PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
‘Phytophthora’ root rot
An example of a soilborne fungal disease
Phytophthora, one of the world’s most
damaging disease organisms affects a broad
range of plant species costing millions of dollars each
year in Australia. This introduced soilborne fungus
became important initially because of its occurrence in
the jarrah forest in WA (Keane et al. 2000, Shearer et al
2009) and the seriousness of the disease on many
ornamental plants and fruit crops. Threatened species
may be at high risk of extinction. Many Phytophthora
species and other root rotting fungi cause major yield
losses in Australia annually. Many investigative and
information groups have been formed, eg
Dieback Information Group www.dieback.org.au/
Centre for Phytophthora Science and
Management www.cpsm.murdoch.edu.au/
Biological Crop Protection www.biolcrop.com.au/
Soilborne Diseases Symposia held regularly by the
Australasian Plant Pathology Society www.apps.net.au/
Phytophthora Online Course: Training for Nursery
Growers (Oregon State University, currently. available
at http://oregonstate.edu/instruct/dce/phytophthora/
Scientific name
Phytophthora root rot (Phytophthora cinnamomi (Pc),
Phylum Oomycota) is often called ‘dieback’ but do not
confuse ‘Phytophthora root rot’ caused by Pc with
dieback caused by other agents, eg Armillaria root rot,
Christmas beetles and other foliage-feeding insects,
drought, etc. Additionally, diseases called ‘Phytophthora
root rot’ may be caused by species of Phytophthora
other than P. cinnamomi, eg Phytophthora root rot of
lucerne is caused by P. megasperma. There are more
than 60 described species of Phytophthora, many of
which have been imported into Australia.
Host range
Wide host range, including ornamentals, eg azalea,
native plants, eg Proteaceae, Epacridcaeae, Myrtaceae
especially eucalypts (jarrah), susceptible commercial
floriculture taxa include waxflower, banksia, boronia,
crowea, rice flower, waratah, thryptomene; fruit, eg
apple, avocado. peas, orange, grape, vegetables, field
crops and weeds. Most states have host ranges for
their state, eg Reid (2006) has provided a list of the
main species of importance to horticulture in WA.
Symptoms and impacts
Soil diseases affecting roots and crowns are often unnoticed
for years. In addition to attacking mature plants,
this fungus can attack seeds and seedlings (page 371).
Above ground symptoms (on shrubs, trees).
Leaves may develop brown tips and margins.
Generally a wilting, yellowing or dying back of
foliage and a general unthrifty appearance prior to
death of the plant, may be present on only one side
of the plant. Damage to roots and water conducting
vessels prevent plants from taking up enough water
from the soil. Many of these symptoms may be
caused or exacerbated by other soil diseases, nutrient
deficiencies or toxicities and a range of
environmental stresses, which may be operating
at the same time.
Plant may die during the dry summer months as
diseased root systems cannot supply adequate water
for plant survival.
Large trees may take years to die.
Collar rots and stem cankers. If the
bark is removed at ground level or from stem
cankers, underlying tissues are often brownish due
to the fungus attacking these areas.
Below ground symptoms.
On removing plants from soil, affected roots are
black or brown, rotted and outer areas may come
away leaving a thread-like vascular system.
Root system is reduced preventing uptake of water
and nutrients. Tip out pots to assess root health,
examine the collar region, wash roots from potting
medium and examine under a dissecting microscope
against a white background.
Impacts.
Phytophthora has been listed as a key threatening
process to native vegetation in parts of Australia,
whole ecosystems being affected. Many crops are
seriously affected.
Phytophthora spp. Many species cause damping-off of
seeds, seedlings, cuttings, also root,
collar and trunk rots of a wide range of
plants, nursery plants. A few species
attack fruit, leaves, etc. Nursery plants.
P. cinnamomi Wide range of plants (native, exotic)
P. cactorum Apples, pears, certain native plants
P. citricola Citrus, some genera of native plants
P. citrophthora Citrus, causing collar, crown, stem,
root and fruit rots, also some other
fruits, some vegetables, etc
P. cryptogea Apples, some genera of native plants,
gerbera
P. drechsleri Proteaceae, many genera of native
plants, nursery plants
P. megasperma Wide range of plants, eg lucerne,
Brassicas and other vegetables, etc
P. nicotianae Many genera native plants, stone fruit,
strawberry, tomato, nursery plants
P. palmivora Wide range of exotic species, durian,
P. infestans Late blight (Irish blight) of potatoes,
tomatoes and other Solanaceae occurs
in some states and some strains are
still a serious disease in some parts of
the world. Not discussed in this text.
Fig. 204. Some of the many symptoms
and diseases caused by Phytophthora spp.
Combinations of causes:
Fusarium oxysporum f.sp. zingiberi (Foz) and soft
rot bacterium (Erwinia chrysanthemi) have played a
part in the poor crop establishment of ginger in Qld.
Macrophomina phaseolina and root knot nematode
( Meloidogyne incognita) play a part in root disease
of chick pea.
Root colonization by arbuscular mycorrhiza fungi
also increases in the presence of Pseudomosas putida.
364 Fungal diseases - Examples of fungal diseases