PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
DISEASE
CYCLE
ALMOST ALL FUNGI WHICH INFECT PLANTS spend part of their lives on host
plants and part in the soil, or in or on plant debris in the soil.
HOST ONLY
These fungi spend all their vegetative life cycle on the host plant. Spores may land on
soil or plant debris where they remain until carried to a host where they can germinate,
grow and complete their life cycle, eg powdery mildews.
HOST AND HOST DEBRIS
These fungi grow parasitically on their hosts and continue to grow on the dead tissues
of their hosts as saprophytes to complete their life cycle. They can only grow on the
organic matter of their host, eg apple and pear scab.
HOST, HOST DEBRIS, OTHER DEBRIS AND SOIL
These fungi grow parasitically on their hosts but continue to grow on the dead tissues
of the host after it has died. These fungi also grow out of the host plant into the soil or
other decaying plant material, where they can grow and multiply as a saprophyte, eg
Sclerotinia, Sclerotium.
WHY IS KNOWLEDGE OF THE DISEASE CYCLE IMPORTANT?
Knowledge of the disease cycle is essential for the implementation of effective
control measures, including:
Timing of pesticide applications.
Application of preventative protectant treatments.
Whether leaves should be gathered and destroyed.
Whether seed from diseased plants should be used.
OVERWINTERING,
OVERSUMMERING
Resistant spore
Rhizomorphs
Sclerotia
CARRYING OVER THE FUNGUS FROM ONE SEASON TO THE NEXT
Fungal structures. Hyphae may grow into different structures which ensure
survival and spread of the fungus but which make control difficult. Dormant
stages are stimulated into growth when the roots of a susceptible host plant is in
close proximity and conditions are favourable, eg
– Thick-walled spores, eg chlamydospores, oospores of Phytophthora.
– Sclerotia consist of a mass of hyphae bunched together formed by some fungi,
eg Rhizoctonia, Sclerotium, Sclerotinia. Sclerotia are pale at first but they
darken as the hyphae on the outside dry out forming a hard skin that protects the
hyphae inside, enabling them to start growing again sometimes years later when
conditions are favourable. They are formed either inside or on outside of plants.
– Rhizomorphs are bundles of parallel hyphae (about the thickness of a shoelace)
formed by some fungi, eg Armillaria, that can grow through the soil to new
hosts. However, Armillaria does not appear to readily for m rhizomorphs in
Australia.
ON OR IN THE HOST PLANT
Bud scales
- Peach leaf curl of stone fruit
Twigs, branches
Trunks
- Brown rot of stone fruit, powdery mildews
- Wood rots
Roots - Phytophthora root rot, Rhizoctonia stem rot
ON OR IN PLANT DEBRIS
Leaves
Fruit
- Apple scab
- Brown rot of stone fruit
Trunks - Wood rot
Roots - Phytophthora root rot
SOIL
As mycelium, sclerotia or thick walled resting spores, eg Phytophthora, Rhizoctonia,
Fusarium and Verticillium.
SEED
Many fungal diseases are seedborne. They are either within or on the surface seed, eg
rust spores adhere to the outside of seed of infected plants.
‘OVERWINTERING’ IN SEVERAL WAYS
Some fungi can overwinter in several ways, eg
Phytophthora root rot on host plants, in plant debris, soil and as thick-walled spores.
Botrytis blight as spores on leaves of rooted cuttings, discarded plant debris, etc.
Fungal diseases - Biology, identification and classification 325