Plant Protection 3 : Selected Ornamentals, Fruit and Vegetables

Annuals and
Herbaceous Perennials
PESTS AND DISEASES
Parasitic
Virus and virus-like diseases
Bacterial diseases
Bacterial leaf spots
Fungal diseases
Damping off
Downy mildews
Flower blights, flower rots
Fungal leaf spots
Powdery mildews
Root, stem and crown rots
Rusts
Wilts
Nematode diseases
Insects and allied pests
Aphids
Bugs
Caterpillars
European earwig
Greenhouse whitefly
Leafhoppers
Leafminers
Mealybugs
Mites
Thrips
Weevils
Snails and slugs
Vertebrate pests
Non-parasitic
Chemical injury
Environment
Genetic abnormalities
Nutrient deficiencies, toxicities
WEEDS
Annual and perennial herbaceous ornamental
plants are affected by the same types of diseases,
pests and weeds as vegetables.
PESTS AND DISEASES
Parasitic
VIRUS AND VIRUS-LIKE DISEASES
Host range: Many annuals are susceptible to at
least one or several virus diseases. Some viruses
only attack one species, eg carnation mottle virus,
while others, eg cucumber mosaic virus and tomato
spotted wilt virus (TSWV), infect many species.
Others, eg tobacco mosaic virus may have
different strains each of which has a different host
range. Impatiens necrotic spot virus (INSV) is
closely related to TSWV, has a wide host range but
has not been detected in Australia (Hill 1994).
With the introduction of western flower thrips
(Frankliniella occidentalis) both TSWV and INSV
may become more important diseases.
Symptoms: Symptoms vary with virus, cultivar,
growth stage and temperature and usually appear
one to several weeks after infection. Symptoms
may be more obvious during spring and autumn.
Leaves develop mosaic patterns, pale yellow,
green or brown ring-like markings, straw-coloured
spots, or be distorted (Fig. 1). Stems may develop
black streaks. Flowers may be distorted, green or
variegated (Fig. 2). Plants may be stunted, yellow,
have reduced flower quality and yield, but do not
usually die. Some grow reasonably if cared for,
but may deteriorate and are a source of infection.
Diagnostic tests are available for some species.
Overwintering: Infected hosts, including weeds.
Some may be generally seedborne, others may be
seedborne only on certain hosts. A few may
overwinter in infected crop debris.
Spread: All viruses are spread by vegetative
propagation and grafting from infected plants.
Some are also spread by sap-sucking insects, eg
aphids, leafhoppers, thrips. More than 20% of
virus diseases are seedborne but in a variable
percentage. Some spread also by mechanical
transmission of plant sap by foliage contact and
on hands, clothes and tools during plant handling,
eg pruning, flower cutting, and a few also by
pollen. Also by introduction of infected seedlings,
cuttings and some by debris from infected plants.
Conditions favouring: Repeated vegetative
propagation from infected plants. Weather may
favour an increase in vectors. At certain times of
the year, especially after hot dry weather, vectors
migrate from drying weeds and other hosts where
they breed and feed, to ornamentals and other hosts.
Control: As there is no cure for infected plants,
the aim is to prevent infection. To minimise
losses, plant virus-tested planting material and
practice strict hygiene. Insecticides may be
necessary to control vectors in commercial crops.
Cultural methods: Rotate crops if the virus
overwinters in plant debris, eg tobacco mosaic.
Sanitation: Before planting remove volunteer hosts
and weeds. After planting remove virus-infected
plants and weeds in the crop and nearby. If viruses
are sapborne, maintain strict hygiene during
propagation, pruning, handling and harvesting.
Wash hands with hot soapy water, sterilise tools
before and after handling particular groups of
plants. Work 'clean' areas prior to infected plants.
Handle new, less infected younger plants before
older, and possibly infected plants. Train staff in
how viruses are spread. See Nurseries N 51, N 55.
Biological control: It will be possible to control
some vectors biologically in the future.
Resistant varieties: Varieties with resistance to
virus diseases should be used where possible.
Plant quarantine: Isolate susceptible seed and
cutting beds and crops from infected hosts.
Disease-free planting material: Plant certified
virus-tested planting material (Fig. 3), otherwise
select vegetative propagation material and seeds
from symptomless plants. Yields of crops derived
from virus-tested planting material are greater than
from virus-infected crops. As most viruses
infecting annuals have insect vectors, virus-tested
planting material needs to be re-purchased regularly.
Infected seed may be treated with hot water.
Physical and mechanical methods: Grow where
practical in insect-proof greenhouses and control
insect vectors inside. If virus is soil-borne
(unusual), pasteurise soil.
Pesticides: Insecticides may be applied to control
vectors and reduce re-infection of virus-tested stock
and spread of virus within commercial crops,
seedbeds and especially greenhouses. Some vectors
have developed resistance to some insecticides.
A 4
ANNUALS AND HERBACEOUS PERENNIALS