DISEASES OF WHEAT
Black or stem rust
Causal Organism: Puccinia graminis
tritici
Symptoms
Symptoms are produced on almost all aerial parts
of
the wheat plant but are most common on stem, leaf sheathes and upper
and
lower leaf surfaces. Uredial pustules are oval to spindle shaped and dark orange-red (rust) in color. They
erupt
through the epidermis
of
the host and are surrounded by
tattered host tissue. The
pustules
are dusty in appearance due to the vast number of spores produce. Spores are
readily released when touched.
Teliospores are produced in the same pustule. The color of the pustule changed
from rust color to black as teliospore production progresses. If a large number
of pustules are produced, stems become weakened and lodge. Symptoms are very different on the alternate host. Pycnia (spermagonia)
produced on the upper leaf
surface appear
as
raised orange spots. Small amounts of honeydew that attracts
insects are produced in this structure. Aecia, produced on the lower
leaf surface, are yellow. They are bell-shaped and extend as far as 5 mm from the leaf surface. (Plate 1)
Epidemiology of wheat rust in
India
• K.C.Mehta (1940) – alternate hosts are non functional
– Scarcity of germinable teleuto material available from wheat crop in greater
Plain & even in hills due to exposure to the hottest part of the year (April- June)
– Viable teleutospores are likely to be formed on crops at higher altitudes. If
at all these germinated and infected barberis in monsoon such infection
would be of little consequence as there is hardly any crop at that time.
– Black rust had been found to break out in plains as early as December
– January i.e. 3-4 months prior to the earliest possible infection of barberis in
the
hills and these are none in the plains.
– There was no case on record, nor any evidence could be obtained during the studies of an outbreak if black rust starting from barberis.
– At higher altitude rust can not spread from plant to plant during winter due
to severe
cold.
– Therefore, it was obvious that outbreak
of
black rust originated largely from uredospores that oversummer in the hills.
Disease Cycle
Primary infection is mainly through barberry, i.e., Berberis vulgaris which play
a role in USA, Europe and Australia, where as in India they are not known to play
any role in the perpetuation of the fungus. The source of inoculum for black rust
comes from south, i.e., Nilgiri and Pulney hills. In
plains of North
India during
summer months the uredospores cannot survive because of the high temperatures. The possibilities of the
fungus surviving on ratoon tillers or self sown wheat plants, late and off season wheat crops and certain grasses growing in cool areas particularly in the foot hills of Himalayas in the North, the Nilgiris and Pulney hills in the South appear to be great.
The grasses, viz., Briza minor, Bromus patula, Brachipodium
sylvaticum and Avena
fatua, harbour the fungus in the off-season. It is believed that the fungus over
summers on the wheat plants and grasses in the hilly areas and spreads to the plains in the main wheat crop season. In the central Nepal, the wheat crop sown in
August and harvested in December, January becomes infected by P. graminis tritici
from October. This may be a source of inoculum for the main crop sown in the plains, which becomes infected from February each year.
Nagarajan
and Singh (1975)
Indian stem rust rules (ISR)
• ISR- I – The occurrence
of tropical
cyclone (Strong depression in Bay of Bengal in Arabian sea 65-81 o East,
10-15oNorth)
• ISR-II
–
A
persistent
high
pressure over South central
India.
• ISR-III – A deep trough extends into south India caused by the onward
movement of western disturbances.
• If ISR –I is not satisfied central India remains free from stem
& leaf rust epidemics
• If ISR II & III are satisfied than rust occurs late in crop season & with low intensity.
Brown or Leaf rust
Causal Organism: Puccinia recondita
Symptoms
The most common sites for symptoms to appear is on leaf blades and sheaths, however glumes and awns may occasionally become infected and exhibit symptoms. Uredia are
seen
as small brown blisters or pustules scattered on
host
tissue. The epidermis
covering the pustule
ruptures and the structure takes on a dusty appearance.
Brown spores are easily dislodged and may
cover clothing,
hands or implements. When the infection is severe leaves dry out and die. Since inoculum is
blown into a given area, symptoms are often seen on upper leaves first. As plants
mature, the brown urediospores are replaced by
black teliospores. Pustules containing these spores are black and shiny since the epidermis does not rupture.
