Forest Pathology in New Zealand No. 15
(Second Edition 2009)
Uromycladium rusts of Acacia
M.A. Dick
(Revised by M.A. Dick)
Causal organisms
1. Uromycladium acaciae (Cooke) P. Sydow & Sydow
2. U. alpinum McApline
3. U. maritimum McAlpine
4. U. notabile (Ludwig) McAlpine
5. U. robinsonii McAlpine
6. U. simplex McAlpine
7. U. tepperianum (Saccardo) McAlpine
Note: In 1972 the rust fungus Uromyces digitatus was identified as causing small
light brown pustules on the upper and lower surfaces of Acacia mearnsii in
Whangarei and Kerikeri. No other reports of this fungus have since been
received.
Fig. 1- Gall caused by Uromycladium notabile on branch of Acacia dealbata.
Introduction
Seven of the eight described species of Uromycladium have been recorded in
New Zealand where they are restricted in host range to a number of species of
Acacia. (Elsewhere there are occasional records on Albizia.).
These rust fungi have a complex life cycle which can involve the formation of up
to three types of fruiting body and four spore types (Fig. 2). In all species, initial
infection occurs when basidiospores germinate on a leaf, phyllode, or branch and
penetrate the surface, causing the formation of a spot of diseased tissue on the
host. Where there is a full cycle, small fruiting bodies (pycnia) develop within the
spot. Following fertilisation of the pycnia by spores (pycniospores) from other
pycnia, the next stage, uredinia, can be formed. Uredinia produce urediniospores, which are the vegetative "repeating" stage of the fungus. These spores
are readily wind- dispersed to other plants where germination, infection, and
further production of uredinia may occur. Several crops of urediniospores may
be thus produced during the growing season. Later in the growing season telia
are formed. These fruiting bodies produce wind-dispersed teliospores, which (in
contrast to the urediniospores) can overwinter. Teliospores cannot themselves
infect the host, but germinate to produce basidiospores which can. Pycnia then
develop on a diseased spot arising from a basidiospore infection, beginning the
cycle anew.
Fig. 2 - Life cycle of Uromycladium species.
1. Uromycladium acaciae
Fig. 3 - Brown, powdery pustules of U. acaciae
on lower leaf surfaces of A. mearnsii.
Type of injury
Leaf spots, and branch and stem distortions.
Diagnostic features
Small, brown, powdery patches on the lower surfaces of leaves (Fig. 3).
Distorted, depressed, or slightly swollen areas on branches and stems.
Elongated, chocolate-brown, powdery masses on the branch distortions.
Hosts
Acacia dealbata (Records on A.baileyana, A. decurrens, A. mearnsii and
A. paradoxa are regarded as doubtful).
Distribution
Found from Auckland to Southland.
Disease development
The pustulate fruiting bodies (telia) appear primarily on the lower surfaces of
leaves, either scattered singly or in small groups up to 5 mm in diameter.
Infection on branches or stems causes depressed cankers or swollen distorted
areas from which the telia erupt. When the telia are mature the host epidermis
splits, exposing the chocolate-brown, powdery masses of teliospores which are
wind-dispersed.
Economic importance
This disease does little damage and is not considered to be of any importance.
Control
Not considered necessary.
2. Uromycladium alpinum
Fig. 4 - Spore masses webbing the pinnules.
Type of injury
Newly developing spring flush becomes distorted and chlorotic. Pinnules are
subsequently cast.
Diagnostic features
Masses of chestnut brown spores on young foliage.
Pinnae often webbed together (Fig 4).
Hosts
Acacia baileyana, A. dealbata, A. mearnsii.
Distribution
Widespread in the North Island. Also recorded from Marlborough and
Canterbury.
Disease development
The disease is characterised by vast masses of chestnut brown spores produced
in mucus which coat the pinnules and often web the pinnae together. Pinnules
are subsequently cast. Newly developing spring flush becomes distorted and
chlorotic.
Economic importance
Uromycladium alpinum has caused considerable losses In nursery seedlings of
A. dealbata and A. mearnsii. Indications are that A. dealbata is less susceptible
to the disease than A. mearnsii.
Control
Not considered necessary in adult trees. In nurseries control has been achieved
with applications of tolyfluanid.
3. Uromycladium maritimum
Fig. 5 - Lesions of U. maritimum on phyllodes of A. longifolia var. sophorae.
Type of injury
Conspicuous lesions on phyllodes (flattened petioles taking the place of leaves)
and stems.
Diagnostic features
Small, black, elongated lesions on both sides of phyllodes (Fig. 5).
Elongated or circular, slightly raised spots up to 5 mm in diameter on
branches and stems
A sooty coating on phyllodes and twigs.
Hosts
Acacia floribunda, A. longifolia subsp. longifolia, A. longifolia subsp. sophorae,
A. notabilis.
Distribution
Throughout the North Island, except Northland, and in Nelson.
Disease development
Reddish-brown, elongated lesions up to 5 mm long develop on both sides of the
phyllodes and on branches and stems. Small, circular pycnia in the centre of the
lesion become surrounded by the elongated uredinia. Urediniospore production
is profuse and the dark brown spores coat the lesions and often the twigs and
leaves with a sooty covering which frequently becomes encrusted. Teliospores
are produced later, intermixed with the urediniospores.
Economic importance
Of no economic importance.
