Biology of phytophthora blight and its characters

1. Dinesh K. Pancheshwar
Scientist
Plant Pathology
College of Agriculture, JNKVV,
Balaghat

2. Black Shank of Tobacco
Caused by fungus: Phytophthora nicotianae
synonyms: Phytophthora parasitica var. nicotianae,
Phytophthora nicotianae var. nicotianae

3. Black shank can be a devastating root and crown rot
disease of all types of tobacco, with losses in
individual fields reaching 100%. The disease was first
described from Indonesia in 1896, but has since
spread to most major tobacco growing areas
worldwide.
Tobacco is grown in 0.4 million hectares in India, accounting for
approximately 0.27% of the net cultivated area, and around 80% of tobacco
is grown in the states of Andhra Pradesh (44%), Gujarat (24%) and
Karnataka (15%).

4. Symptoms of black shank on tobacco with
characteristic yellowing and wilting of leaves.

5. Symptoms and Signs
Black shank affects tobacco plants at all the growth stages.
Disease begins on young seedlings or transplants once soil
temperatures rise above 20°C.
The most common symptom of the disease is a root and
crown rot, but the pathogen may also infect leaves if they
come in contact with infested soil during rainy periods.
Signs of the pathogen are infrequently observed on plant
stems and around leaf lesions, but hyphae often are readily
observed in pith tissues upon splitting of the stem.

6. Since other fungi, especially species of Fusarium, are often
present in these necrotic pith tissues, microscopic
observation of pith cells may help confirm the presence of
characteristic hyphae of Phytophthora.
In the field, diseased plants are often associated with wet
soil and losses may reach 100% in susceptible cultivars in
years favourable for disease development.

8. Symptoms of black shank on a young tobacco
transplant. Note the wilting and yellowing of lower
leaves.

9. Root and stem symptoms of black shank. Note the
total loss of the root system and the necrotic stem
lesion

10. Exterior and interior appearance of a stem lesion of
black shank.
The disking does
not extend beyond
the stem lesion
visible on the
outside of the
plant.

11. Leaf lesions caused by Phytophthora nicotianae on
a leaf of tobacco.

12. Hyphae of fungus in the pith of infected tobacco
stem.

13. Hyphae of fungus in parenchyma cell from the pith of
infected tobacco plant.
Irregular width
and lack of
septations
typical of
hyphae of P.
nicotianae.

14. Total loss of tobacco crop to black shank.

15. Symptoms of black shank on a young tobacco seedling
(left) and growth of Phytophthora nicotianae from an
infected seedling (right).

16. Early stages of the black shank disease on a tobacco
seedling. Note the wilting of leaves and the
development of a stem lesion below the soil level
(arrow).

17. Biology of Pathogen
Phytophthora nicotianae is a fungus-like organism in the
Kingdom Straminipila (sometimes written as Stramenopila),
Phylum Oomycota,
Class Oomycetes.
All stages of the organism are diploid; whereas, most true fungi are haploid.
The organism can grow vegetatively at temperatures between 5°C and 37°C, with
optimal growth occurring between 26°C and 32°C.
Extended exposure to temperatures above 40°C is lethal to the organism.
•

18. Hyphae
Hyphae are hyaline (colorless, transparent),
aseptate or coenocytic, and typically irregular in
width (3-11µm) with few to numerous hyphal
swellings. With age, hyphae acquire
pseudosepta and colonies become light yellow.

19. Note the absence of septations, irregular width,
and presence of hyphal swellings

20. Asexual reproduction
Sporangia Sporangia are ovoid, pear-shaped, or spherical,
and have very conspicuous papillae. Sizes of sporangia vary (18-70
x 14-39 µm) with isolate and the growth medium. Germination is
either direct by production of hyphae or indirect by the production
of five to 30 zoospores

21. A.Direct germination of sporangium by means of hyphae.
B.Indirect germination of sporangia by means of motile zoospores.