Yield loss often occurs
as
a result of infection by Puccinia recondita f. sp. tritici.
Heavy infection which extends to the flag leaf results in
a shorter period of grain fill and small kernels. (Plate 2)
Pathogen
The fungus, Puccinia recondita, is heteroecious. The uredial and telial stages
appear on wheat and some other grasses and aecial and pycnial stages on species
of Thalictrum. In India, the role of Thalictrum javanicum and T. flavum as alternate
hosts has not been precisely
determined. In Russia, Isopyrum
fumaroides is known
to act
as a natural
alternate host. The uredospores are brown,
spherical
and minutely echinulate with 7-10 germ pores. Telia are rare, but when formed are
found
mostly on the lower surface of the leaf and do not rupture. Teleutospores are smooth, oblong, thick walled and brown with a rounded and a prominent thickened apex.
Disease Cycle
Yellow or stripe rust
Causal Organism: Puccinia striiformis
Symptoms
Mainly occur on leaves then leaf sheaths and stem. Bright yellow pustules
arranged in linear rows (Uredia) appear on leaves at early stage and at maturity, pustules are dull black lesions
(telia) and hence the name stripe rust. (Plate 3)
Pathogen
Uredosori appear as yellowish brown pustules chiefly on the leaves. Initially
the
green colour of the leaves fades into long
streaks and the uredia appear along them.
The uredospores are almost
around or oval in shape and bright orange in colour. The teliospores are bright organge to dark brown, two celled and flattened
at the top. Sterile
paraphysis is also present at the end of sorus.
Favourable conditions
The disease appear between the temperature range of 2-23oC with optimum
8-13oC and heavy
rainfall during November–December.
Temperatures of 10-15˚C
and
a relative humidity of 100% are optimal
for
spore germination, penetration and
production of new, wind-dispersed spores. The fungus is inhibited by temperatures
over 20˚C although strains tolerant of high temperatures do exist.
Disease Cycle
P. striiformis requires living green plant material in order to survive. The fungus survives during the winter as dormant mycelium or active sporulating lesions
on volunteers or early autumn-sown crops which provides an excellent source of
yellow rust over wintering inoculums during off season. Yellow rust within plant tissue can survive very low temperatures so once infected the fungus will usually survive in himalyan hills. In the spring, particularly in cool moist weather, the fungus
starts to grow and
produces
active sporulating lesions.
The complete cycle from infection to the production of new spores can take as little as 7 days du
spore
type, the basidiospore,
but no alternate host
has
been found.
Although the teliospores
seem
to have no function in the
disease cycle they may
contribute to the development
of new
races through sexual
recombination.
No alternate host for the fungus
is known nor
it is
known how fungus completes its
life cycle.
The fungus
overwinters in its
uredial stage in England
and other
countries. Uredospores and dormant mycelium survive
on
wild hosts and self sown
wheat crops
in
the hills.
Inoculum from this area through wind currents reaches foot hills (Gurdaspur, Ropar
in Punjab, Pantnagar in Tarai (Uttarkhand) by the end of December & early January
and gets established. and establishes foci of infection and from this region spreads in northern parts of the country
during January
& February. In North India,
uredospores or teleutospores do not survive during
the
hot summer
months and every
year fresh inoculum in the
form of uredospores comes
from the hills to the
plains and uredospores of P. striiformis is not a regular long distances spreader and
from the over wintering area in Hindukush, Sulaimanranges & Himalayas the
pathogen spreads upto a few hundred kilometers.
It may overwinter on volunteer wheat plants at an altitude of about 1500 to 1800
meters in the Himalayas. The uredospores germinate after a period of dormancy
and
form a source of inoculum for early sown wheat crop. Some weeds like Agropyron semicostatum, Bromus catharaticus, Bromus japonicus and Hordeum
murinum also serve as primary source of inoculum. Secondary infection is by
wind borne uredospores.
Management
ü Eradication of self sown wheat plants and weed hosts in hills.
ü Adjust time of sowing to avoid late sowing.
ü Balanced application of nitrogenous fertilizers.