Control
Not considered necessary.
4. Uromycladium notabile
Type of injury
Large galls on branches and stems cause dieback, and sometimes tree death.
Diagnostic features
Large, convoluted, brown galls on branches, stems, and seed pods
(Fig. 1).
Smaller blister-like galls on leaves and petioles.
Dieback of branches beyond the galls.
Death of young trees.
Hosts
Acacia baileyana, A. dealbata, A. decurrens, A. mearnsii, A. parramattensis.
Distribution
Throughout New Zealand.
Disease development
The fungus attacks stems, branches, mature seed pods, and leaf petioles
forming inflated, convoluted galls. These galls are usually 2-8 cm in diameter, but
occasionally grow up to 35 cm across. Galls continue to grow for several
seasons and restrict water conduction within the tree. As a result, branches
beyond the galls make little growth, produce few leaves, and begin to die back.
When infection is severe, whole trees may be killed outright in a season or two.
The olive-brown galls become completely covered with cinnamon- or chocolatebrown urediniospore and teliospore masses, which are produced on the surface
season after season. Minute, black, pustulate pycnia containing pycniospores
can be found associated with both uredinia and telia. Galls on leaves and
petioles are much smaller than those on branches, appearing as raised, blisterlike, green to purplish structures. Both urediniospores and teliospores are winddispersed.
Old galls are frequently attacked by insects and extensive tunnelling by larvae
can be found. The insects, however, live at the expense of the gall and do not
affect the host tree.
Economic importance
By the 1920s this fungus had heavily infected A. decurrens which had been
planted in the hopes of starting a tannin industry, and the 1800 ha established for
this purpose were felled and cleared. Susceptible host species are now grown
only as ornamentals.
Control
No control known.
5. Uromycladium robinsonii
Fig. 6 - Lesions of U. robinsonii on phyllodes of A. melanoxylon.
Type of injury
Tiny galls on phyllodes, juvenile leaves, and petioles sometimes cause death and
casting of phyllodes and leaves. Depressed cankers on twigs and small branches
cause dieback in severe cases.
Diagnostic features
Brown, powdery pustules on upper and lower surfaces of phyllodes and
leaves, and on petioles (Fig. 6).
Elongated, brown-black sunken cankers up to 4 cm long on twigs and
small branches.
Dieback of the tree crown.
Host
Acacia melanoxylon, A. longifolia subsp. sophorae
Distribution
Widespread in the North Island. Recorded south to Canterbury in the South
Island.
Disease development
Infection commonly occurs on phyllodes, leaves, and petioles. The less common
infection of twigs and small branches leads to elongated, brown-black cankers up
to 4 cm long, which become sunken due to the collapse of dead host tissue. In
severe cases, cankers may girdle twigs and branches, causing dieback of parts
of the crown.
Chestnut-brown, powdery pustules containing urediniospores, teliospores, or
both are formed on upper and lower phyllode/leaf surfaces and on cankers. They
may be scattered singly, but more frequently form circular groups up to 2 mm in
diameter. Pycnia and their pycniospores are found within these circles.
Economic importance
Although generally considered to be of minor importance, U. robinsonii has
recently caused the dieback of up to 80% of the crown of trees in a 4-year-old
plantation, in Whakarewarewa State Forest.
Control
No control known.
6. Uromycladium simplex
Type of injury
Minute pustules on phyllodes.
Diagnostic feature
Brown, powdery pustules scattered over both surfaces of the phyllodes.
Host
Acacia pycnantha.
Distribution
Canterbury.
Disease development
Brown, pustulate fruiting bodies(uredinia) up to 1 mm in diameter develop in an
irregular pattern on both sides of the phyllodes. They are frequently scattered so
densely that they merge together. The uredinia first produce a rusty brown,
powdery mass of urediniospores, and later in the season also form teliospores.
Initially, teliospores can be found intermixed with the urediniospores.
Economic importance
Of rare occurrence and no importance.
Control
Not considered necessary.
7. Uromycladium tepperianum
Type of injury
Galls on branches and stems causing dieback and sometimes tree death.
Fig. 7 - Galls of U. tepperianum on twigs of A. verticillata
Diagnostic features
Distorted, inflated, brown galls on branches (Fig. 7), stems, and seed
pods.
Smaller galls on phyllodes and petioles.
Dieback of branches beyond the galls.
Occasional death of young trees.
Hosts
Acacia paradoxa, A. podalyriifolia, A. ulicifolia, A. verticillata, A. riceana.
Distribution
Throughout the North Island and Canterbury.
Disease development
As with U. notabile, perennial galls develop on branches and stems and inhibit
growth. Branches eventually die back, and small trees die if galls have formed on
the stem. Smaller swellings form on the phyllodes and seed pods. Every spring
and summer the surface of the galls becomes covered with powdery, cinnamoncoloured masses of teliospores. Minute, dark brown pycnia are scattered over
the galls and swellings in association with the telia. Teliospores are windspread.
No urediniospore stage is formed. Insect larvae commonly invade the galls,
boring through them and leaving conspicuous tunnels.
Economic importance
In New Zealand the host trees are grown only as ornamentals, so the disease is
of no economic importance to plantation forestry.
Control
Not considered necessary.
BIBLIOGRAPHY
Cunningham, G.H. 1931: "The rust fungi of New Zealand". Published by the
author.