22. C. Encysted zoospores
germinating and infecting root
Zoospores attach to a host root, encyst, and directly penetrate the host
epidermis.
A. Motile zoospores
B. Zoospores on root surface

23. Asexual Phytophthora nicotianae chlamydospore
thick wall that aids in survival

24. Sexual reproduction
Fungus is heterothallic, requiring two mating types (A1 and
A2) for the production of oospores. Some single-strain
cultures may develop oospores with age.
Oospores are thick-walled, between 13 and 35 µm in
diameter, almost filling the spherical oogonium cavity
(between 15 and 64 µm in diameter).
The antheridium is amphigynous (the oogonium grows
through the antheridium at mating), spherical and remains
permanently attached to the oogonium. It functions to
contribute a nucleus during fertilization of the oogonium.

25. Antheridium is amphigynous and remains permanently
attached. The oospore is spherical and almost fills the
oogonial cavity.

26. Black Shank of Tobacco Disease Cycle

27. Black shank is a polycyclic disease. The number of cycles of
infection per growing season and the extent of spread
within a field are related to environmental conditions and
the level and type of resistance in the cultivar planted.
Chlamydospores in the soil and infested crop debris serve as
the primary inoculum that initiates epidemics.
Chlamydospores germinate in warm moist soil to produce
one or several germ tubes that either directly infect the
plant or produce a sporangium.

28. Chlamydospore germinating by the production of
hyphae

29. Chlamydospore (center) germinating to produce
sporangia (arrows)

30. Saturated soil stimulates the release of motile zoospores from sporangia, and
these are the primary infective propagules. Zoospores swim through
saturated soil pores or move in surface water over greater distances. Active
movement is directed toward nutrient gradients that occur around the root
tips and wounds on the host plant. Once the zoospore contacts the root
surface, it encysts (produces a cell wall), loses the flagella, then germinates to
form a germ tube that directly penetrates the host epidermis. The pathogen
continues to colonize the roots and stem via hyphae. Sporangia serve as
secondary inoculum, and can form within 24 hours of inoculation under
moderate temperature and moist soil conditions. Root and stem colonization
results in typical root rot and black shank symptoms. New chlamydospores
form on and in the root as disease progresses and can either germinate to
initiate new infections or serve as a survival structure until the next tobacco
crop is planted.

31. In surface water, zoospores
of Phytophthora nicotianae may
move long distances within a field
and to adjacent fields
Hyphae of fungus colonizing
tobacco roots

32. Sporangia on root surface
Chlamydospores in root
tissue

33. Phytophthora Blight of
Castor
Disease caused by fungus:
Phytophthora parasitica

34. •The disease is known to exist in India since 1909 when it
was reported from Pusa, Bihar.
•It generally appears during rainy season, i.e., about the
end of June and continues up to September.
•The disease is severe in low lying and badly drained fields
and it is in such areas that it destroys nearly 30-40%
seedlings.

35. Symptoms
The disease appears circular, dull green patch on both
the surface of the cotyledon leaves.
It later spreads and causes rotting.
The infection moves to stem and causes withering
and death of seedling.
In mature plants, the infection initially appears on
the young leaves and spreads to petiole and stem
causing black discoloration and severe defoliation.

36. •The leaf spots turn yellow and then brown and
concentric zones of lighter and darker brown colour
are formed.
•The outer border is not well defined and is greenish
above and brownish-grey below.

37. Dead seedling
Spot on older leaf
Leaf blight symptom

38. Biology of the Pathogen
The causal organism, Phytophthora colocasiae., consists of inter
and intra-cellular mycelium which develops inside the host tissue.
After a few days growth numerous branches emerge from the
lower epidermis of the leaf generally through stomata as
sporophores / sporangiophores either singly or in twos or threes.
A single colourless ovoid or roundish sporangium is borne at the
tip of sporophore.
A ripe sporangium liberates zoospores when put in water.
The number of zoospores varies from 5 to 45 in each sporangium.
The zoospores germinate readily by one or rarely two germ tubes.

39. These are formed freely during hot and dry months when
sporangia are scanty and retain the power of germination for many
months.
Oospores have also been produced on artificial cultural media.
The oospores remain viable for a long period and it is considered
possible that these might serve as a source of carrying the parasite
over from one crop to the next.
Secondary infection spreads rapidly through sporangia provided
the weather conditions are favourable.

