PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
Diagnostics. Current focus is on detection and
diagnostics of soilborne diseases (nematodes, bacteria,
fungi, etc) before planting the crop. Pscheidt (2009)
provides a good summary of the Diagnosis and Control
of Phytophthora Diseases (avail online).
Symptoms. Phytophthora infection may be present
but not observed because root replacement may keep
up with the rate of root death.
– Pc cannot be easily distinguished by growers from
symptoms alone and the disease is often misdiagnosed.
Laboratory analysis necessary. However, growers of
crops which Pc commonly infects and which produce
reliable visible symptoms, eg azalea, jarrah, avocado,
quickly become familiar with symptoms of disease.
– Other soilborne disease on some plants, eg
Pythium, Fusarium, Cylindrocladium, Rhizoctonia
and Thielaviopsis (Chalara) are difficult to distinguish
from Pc as the cause of root rot on initial examination.
– Diseases and pests of the upper part of the
plant, eg trunks, etc, can be difficult to determine.
– Non-pathogenic causes such as anaerobic
conditions in the root zone caused by excessive
watering, poor quality potting mix, or herbicide
injury can cause similar breakdown of roots.
– Indicator species, eg grass trees (Xanthorrhoea)
found dead or dying indicate that Pc is in the area.
– Identification of unknown fungi in the soil can be
difficult if there are insufficient fungal hyphae for
proper identification.
The detection and identification of Phytophthora,
and other root rots in plants, soils, potting mix, sand
and other materials is a major part of the work of
laboratories diagnosing plant diseases. However, no
one piece of information is enough to conclusively
diagnose a Phytophthora disease, the presence of the
fungus may only be part of a broader or deeper
problem or not related at all. Evidence from the field,
sick plants and identification in a laboratory must all
indicate the same problem.
Consult a diagnostic service to confirm or reject
a preliminary diagnosis. Association of a fungus with
symptoms does not prove that it is the primary cause
of the symptoms, it may be a secondary invader of
tissue damaged by one or several other agent. Several
diagnostic tests have developed to diagnose PC.
– Soil. Baiting for disease organisms, eg Pc, involves
placing a soil sample in a container, flooding it with
water and adding susceptible plant parts as bait, eg
lupin roots, cotyledons of Eucalyptus sieberi). If
zoospores are present they will infect the bait which is
then placed onto agar, spores are produced and
identified by either microscopic examination or more
recently by DNA tests. A negative result from baiting
indicates freedom from Pc. Sometimes this may be a
false negative when populations of Pc are low.
– Roots of infected plants may be directly placed onto
selective agars, spores produced similarly identified.
– Microscopic examination to distinguish spore
structures in infected tissue, on agar cultures or baits.
Taxonomic keys identify species. If spores are lacking,
diseased tissue can be kept in a high humidity chamber
for a few days or cultured to promote spore formation.
Spores of some species of Phytophthora, Pythium and
Cylindrocladium, or the characteristic hyphae of
Rhizoctonia, can be identified this way.
– Non-DNA test kits for some soilborne
diseases. Alert Fungal Disease Detection Kits
have been used by commercial growers to detect some
soil fungi including Pc, Pythium and Rhizoctonia.
These kits allow early detection and confirmation of
disease avoiding unnecessary chemical applications
while maintaining good crop quality. Test kits can be
expensive. ELISA tests are quick and efficient and
mostly laboratory-based, some can be used on-site.
The fungus reacts with chemical reagents to cause a
detectable color change.
– DNA-based tests. Phytophthora IDENTIKIT TM is a
DNA-based diagnostic test that accurately and identifies
the pathogen from infected plant material, baited soils
and water. It overcomes the limitations of the traditional
baiting method in that failed negatives are eliminated
and large numbers of samples can be processed in a
short time. Such tests will benefit management of
eucalypt dieback.
A single soil sample, using a DNA extraction
process, can now identify and quantify a range of
fungal and nematode disease organisms and predict
the likely extent of the losses well before a crop is
even planted, eg Fusarium, Rhizoctonia, Mycospherella,
Guaeumannomyces graminis, Phoma, nematodes, etc.
Results have to be interpreted accurately at field level.
Growers can change cultivars, crops, modify cropping
programs where risk of crop loss is high.
Nonspecific
symptoms
Baiting
followed
by
spore Microscopic
production
DNA-based
examination
on agar
diagnostic
of spores tests
Some methods used to diagnose Phytophthora spp.
Disease cycle
See Fig. 206, page 367.
‘Overwintering’
Pc as spores (up to 9-10 years) and/or mycelium in
the soil or media up to 20 years.
Pc can be recovered from tap roots 1-2 m deep.
As spores and/or mycelium in infected plants, on
root and stem debris from infected plants.
Other soilborne fungi can 'overwinter' as sclerotia, etc.
Spread
Water. Zoospores spread in surface drainage water
from contaminated areas, in recycled irrigation water
and from infected to healthy plants in running or
splashing water. Run off and subsoil seepage may
carry spores onto a site. Rate of spread in bushland
downhill may be 0.7-3.6 m/yr but more after fires etc.
Aerial spread. Contaminated wind-blown dust
may contaminate stored media. Other species, eg P.
infestans, may be spread by irrigation splash and
wind blown driven rain.
In soil in containers, on tools, machinery, vehicles,
bicycles, boots, other equipment; re-using infected
soil as a potting mix; in gravel from surrounding
forest areas. Pc readily contaminates pots and potting
mixes allowed to contact soil, and in the past has been
detected in some brands of imported peat.
Plants. Movement of infected nursery plants, plant
material, tube stock, seedlings. P. ramorum was
spread widely in the USA through the shipping of
infected stock from nurseries.
Infected propagation material, eg tube stock,
tubers, plugs, seeds. Cuttings can be a source of
infection if taken too close to ground level.
Possibly by soil animals. Fungus gnats present in
moist organic matter may spread Chalara.
Bush regenerators may unwittingly contribute to
the spread of disease through soil disturbance and
planting stock from infected nurseries.
Vertebrate pests, eg feral pigs, horses.
Many plants become infected in garden or bush via
a nursery (like weeds) and then may spread in
water run off into neighbouring bushland and
through dumping plants in the bush.
Pod-boring beetles overseas are attracted to
disease lesions and rapidly generate and spread
secondary inoculum in epidemics of pod rot.
366 Fungal diseases - Examples of fungal diseases