plant virology

1. INTRODUCTION TO PLANT VIRUSES
Virus:
Virus is set of one or more nucleic acid template molecules normally
encased in coats of protein or lip-protein and organize its own
replication only within specific host cells
Virion:
A complete mature viral particle is called virion
Prion:
Smaller infectious protein particles encoded by a chromosomal gene
of the host cells
Viroids: Small, low-molecular-weight ribonucleic acids (RNAs) that
can infect plant cells, replicate themselves and cause disease. OR
Naked, single-stranded circular molecule of infectious RNA called
viroid
Virusoids:
The extra-small circular RNA component of some isometric RNA
viruses

2. Importance of Plant Viruses
 Plant viruses are viruses that multiply by using
the material synthesized by the host to
manufacture new virus particles and cause
infection
 Beneficial Aspects:
 Used as color breaking agent in ornamental
plants. Ornamental plants become attractive e.g.
Tulip mosaic virus
 Used as a vector in genetic engineering, CuMV
 Viruses used as a cross protection

3. Harmful Aspects
 Causes harmful diseases in humans, animals,
and plants
 Cause reduction in the growth of the plants.
 Reduction in yield and ultimately failure of the
crop
 Affect the quality and quantity of the production
 Make the plants more vulnerable to other
organisms (Insects) e.g. TMV, SCMV, CLCuV,
CTV

4. COTTON LEAF CURL DISEASE
CLCuV, ssDNA
Genus: Begomovirus, Family: Geminiviridae
 Occurrence & Importance:
 First reported in Nigeria in 1912
 In 1959 from Philippines
 In 1967 in Pakistan near Multan
 Epidemic form in 1993-95 in Pakistan
 Most serious disease of cotton
 Cause losses from 22.38-68.50%

5. Symptoms
 Thickening of veins
 Color of infected plants become dark
green
 Upward or downward curling of leaves
 Leaf enation
 Twisted leaf petioles
 Stunting of plants

6. Symptoms

7. Symptoms

10. Disease Cycle
 Not seed or soil-borne
 Over winter on alternate hosts (tomato,
tobacco, beans, lehli, okra, datura etc
 Transmitted by whitefly (Bemisia tabaci)
 30 Min. feeding on infected plants
 24 hours latent period
 30 Min. inoculation feeding period
 Retention period: through out life of
whitefly

12. Epidemiology
 Minimum temp. 27-28 0C
 Maximum temp. 38-40 0C
 Relative humidity: 56-60%
 Susceptible host on large area
 Less rainfall or no rainfall
 Abundance of whitefly population

13. Management
 Tolerant varieties like Allah Din One (A-
One), MNH-886 and FH-142
 Destruction of alternate hosts
 Uproot & burry first observed infected
plants
 Control insect vector (Imidacloprid, polo)
 Use of nutrients also help to reduce the
disease incidence

14. BANANA BUNCHY TOP DISEASE
(BBTV), ssDNA VIRUS
(Genus Babuvirus, Family Nanoviridae)
 Most important disease of banana crop
 Occur in all banana producing countries
(Fiji, Indonesia, Malaysia, Australia, Sri-
lanka, Pakistan, India)
 Infected plants produce no fruits

15. Symptoms
 Dark green streaks seen on petioles &
veins of new leaves looking against light
 Leaves arise in clusters
 Margins of leaves become chlorotic &
rolled upward
 Top of plant become narrow, upright &
closer appearing bunchy
 Plants show stunted growth
 Inflorescence fails to emerge

17. Disease cycle
 Vector: Black banana aphid (Pentalonia
nigronervosa)
 Also transmitted through propagative
materials (rhizomes, suckers, tissue-
cultured meristem)
 Aphid feed 17 hrs to become infective
 Maxi. Vector concentration at base of stem

18. Disease cycle

19. Epidemiology
 18-20 0C temperature
 41-84% Relative Humidity

20. Management
 Quarantine measures
 Use of virus free propagative material
 Locating new plantation away from old
 Destroying all volunteer banana plants
 Uprooting & burning of infected plants
 Control vector with insecticidal soap water spray
or insecticides (Prentox Diazinon AG500)
 Cultivate resistant varieties (Gross Michel)

21. Potato Leaf Roll Disease
(PLRV), ssRNA
Genus: Polerovirus, Family: Luteoviridae
 Occur world wide
 Most destructive virus of potato
 Yield losses more than 50%

22. Symptoms
 Appear mostly on lower leaves
 Leaflets roll upwards
 Margins & tips of leaves become yellow
 Leaves become leathery & brittle
 Plants stunted & have upright growth
 No. of tubers/plant & size is reduced
 Phloem necrosis occur in tubers