40. The sporangia are easily disseminated by the wind and germinate
readily on the leaves producing zoospores which penetrate by
means of germ tubes either through stomata or directly and
produce diseased spots within 24 hours and the next crop of
sporangia appear in about two days.
The fungus also causes infection on young potato, tomato and
brinjal plants and seedlings of several garden annuals.
It also produces infection on the leaf of Sesamum.
These plants might serve as collateral or alternative hosts of the
fungus and help in the perpetuation of the disease.

41. The pathogen produces non-septate and hyaline
mycelium. Sporangiophores emerge through the
stomata on the lower surface singly or in groups.
They are unbranched and bear single celled, hyaline,
round or oval sporangia at the tip singly.
The sporangia germinate to produce abundant
zoospores. The fungus also produces
oospores and chlamydospores in adverse seasons.

42. Favourable Conditions
•Continuous rainy weather.
•Low temperature (20-25˚C).
•Low lying and ill drained soils.

43. Disease cycle
The pathogen remains in the soil as
chlamydospores and oospores which act as
primary source of infection. The fungus also
survives on other hosts like potato, tomato,
brinjal, sesamum etc. The secondary spread
takes place through wind borne sporangia.

44. Management
•Remove and destroy infected plant residues.
•Avoid low-lying and ill drained fields for sowing.
•Seed dressing with 4g Trichoderma viride
formulation can reduce disease incidence.
•Treat the seeds with thiram or captan at 4g or
metalaxyl at 3g per kg seed.
•Soil drenching with copper oxychloride @3g /lit or
metalaxyl 2g/lit is also useful.

45. IDENTIFICATION OF PHYTOPHTHORA

46. ASEXUAL PHASE
Sporangia papillation and caducity are important characters for identification, as are the
sporangiophore shape, and the presence/absence and shape of hyphal swellings and
chlamydospores.
Sporangia papillation and caducity
There may be a thickening at the apex of the sporangium. In caducous species, the sprorangia
break off readily. In noncaducous species, the sporangia are retained on the sporangiosphore at
maturity. The length of the pedicel remaining on caducous sporangia may also be helpful for
identification.

47. Papilation in Phytophthora

48. Sporangia proliferation
Sporangia may exhibit internal proliferation or external proliferation. External
proliferation can be extended or nested, or both.

49. Sporangiophore shape
The sporangiophore can be unbranched (simple) or branched. Branched
sporangiophores may be umbellate, simple sympodial, or compound
sympodial..
One species, Phytophthora litchii, has a specialized type of compound sympodial sporangiophore that appears
erected, very similar to those of downy mildews

50. Chlamydospores and hyphal swellings
Chlamydospores and hyphal swellings may be present or absent. When present, they can be intercalary (in
the middle of a hypha) or terminal (at the end of a hypha), and can vary in shape and distribution.

51. Sporangia shape
Sporangia vary widely in shape, but this character is not very helpful for
identification.

52. SEXUAL PHASE
Aspects of the sexual phase that are helpful for identification include whether the species is homothallic
or heterothallic, the shape of the gametangia (antheridia and oogonia), and the shape of the oospore.
Antheridium
The antheridium may be paragynous (next to the oogonial stalk) or amphigynous (around the oogonial
stalk).

53. Oogonium
The oogonium may be smooth or ornamented and may have a tapered base.

54. Homothallism vs. heterothallism
Homothallic species are self-fertile and non-outcrossing, with both sexual mating types in a single culture.
Heterothallic species have separate mating strains or types, so they are sterile without the interaction of different
thalli of opposite mating types.

55. Oospore
May be plerotic (no space between oospore wall and oogonium wall; the oospore
fills the entire oogonium)
or aplerotic (with space between the oospore and oogonium walls), or slightly
aplerotic.

56. COLONY MORPHOLOGY
The shape of the colonies produced on particular media may also be helpful for
species identification.

57. THANKS