PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
CONDITIONS
FAVOURING
EXPRESSION OF SYMPTOMS
Severity symptoms of individual plant viruses may vary with the crop variety, locality
and from one season to another.
Temperature. Viruses producing yellow or leaf roll symptoms are most severe in
the summer whereas mosaics or ringspots are most pronounced in the spring. New
growth produced during summer on mosaic- or ringspot-infected plants usually
shows only mild symptoms or are completely free from symptoms, eg apple mosaic
virus infection causes more pronounced symptoms during cool springs.
Masked virus diseases produce symptoms only under certain conditions of light
or temperature. Latent virus diseases do not show any symptoms in some
infected hosts, eg tobacco mosaic virus infection in African violet.
VEGETATIVE PROPAGATION
Because virus diseases in host plants are transmitted in vegetative propagative material
such as bulbs, corms, cuttings, root stock and scions, plants propagated by this means are
prone to carry virus diseases, eg carnations, daffodil, potato. Viral infection builds up
over generations eventually making some cultivars unproductive.
VECTORS AND AVAILABLE HOSTS
If spread by vectors, large populations of vectors and hosts favour infection.
HANDLING
Virus diseases spread by sap transmission are spread during handling, eg cucumber
mosaic virus is usually spread through a crop of cucumbers during the first picking.
ENVIRONMENT
Vegetative propagation
Vector
Handling
Light/temperature
SUSCEPTIBLE
HOST PLANT
VIRUS DISEASE
PRESENT IN AREA
Fig. 140. Virus disease triangle.
INTEGRATED DISEASE MANAGEMENT (IDM)
MAIN STEPS
You need to know how
the virus is spread to
carry out the right
control measures
at the right time
CONTROL METHODS
Legislation
Cultural methods
Sanitation
Biological
Resistant varieties
Plant quarantine
Disease-tested material
Physical/mechanical
Pesticides
Organic, BMP etc
X
1. Plan in advance an IDM program that fits your situation. Keep records of the crop,
eg source of planting material, planting/sowing dates, temperature, irrigation,
fertilizers and pesticides.
2. Crop/region. List the problems your crop/region gets. IDM programs are available
for different species of viruses on a range of crops.
3. Identification can be difficult. Be familiar with local virus diseases. Consult a
diagnostic service if necessary (page xiv). Be aware that virus symptoms may mimic
those of other diseases, eg nutrient or spray injury (page 275). Understand the life
cycle, spread, etc of the virus. Obtain Fact Sheets on your virus.
4. Monitoring. Know when, where, what and how to monitor. Growers of
susceptible crops should regularly monitor crops or indicator plants for symptoms
of virus. There are specific tests for some viruses. Vectors which spread the virus
can be monitored using sticky traps.
5. Threshold. This depends on the virus, the crop and the region and any legal
requirements. You may need to calculate your own threshold, domestic or
commercial for economic or aesthetic damage tolerance. There may be nil tolerance
for quarantine or other situations.
6. Action/control. For all practical purposes infected plants in the field cannot be
freed of virus after infection. Commercial growers can prevent initial infection by
selecting resistant/tolerant varieties and planting virus-tested material when ever
possible. Commercial growers can control insect vectors and weed hosts. Parent
stock must be tested regularly and kept virus-free and measures taken to prevent
subsequent spread. Home gardeners can purchase good quality stock/seed (usually
free of virus) and tolerate any subsequent virus or rogue out seriously affected plants.
7. Evaluation. Just how effective was virus control? Commercial growers should
test parent stock plants for virus every year. Recommend improvements if required.
Virus and virus-like diseases 283