Information Sheet
IS13001
Do you know your rusts?
There are two well-known rust diseases of sugarcane – orange
rust caused by Puccinia kuehnii and brown rust caused
by Puccinia melanocephala. During the 1970s brown rust
spread to many sugar industries throughout the world and,
consequently, was known as ‘common’ rust. It appeared in
Australia in 1978. Orange rust, although present in Australia
since the 1890s, was considered a minor disease and was
confined to the Asia-Pacific region.
Of course, things changed in 2000 when an orange rust
epidemic swept through the industry – the ‘rare’ rust became
‘common’ in Australia. To avoid confusion ‘common rust’ was
given the more descriptive name of brown rust. The two
rusts can be distinguished by symptoms, environmental
requirements and spore characteristics. Both diseases show
elongated lesions on the leaf. In a fresh infection, the
underside of the leaf shows masses of spores erupting from
the lesion, known as a pustule.
Orange rust occurs in all cane-growing districts in Queensland
and New South Wales and the disease is prevalent in humid
summer conditions. Orange rust pustules are orange when
fresh, shorter than those of brown rust and tend to form in
clumps, more towards the leaf base (Photo 1).
Brown rust occurs during dry weather with cool nights and
dews. Brown rust is most often seen in the spring in northern
districts, and in late spring and into the summer in the south.
Brown rust pustules are brown and are more elongated.
Photo 1: Orange rust pustules in clumps.
The pustules are spread evenly, located more towards the
leaf tip (Photo 2). Orange rust spores are larger than brown
rust and golden-orange in colour (Figure 1). By looking at
the spores under the microscope, you can easily see that the
spores have a thick wall at the apex and have large spines.
Brown rust spores are smaller than orange rust, and are
red-brown in colour (Figure 2). When observed under the
microscope, brown rust spores can be distinguished from
orange rust spores because the entire wall of the spore is
of uniform thickness, the spines are very small but more
numerous and several pores will be prominent. The pustules
also contain some very distinctive structures known as
paraphyses (see Figure 2).
During an epidemic in Mackay, losses due to orange rust in the
susceptible variety Q124 were up to 45%. Since that time, with
lower infection pressures, the losses are more likely 15-20%
in susceptible varieties. Losses in other districts are likely to
be similar to this latter figure, and will vary with the weather
experienced each year. The best conditions for orange rust are
a long and extended wet season.
Brown rust yield loss research was carried out soon after
the disease entered Australia, in the late 1970s to the early
1980s. Trials were undertaken with fungicides and in the most
susceptible varieties losses were around 25%. As with orange
rust, losses will vary with season and weather conditions.
Photo 2: Brown rust in Q190A. Pustules are more towards the tip.
Apical
thickening
Pores
Paraphyses
Large
spines
Figure 1: Orange rust spores. Optimum temperature for
orange rust spore germination is 17-23oC.
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Figure 2: Brown rust spores. Optimum temperature for
brown rust spore germination is 11-27oC.
Information Sheet
IS13001
Both orange rust and brown rust are controlled by planting
resistant varieties. Any variety coming through the SRA
breeding program that is highly susceptible to either rust is
discarded. Rust fungi in other crops are known to mutate so
that they can attack previously resistant varieties. This is what
we believe happened when Q124 suddenly became
susceptible to orange rust.
variety on a farm is a good strategy to limit the risks from
disease outbreaks.
Fungicides were used to control orange rust as an interim
control measure while Q124 was being replaced by resistant
varieties. Aerial application of fungicides is effective for control
of orange and brown rusts but is generally not required if
resistant varieties are available.
Changes in susceptibility of sugarcane varieties has been rare
in the past but it is possible that we may see it again in the
future. Planting no more than 30-40% of any one
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Below: Orange rust
Below: Brown rust
A close-up of a diseased leaf showing the lesions of orange
rust.
Brown rust on sugarcane leaf.
Diseased leaf with orange rust lesions.
In brown rust the initial yellow spots on the leaves increase
in size and turn a rusty brown.
A diseased crop of Q124 – dead leaves on standing cane
caused by orange rust.
Brown rust in a field of young Q117 plant cane.
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