PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
Rusts
Rust diseases derive their name from the orangebrown
spore masses which many rust fungi produce
on their hosts. The cereal rusts occur universally
wherever susceptible hosts grow. The Romans
considered the cereal rusts so important that they
believed that the Gods Robigo and the Robigus were
responsible for them and planned annual festivals to
please them. GRDC Rustlinks is the main online
source of information for cereals rusts.
www.grdc.com.au/rustlinks
Scientific name
Rusts (Order Uredinales, Phylum Basidiomycota)
are a specialized group of fungi which produce a
range of spore states (Table 63).
Host range
Ornamentals, fruit, vegetables, field
crops and weeds may be attacked. Generally a
particular species of rust can attack only certain
host species or only certain varieties. Rust fungi
that are morphologically identical but attack
different host genera or species are called form
species (f.sp.), eg Puccinia graminis f.sp. tritici.
Native rust fungi are continually being detected.
Rose rust (Phragmidium mucronatum)
Sunflower rust (Puccinia helianthi)
Stripe rust (P. striiformis)
Wheat leaf rust (P. recondita f.sp. tritici)
Rust (P.grevilleae) on Proteaceae
See also page 322.
Symptoms
Leaves, stems and fruit may be attacked. Sepals,
and occasionally other flower parts, glumes in
cereals and grasses. After infection it may be some
time before symptoms appear – plants can be
dispatched infected but without symptoms.
Leaves, stems
The leaf upper surface becomes speckled due to
a yellow zone which forms around infection
zones. The small yellow patches may run together.
On leaf under surface, corresponding yellow,
orange or rusty-brown spores masses
(urediniospores) develop. Later in the season, dark
or black spores may form in pustules (teliospores).
White rust of chrysanthemum produces pinkishwhite
waxy pustules. Some rusts produce spores
on both leaf surfaces.
When infection is heavy, premature and
repeated leaf fall seriously weakens the plant. If
stem infections are heavy, stems may be ringbarked
causing dieback of the upper portion.
– Wattles. Rust galls (Uromycladium spp.) develop
on flowers, stems and foliage (Fig. 192).
– Young poplar trees in nurseries may die.
– French bean (Phaseolaris vulgaris). Spore masses
on leaves may be black rather than red.
Fruit
Lesions may develop on peach fruit, bean pods, etc.
Diagnostics. Presence of rust spores
The powdery rust spores can be removed by
running the thumbnail across mature lesions, this
indicates the presence of characteristic rust spores.
May be confused on leaves with various leaf
spotting diseases on some hosts.
Rust on fruit and twigs can be difficult to recognize
without microscopic examination and experience.
Microscopic examination of spores by an expert
may be required to confirm identification (se page xiv).
Observation of the intricate structure of the spores is
often needed to enable accurate identification.
In the early stages of infection only pin point spots
may be present on leaf under sides. Experience is
needed to detect the early stages of infection.
For some rusts, there are keys and DNA tests.
Fig. 190. Geranium rust (Puccinia pelargoniizonalis).
Rust pustules on leaf undersurfaces.
PhotoNSW Dept of Industry and Investment..
Fig. 191. Bean rust (Uromyces appendiculatus).
Under surface of bean leaflet with reddish-brown pustules
surrounded by a pale border. PhotoNSW Dept of Industry and Investment.
Fig. 192. Gall rust (Uromycladium spp.)
on wattle stems. PhotoCIT, Canberra (P.W.Unger).
Fig. 193. Rose rust (Phragmidium mucronatum). Left: Upper
surface of leaf with yellow areas. Right: Undersurface of leaf
showing orange urediniospores and black teliospores.
PhotoCIT, Canberra (P.W.Unger).
Fungal diseases - Examples of fungal diseases 351