likelihood of hazard
organisms escaping the facility and limit the ability of local organisms to vector hazard
organisms from the imported plants and into the local environment (in New Zealand) or
from the local environment on to the material to be exported (country of origin). The level
of security provided by the facility should therefore be sufficient to contain such propagules
as airborne fungal spores, pollen, and vector insects. Currently the only measures known to
provide this level of containment are to ensure all outward flowing air vents are filtered
with a High-Efficiency Particulate Air (HEPA) filter, and a negative air pressure (15 Pa) is
DRAFT Import Risk Analysis: Viruses, Viroids, Phytoplasma, Bacteria and Diseases of
Unknown Aetiology on Malus Nursery Stock
August 2009 Page 21 of 203