ü Grow resistant varieties viz., WH 283, WH 542, WH
896, WH 912, WHD 943,
HD 2967, WH 1105, Raj
3765 and DPW 621-50.
ü Avoid cultivation of susceptible varieties PBW 343 and HD 2851.
ü Spray twice or thrice with
zineb @ 0.25% or
Mancozeb
@
0.25%
or propiconazole (Tilt) @ 0.1%, at 15 days interval.
Loose smut
Causal Organism: Ustilago tritici
Symptoms
It is very difficult to detect infected plants in the field until heading. At this
time, infected heads emerge earlier than normal
heads. The entire inflorescence is commonly
affected and appears as
a mass of olive-black spores, initially
covered by a thin gray membrane. Once the membrane
ruptures, the head appears
black
powdery mass of teliospores. Spores
are
dislodged, leaving only the rachis intact. In some cases remnants of glumes and awns may be present on the exposed rachis. Smutted heads are shorter than healthy heads due to reduction in the length of the
rachis and peduncle. All or a portion of the heads on an infected plant may exhibit
these symptoms. Other symptoms may be detected if the field is examined closely. Chlorotic streaks may also be visible on the flag
leaves. (Plate 4)
Pathogen
Teliospres of the fungus are pale, olive brown, spherical to oval
in shape.
These smut spores germinate and produce promycelium or sporidium.
The
promycelial cells fuse and give rise to germ
tubes that enter the ovary through the stigma and become
established in the embryo remaining
dormant until seed germination.
Disease Cycle
It is internally seed borne and is systemic. The
fungus is carried over in the seed as dormant mycelium. When the planted seed germinates the mycelium
becomes active. It grows along with the plant at the juncture of earhead formation
and the mycelium reaches the ovaries and transforms the ovaries into a mass of
black smut spores. Teliospores (smut spores) spread occurs through wind. The
sporidia infect the healthy flowers. The mycelium enters the ovary and remains in the seed as dormant mycelium.
Management
ü Remove the disease plants immediately with precaution.
ü Grow resistant varieties WH 896, WH 912 and WHD 943.
ü Hot water treatment: Soak the seed in cold water for 4 h and then immerse the seed in hot water at a temperature of 132 0F or 520C for about 10 minutes. Dry the seed in shade before
sowing.
ü Solar seed treatments: Soak the seed in water for 4 hours (8 AM to 12 Noon) and expose the seed to the hot sun for 4 to 5 hours (from 12 Noon to 5 PM) on cement or rocky surface.
This can be practiced in the areas where the
summer temperatures are high (42-440C) during May-June
ü Seed treatment with systemic
chemicals like Bavistin
@
0.2% or tebuconazole (Raxil DS @0.1%) or Vitavax @0.2% or Benlate @ 0.2%
Flag smut
Causal Organism: Urocystis
agropyri
Symptoms
Masses of black teliospores are produced in narrow
strips just beneath the epidermis of leaves, leaf sheaths and occasionally the culms. Diseased plants often are stunted, tiller profusely and the spikes may not emerge. A severe infection usually
induces the leaves to roll, producing an onion-type leaf appearance. The epidermis of older diseased plants tends to shred, releasing the teliospores (Plate
5).
Flag smut of wheat
occurs in the leaf blades,
forming black stripes running
lengthwise. In the early stages, these stripes are somewhat lighter than the green
color of the normal leaf; later they become lead-colored and finally
black because of
the
presence of the dark-colored teliospores produced by the fungus. They
are commonly more noticeable in the upper leaves, and may be seen even before
jointing is apparent in the plants. The stems (culms) often show these black stripes also. Infected plants usually
are
more or less dwarfed. The leaves and sheaths
become twisted in some cases, and the infected culms rarely
head out or produce seed.
Where heads do appear on infected culms, the black stripes may be present on the glumes at the base of the head and usually are present on the culms just
below the head.
Pathogen
Aggregated spore balls, consisting
1-6 bright globose,
brown smoth walled
spores surrounded by a layer of flat sterile cells.
Favourable conditions
Temperature of 18-24˚C, relative humidity 65% and above. Low soil moisture
and cool soil temperatures.
