PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
Disease cycle
In Australia, many rust diseases appear to produce
only urediniospores (stage 11 - asexual repeating
stage), eg chrysanthemum rust (Puccinia
chrysanthemi) (Fig.195 below).
‘Overwintering’
As infections on perennial hosts, old
diseased crops left standing after harvest.
As thick walled spores (urediniospores and
teliospores) on infected crop debris, in the soil
and on seed and crop regrowth volunteers.
Many heteroecious rusts can survive from
season to season in countries with mild winters,
such as most parts of Australia, in the repeating
stage (urediniospores), which can lodge in bark
crevices, on buds or on plant debris until the
following season when the wind will blow them
onto new host leaves and other plant parts.
Poplar rust, stone fruit rust and the cereal rusts
are examples of heteroecious rusts which can
successfully ‘overwinter’ by this means.
In heteroecious rusts, the fungus can be
going through further spore stages on the
alternate host. In Australia these rusts seem to be
able to ‘overwinter’ as urediniopsores.
Oversummers on volunteer plants.
Spread
Spores are spread by wind and water splash from
infected host plants and infected host plant
debris to other susceptible host plants.
Spores may adhere to the surface of seed from
infected host plants and infected leaves. The
inoculum can survive in the soil and is often
spread via splashing water.
Overseas, barley stripe rust on susceptible
varieties may spread via unwashed clothes or
shoes worn in infected crops.
Movement of infected host plants.
Conditions favoring
Most problems occur in field plantings.
A film of water on leaves for a period of
about 4-5 hrs is necessary for downy mildew
spores to germinate and infect a plant but
symptoms may not develop until much later so
that plants which appear healthy and are
dispatched can subsequently develop rust.
For each species of rust or even each race
there is a particular regime of temperature and
humidity which determines infection, it is not
possible to generalize, eg
– Poplar rust – High humidity. high temperatures.
– Stem rust (cereals) - High humidity and moderate
temperatures.
– Bean rust - Cool, damp weather, fogs, mists.
– Sorghum rust - Warm humid weather favours leaf
infection, disease development, spore production.
Fig. 195. Disease cycle of chrysanthemum rust (Puccinia chrysanthemi).
Fungal diseases - Examples of fungal diseases 353