25. Disease cycle
 More than 10 species of aphid act as
vector (Myzus persicae)
 Infected tubers are primary source of
infection
 Aphid feed for several hrs (48-54 hrs)
 Alternate hosts are Datura stramonium

26. Epidemiology
 15-20 0C temperature
 40-65% Relative Humidity

27. Management
 Obtain certified seed
 Plant early in the season
 Pulled out sick plants as soon as located
 Detop plants in last week of December
 Apply systemic insecticides (aldicarb,
carbofuran)

28. Tobacco Mosaic Virus
(TMV) ss-RNA
Genus: Tobamovirus, Family: Virgaviridae
 First virus ever discovered in history
 Known to occur in all the tobacco growing
countries of the world
 55% reduction in yield has been reported
 Sub stranded produce fetch low price in
the market

29. Symptoms
 Leaves show mild veins clearing
 Mottle and mosaic pattern
 Light discoloration along the veins of
young leaves
 Wrinkling, crinkling, twisting margins and
narrowing of leaf lets
 Dwarfing of entire plant

31. Disease cycle and epidemiology
 Survive in herbaceous and woody plants
 Tomato, Potato, pepper grown indoor are source
of infection
 Virus present in plant debris, soil, cloths and
even manufactured tobacco cigarette and cigar
 Contact with the wounded plants
 No insect vector, Mechanical transmission
 Wide host range of 500 species of flowering
plants
 Temp. 28-32 ºC, RH 44-56%

32. Management
 Use clean and healthy seed in the nursery
 All workers should disinfect their hands
 Avoid chewing of tobacco in the field
 Rouging out infected plants
 Crop rotation with maize and wheat, avoiding
tomato and pepper
 Healthy leaves harvest first than infected one
 Field sanitation practices

33. Sugarcane Mosaic Virus Disease
(SCMV) ss-RNA
Family: Potyviridae Genus: Potyvirus
 First recorded in Java in 1892, in Punjab in
1926-27
 Occur in every sugarcane growing
country
 Estimated yield losses vary greatly upon
cultivar and environmental conditions

34. SYMPTOMS
 Contrasting shades of green, islands of normal
green or yellowish chlorotic areas on the leaf
blade
 leaf reddening or necrosis
 Chlorotic areas most evident at base of the leaf
 Chlorotic areas may also be present on the leaf
sheath, but rarely on the stalk
 Young, rapidly growing plants are more
susceptible to infection

36. Disease cycle and epidemiology
 There are three principal modes of spread
of SCMV:
 (1) by aphid vectors,
 (2) by infected seed cane and
 (3) by mechanical inoculation.
 Only aphid vectors and infected seed cane
are important in the field.
 Temp. 35-38 ºC, RH 68-78%

37. Management
 Resistant varieties (CPF 242, HSF 240, SPF
234) is the most effective method to control
 Use disease free seed canes
 Management practices targeting insect vectors
 Control of mosaic through heat treatment only
practical in quarantine situations
 Some plants recover from mosaic by nutritional
application

38. Citrus Tristeza Virus (CTV)
or Quick decline, ss-RNA
Family: Closteroviridae Genus: Closterovirus
 Biggest threats to citrus production in globe
 More than 30 virus, virus-like diseases of citrus
known in the world, of which CTV is the most
destructive.
 The first tristeza disaster was reported in 1930's
in Argentina, where 90% of the citrus was
planted on sour orange rootstock

39. Symptoms
 Yellowing of leaves
 Dieback of the phloem in the sour orange
rootstock below the bud union
 Rootstock being girdled
 'Slow decline' where trees decline in a period of
years
 Stunting where the virus does not kill the tree,
but the tree does not grow
 Deep pits are present under depressed areas of
the bark

41. Disease cycle
 Vectors of CTV are black or brown citrus aphid
(Toxoptera citricida)
 Due to the aphid introduction, CTV rate of
spread has increased dramatically
 Additionally, growers have aided in the spread of
the virus with the propagation of trees using
infected bud-wood.
 Temperature, 20-27 0C RH, 55-70%

42. Management
 Exclude any introduction of severe strains
 Need for mild strain protection in the future
 Strict control by quarantine must be continued
 Rootstocks that offer resistance to tristeza
decline include the citranges (C-35, Carrizo,
Troyer), Citrumelos (Swingle), mandarins

43. THANKS
Dr. Safdar Ali
Dr. Luqman Amrao