viral disease of field crop

1. Virus Epidemiology and
Ecology
SUBMITTED TO : DR. SHIKHA SHARMA
SUBMITTED BY : DIKSHA KUMARI

2. Ecology
 Ecology describes the factors influencing
the behaviour of a virus in a given
physical situation.
 These factors include host range,
pathogenesis, and host responses.
 It is a fundamental concept based on the
relational properties of the virus and is
not a property of the environment.
Epidemiology
 It is the study of the determinants,
dynamics, and distribution of viral
diseases in host populations.
 It includes a dimensional aspect of factors
determining the spread of virus into a
given situation.

3. Epidemiology of viruses in agriculture
 There are two stages in the infection of a crop :
 Primary infection
 Secondary spread
Primary Infection :-
 The perennating pathogen or its part that cause primary infection.
 The source of primary infections varies depending on whether the crop is grown on season
basis or weather there is overlapping cropping.
 If grown on seasonal basis, the virus must have an overwintering (or oversummering) host.
 This can be wild species, a vegetative organ such as a potato or a seed for a seed –
transmitted virus, or a vector in which the virus persists (e.g. a nematode).

4. Mode of transmission
 Presistant viruses
 Non – Presistant viruses
Aphid Settles on
Suitable Host Plant and
Begins Prolonged Feeding
Aphid Feeds for Extended
Period, Allowing Virus to
Circulate in Aphid's Body
Aphid Continues Feeding and
Transmits Persistent Viruses
such as Leutovirus
Non-persistent viruses by
aphids that do not colonise
the plant
Aphids feeds for very short-
times when seeking a host
But for sufficiently long to
infect the plant

5.  Primary infections with nematodes and fungal transmitted viruses reflect the distribution of
vector in soil. Because of ploughing and other cultural Practices, the infection patches are
often elongates.
 If there are no overwintering hosts in that region, viruses can be brought in by insect vectors
carried on high, level air currents from distant regions.
 Example : Macrosteles fascifrons may be blown each spring from an overwintering region
about 300 km north of Gulf of Mexico through the western United States and into the prairie
provinces of Canada. BYDV and cereal aphids follow similar routes.

6. Planting Date
Example the way time of sowing seed affects
virus incidence occurs with the winter wheat
crop in southern Alberta.
Late Planting
Reduces Virus
Incidence
Early Planting
Increases Virus
Incidence
Late Planting
Reduces Risk of
Overlap with
Maturing Crop
Early Planting
Overlaps
Maturing Crop
Reduced Risk of
Virus Spread from
Maturing Crop
Increased Risk of
Virus Spread from
Maturing Crop
Planting Date

7.  Crop Rotation :
 The kind of crop rotation practical may have a marked effect on the incidence of viruses that
can survive the winter in weeds or volunteer plants. Doncastor and Gregory (1948) showed that
it may take 5 and 6 years to eliminate volunteer potatoes from a field in which potatoes had
been grown.
 Soil Cultivation :
 Nematode and fungal vectors may be spread by movement of soil during cultivation.
 The physical movements of soil tillage and harvesting operations are the major means of spread
spread of Beet necrotic yellow vein virus (BNYVV).
 Nurseries and production fields as sources of infection :
 Nurseries especially where they have been used for some years, may act for themselves as
important sources of virus infection.

8. Secondary Spread
 Many of the factors associated with Primary infection control
the Secondary spread of viruses.
 The cultivation of a single crop, over a wide area continuously
for many year may lead to major epidemics of virus disease,
especially if an air borne vector is involved.
 Soil-borne vectors can also be important from this point of
view – for e.g., with Grape vine fan leaf virus in vine-yards,
where the vines are cropped for many years.
 Monocropping may also lead to a build up of crop debris and
the proliferation of weeds that became associated with the
particular crop.
 Weather conditions such as rainfall, temperature and wind
can play a major role in the secondary development of virus
disease in the field.

9. Emergence of New Viruses
 Movement of plant species between countries and continents has been carried out
with increasing frequency during last 2 centuries.
 Agriculture in India, North America and Australia is almost totally dependent on
introduced crop plants.
 Thus there has been ample opportunity for events such as that outlined for cacao to
occur.
 Many cases of rapid increase of begomoviruses in crops introduced into tropical
countries probably arise from indigenous viruses.

10. Major viral disease of Pulse crops

14. Major viral disease of field crops

19. Major viral disease of ornamental crops
Viral Disease Host Plants Vector Symptoms
Tobacco mosaic virus Many ornamental plants
Mechanical transmission (e.g.,
contaminated tools, hands)
Mosaic patterns on leaves,
stunted growth
Tomato spotted wilt virus Various flowering plants Thrips (Frankliniella spp.)
Yellowing, ring spots, wilting,
necrosis
Cucumber mosaic virus
Wide range of hosts including
ornamentals
Aphids (Aphidoidea spp.)
Mosaic patterns, stunting, leaf
distortion
Impatiens necrotic spot virus Impatiens, other ornamentals Thrips (Frankliniella spp.)
Necrotic spots, ring spots, leaf
distortion
Rose rosette virus Roses
Eriophyid mites (Phyllocoptes
fructiphilus)
Excessive thorniness, abnormal
leaf growth, discoloration
Begomovirus (e.g., Tomato yellow
leaf curl virus)
Various ornamentals including
tomatoes, petunias
Whiteflies (Bemisia spp.) Leaf curling, yellowing, stunting
Carnation mottle virus Carnations Aphids (Aphidoidea spp.)
Mottling, leaf distortion, reduced
flower quality
Lily symptomless virus Lilies
Aphids (Aphidoidea spp.), thrips
(Frankliniella spp.)
Symptomless carrier, reduced
vigor and flower quality in
infected plants
Tobacco rattle virus Various ornamental bulbs
Soil-inhabiting nematodes
(Trichodorus spp.,
Paratrichodorus spp.)
Stunting, yellowing, necrosis, ring
spots

20. Replication of ds DNA