PEST RISK ANALYSIS (PRA) TRAINING Group Exercises Manual

GROUP EXERCISE NO. 2
PEST RISK ANALYSIS TRAINING - GROUP EXERCISES MANUAL
Example No. 1 – Butternut Canker 1
SCENARIO: Several European countries have received requests to import nursery stock plants of
walnut (Juglans spp.) from North America. At their request, the European and Mediterranean
Plant Protection Organization (EPPO) has agreed to conduct a PRA. One of the primary pests of
concern is the butternut canker (Sirococcus clavigignenti-juglandacearum).
PEST DATASHEET:
Identity: Sirococcus clavigignenti-juglandacearum Nair, Kostichka and Kuntz (Fungi:
Ascomycota (anamorphic)). The form-genus Sirococcus is probably heterogeneous and the
affinities of this fungus are not clear. Common name: Butternut canker.
Hosts: In North America Juglans cinerea (butternut) is the only species that is killed by the
pathogen, though other Juglans species and hybrids (e.g. J. ailantifolia var. cordiformis, J. regia
and J. nigra) are diseased to varying degrees. More data is needed on the susceptibility of these
species. In Europe, J. regia is widely planted for its nuts, oil and valuable wood except in the north,
as it needs mild winters without late frosts, and a climate which is not too dry. J. nigra is also
cultivated in Europe and used as rootstocks for J. regia or as an amenity tree, and sold in
nurseries. Laboratory experiments indicated that S. clavigignenti-juglandacearum might be able
to survive on other Juglandaceous hosts (Carya spp.) and possibly other trees (Quercus, Prunus).
Distribution: Origin: Possibly Asia or South America. Present: North America (Canada - Quebec,
Ontario, New Brunswick; USA - north-eastern states). Absent: EPPO region.
Biology and Dispersal: Conidia develop beneath infected bark in sticky masses. Stromatal pegs
lift and rupture the bark and, under moist conditions, millions of conidia are extruded. These
conidia are dispersed by rain splash and wind in little droplets or as aerosols during rainfall. They
are transported by run-off water from infected branches to tree trunks, infecting buds, wounds and
other openings. Infection results in multiple stem cankers. Cankers on twigs start to develop,
usually in the lower crown. Spores require at least 16 h of dew at 20°C to germinate on the bark of
J. cinerea. In infected suckers, twigs and branches, cell walls of the bark are degraded, macerated
and rapidly broken up entirely. The fungal hyphae penetrate the phloem, then the xylem, and
progress finally into other bark and wood tissues. They create new cankers when reaching the
cambium. The fungus is able to sporulate on standing or felled dead trees for at least 20 months.
Outside the host, conidia can survive for at least 8 h in cool and cloudy weather. The pathogen
remains viable in diseased tissue and in culture down to 0°C and below. Dispersal occurs by wind
and rain (spores, conidia), and insects probably also play a role in disease dissemination as vectors
or wounding agents. The fungus may be seed-borne, at least in seeds of J. cinerea and J. nigra.
Movement of scion wood and other propagative material, as well as untreated logs and firewood
with bark, may contribute to dispersal of the fungus.
Damage: S. clavigignenti-juglandacearum is a very aggressive pest that causes cankers on stems,
branches and exposed roots. As cankers coalesce progressively, they girdle branches and stems
which may lead to tree death. The fungus has caused dramatic declines of J. cinerea in the USA (in
some states, up to 80%). Good quality wood of J. cinerea is valuable for woodworking, furniture,
cabinetry and dye production. Nuts are an important source of food for wildlife, and the decline of
J. cinerea in North American forests is perceived as a threat to biodiversity. In central Europe, J.
regia and J. nigra have the most valuable wood and high-quality veneers make profits of more
than 5000 EUR per m 3 . Wood of J. regia is used for cabinetry, musical instruments, crafts and
stocks, and nut and oil production are also important industries in southern Europe.
Control: There is no known effective control against butternut canker, other than the removal of
recently killed and visibly infected trees to locally reduce the amount of inoculum, or the selection
and propagation of resistant trees.
1 NOTE: This example was developed specifically for use in the IPPC PRA training course. The
scenario is not real. The data sheet is based on the EPPO Alert List 2005 and EPPO Data Sheet 2006,
URL: http://www.eppo.org/.
8