PESTS AND DISEASES OF FORESTRY IN NEW ZEALAND
Bacterial leaf spot and canker of Poplar, Willow and Alder
Forest Pathology in New Zealand No. 21
Bacterial leaf spot and canker of poplar, willow, and alder
Based on A.G. Spiers (1990)
Causal organism
Pseudomonas syringae pv. syringae van Hall
Fig. 1 - Leaf of a Populus deltoides x P. ciliata seedling exhibiting leaf spot incited by P.
syringae pv. syringae.
Type of injury
Leaf spotting. Cankers on twigs, branches, and stems of mainly 1- to 2- year-old plants with
associated dieback. Occasional whole-plant death where cankers form low on the stem.
Diagnostic features
• Yellow flecks (1-3 mm diam.) on upper and lower leaf surfaces.
• Dark brown spots of irregular outline on both leaf surfaces. Spots may remain discrete, or
merge forming larger blotches.
• Terminal and side shoot dieback with blackened buds and collapsed, desiccated foliage.
• Rough fissures or sunken, blackened lesions on stems (cankers).
• Bacterial ooze from cankers which, when dry, appears as a white deposit on the stem.
Hosts
Populus spp. - All species present in Now Zealand are susceptible but leaf spotting is most
prevalent on P. ciliata, P. deltoides, P. szechuanica, P. trichocarpa, P. yunnanensis, and their
hybrids.
Salix spp. - All species present in New Zealand are susceptible. Stem and twig dieback is more
common than leaf spotting.
Alnus cordata - Leaf spotting and twig dieback.
Fig. 2 - Collapsed necrotic tissue on a P. deltoides x ciliata leaf after infection by P. syringae pv.
syringae.
Distribution
Throughout New Zealand, particularly in localities with high summer rainfall.
Fig. 3 - Year-old shoot of a P. deltoides x
ciliata seedling exhibiting bud and stem
necrosis after Infection by P. syringae pv.
syringae .
Fig. 4 - Year-old shoot of a P. deltoides x
ciliata seedling exhibiting stem cankering as
a result of earlier infection by P. syringae
pv. syringae .
Disease development
Leaf symptoms first appear in spring when temperatures are rising and moisture levels are high.
Throughout the growing season, during periods of continuous rainfall (>3days), spots are
produced on new leaves. For example, a severe attack of P. syringae occurred on foliage and
stems of poplars growing in a nursery at Aokautere during a 4-month period (November 1985-
February 1986), when rainfall was 64% (194 mm) more than average, and rain fell on 52 of
those 120 days. Depending on the cultivar, spots may either remain discrete, or merge forming
extensive blotches. In poplars leaf spotting is generally more common than stem cankering and is
often seen on plants which do not exhibit any other symptoms.
Formation of stem and twig cankers also depends on high moisture levels. Symptoms vary -
stems may be roughly fissured or have sunken black lesions oozing bacteria. Sometimes the
whole stem is affected, killing the plant; more often only the top one third to one half of the
plant, or only lateral branches, are involved. When there is extensive shoot and twig dieback
with blackened dead foliage the condition is referred to as "blast'. New shoots grow from below
the infected areas on stems and branches. Although stem fissures and cankers may heal and the
plant continue to grow, this often leaves a weak point which makes the plant susceptible to
breakage in high winds.
Some severe outbreaks of the disease occur when sudden frosts follow a warmer wet period.
Approximately 50% of Pseudomonas syringae isolates, tested from poplars and willows in New
Zealand were capable of causing ice-nucleation. Ice-nucleating bacteria initiate the formation of
ice crystals within host cells during frosts, and the combination of bacteria and frost causes more
extensive tissue disruption than if either factor were present independently.
Economic importance
Losses due to this disease are generally not great. Outbreaks of disease due to P. syringae on 1-
and 2-year-old nursery grown poplars are sporadic and very dependent on prevailing weather
conditions. In rare instances, damage can be severe when P. syringae is combined with late
frosts.
Control
Control of disease is generally not warranted. Traditionally, copper-based inorganic compounds
have been used to reduce bacterial populations but with only limited success. There is potential
for biological control with competitive, non-pathogenic strains of other Pseudomonas species.
Fig. 5 - Terminal and side shoot dieback of a P. deltoides cultivar exhibiting the classical
symptoms of "blast" after infection by P. syringae pv. syringae .
Bibliography
Haworth, R.H.; Spiers, A.G. 1988: Characterisation of bacteria from poplars and willows
exhibiting leaf spotting and stem cankering in New Zealand. European Journal of Forest
Pathology 18: 426-436.
Kam, M. de 1982: Damage to poplar caused by Pseudomonas syringae in combination with frost
and fluctuating temperatures. European Journal of Forest Pathology 12: 203-209.
Compiled 1990: revised 2010
