PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
IDENTIFICATION
Fruiting body of a wood
rot fungus, a “conk”
Powdery mildew
Culture
ALERT
TESTING KIT
DNA
CLASSIFICATION
Knowledge of
classification
helps in
in understanding
the biology of
fungi and
their control
SYMPTOMS EXHIBITED BY THE HOST PLANT
Foliage fungal diseases are possibly easier to diagnose in the field than other diseases
but can still be difficult, expert help is often needed and nearly always needed for soil
diseases. The presence of signs and symptoms may be sufficient for a preliminary
diagnosis of some fungal diseases, eg signs of grey powdery coating on leaves
(powdery mildew) or symptoms, eg leaf spots.
DETECTION AND IDENTIFICATION BY EXPERTS
Microscopy detects and identifies some diseases which cannot be cultured, eg
powdery mildew, or when fungicides have been previously applied. Examine affected
tissue directly, under a low powered (dissecting) or high powered (compound)
microscope for mycelium, fruiting structures and spores which may be sufficient for a
useful diagnosis. If spores are lacking, diseased tissue can be kept in a high humidity
chamber for a few days or cultured to promote spore development. Spores of some
species of Phytophthora, Pythium and Cylindrocladium, or the characteristic hyphae of
Rhizoctonia, can be identified this way. Lucid keys www.lucidcentral.org/
Interactive Key to the Fungi of Australia
Key to Common Microscopic Fungi (for schools)
Fungi of Australia
Key to 101 Forest Fungi of Eastern Australia
Isolation and culture from infected material obtains pure cultures of fungi which
can be identified from the spores produced. Suspect plant tissue or seeds are placed on
agar media and the organisms that grow from it identified. Others need to be incubated
under certain temperature, aeration or light conditions to produce spores. Baiting for
disease organisms, eg Phytophthora, Pythium, Rhizoctonia, involves floating plant
material (carrot, lupin baits) on the surface of a representative sample of soil, media or
water and observing the baits for signs of fungal invasion and rotting.
Biochemical tests are used for accreditation schemes. Commercial growers use
Alert Fungal Disease Kits to detect soil fungi, eg Phytophthora, Pythium, Rhizoctonia.
ELISA tests are quick, efficient and mostly laboratory-based, some can be used on-site.
The fungus reacts with chemical reagents to cause a detectable color change.
DNA techniques are used to identify fungi, eg Phytophthora, black sigatoka smut of
banana. The Phytophthora – IDENTIKIT has been marketed. An in-field clubroot
diagnostic test is in the process of development.
Wide range of soil pests and diseases can be identified from a single soil
sample.
CLASSIFICATION is mainly according to types of mycelium (with or without
cross walls, etc) and sexual spores produced and can be complicated (Agrios
2005). Fungi and fungal-like organisms are grouped into various Phyla, eg
Fungal-like organisms (various Kingdoms) which include the following phyla:
– Myxomycota (slime moulds) on lowlying plants (not parasitic on plants).
– Plasmodiophoromycota (endoparasitic slime moulds), eg powdery scab of
potato tubers.
– Oomycota (water moulds), eg Pythium, Phytophthora, downy mildews.
True fungi belong to the Kingdom Fungi which include the following phyla:
– Chytridiomycota (zoospores), eg Olpidium (can transmit virus diseases).
– Zygomycota (spores in sporangia), eg bread moulds (Rhizopus, Mucor).
– Ascomycota (ascospores in a sac), eg powdery mildews, peach leaf curl, yeast.
Imperfect Fungi produce asexual spores, not known to produce sexual spores,
eg some powdery mildews (Oidium sp.).
– Basidiomycota (basidiospores in a club), eg mushrooms, wood rots, rusts, smuts.
Sterile Fungi are not known to produce any kind of spores, eg Rhizoctonia.
WHAT IS IT'S PROPER NAME?
When the sexual state of an Imperfect or Sterile Fungus is found, it is usually given
the name of the sexual stage. However, if the name of the asexual or sterile stage is
common and well known, it may continue to be used in preference to the new name.
Powdery mildew of pea
Imperfect Fungi - Oidium sp.
Ascomycota - Erisyphe pisi
Rhizoctonia root rot
Sterile Fungi - Rhizoctonia solani
Basidiomycota - Thanatephorus cucumeris
WHY IS KNOWLEDGE OF THE FUNGAL GROUP IMPORTANT?
Most fungicides are selective, ie they are effective against one group of fungi but not
another. Fungicides belonging to the same chemical groups tend to be effective
against similar groups of fungi. There are exceptions and some of the newer
fungicides are effective against both downy and powdery mildews.
Fungal diseases - Biology, identification and classification 319