Hosts / Distribution
Bread wheat is the primary hosts of flag smut fungi,
and the isolates attacking bread wheat tend to do so exclusively. There are few reports
of
flag smut on durum
wheats and triticales. The disease is found in most winter wheat areas and in cool, fall-sown spring wheat areas.
Mode of Spread
and
Survival
Externally seed and soil borne. Smut spores are viable for more than 10
years.
Management
ü Grow resistant cvs. WH
283, WH 896, WH 912 and WHD 943.
ü Seed treatment with tebuconazole @ 1.0 g
Kg−1, or carboxin and
carbendazim @
2.0
and 2.5 g Kg−1 seed reduced disease to the great extent.
ü
Early or late sowing were found effective in management of the disease.
Karnal bunt
Causal Organism:
Neovossia indica
Symptoms
Symptoms of Karnal bunt are often difficult to distinguish in the field due to
the
fact that incidence of infected kernels on a given head is low. There may be
some
spreading of the glumes due to sorus production but it is not as extensive as
that
observed with common bunt. Symptoms are most readily detected on seed
after harvest. The black sorus, containing dusty spores is evident on part or all of
the
seed, commonly occurring along the crease. Heavily infected seed is fragile and the pericarp ruptures
easily. The foul, fishy
odour associated with common bunt is
also
found with
karnal bunt. The odour is
caused by
the production of
trimethylamine by
the
fungus. Seed that is not extensively infected may
germinate and produce healthy plants. (Plate
6)
Disease Cycle
Karnal bunt is a seed or soil borne, floral infecting disease inoculum (teliospores) on
or
near the soil surface germinates,
producing sporidia, which are
carried by wind to the
floral structures. These sporidia in turn germinate and penetrate the glumes, rachis, or
the
ovary itself. The fungus
enters the newly formed
kernel
and develops in the intercellular
space between the endosperm and seed
coat. The degree of disease
establishment and
development depends
on environmental conditions from spike emergence through grain filling.
Management
ü Grow resistant varieties viz., WH 896, WH 912 and PBW 502
ü One spray of propiconazole (Tilt 25EC@ 0.1%) should be given before or at earhead emergence stage.
ü
Integration of one spray of propiconazole with one spray of bioagent fungus,
Trichoderma viride (0.4% suspension) manage the disease effectively.
ü Chemical control should be adopted mostly in seed production plots. Seed treatment with Thiram @2g /kg seed for
seed borne inoclum.
Hill bunt
- Rough spored bunt
Causal Organism:
Tilletia caries
Symptoms
The fungus attacks seedling of 8-10 days old and become systemic and
grows along the tip of shoot. At the time
of
flowering
hyphae concentrate in the inflorescence and spikelets and transforming the ovary into smut sorus of dark green color with masses of chlamydospores / teliospores. The diseased plants
mature earlier
and
all the spikelets are affected. (Plate
7)
Pathogen
Reticulate, globose
and rough walled. No
resting period. Germinate to produce primary spordia which unite to
form
‘H’ shaped structure.
Disease Cycle
The spores on the
seed surface germinate
along
with the seed.
Each produces a short fungal
thread
terminating in a cluster of
elongated cells and then
produces secondary spores
which infect the coleoptiles of
the young
seedlings before the emergence of the first true leaves. The mycelium grows internally
within the shoot
infecting the developing
ear.
It is externally seed borne and
affected plants develop
apparently
normally
until the ear emerges when it can be seen that grain sites have been replaced by bunt balls.In damp soil, spores usually
germinate and then, in the absence of the host plant,
die.
However, in dry seasons, they may
survive in the soil (especially if they are protected within the glumes
of
shed ears) from the harvesting of one crop to the
sowing of the next.
Wind blown spores, particularly from late-harvested crops, can contaminate neighboring fields which may present bare soil ready for planting the
next crop.
Hill bunt - Smooth spored bunt
Causal Organism: Tilletia foetida
Symptoms
The fungus attacks the seedling of 8-10 days old and become systemic and grows along the tip of the shoot. At the time of flowering, hyphae concentrate in the inflorescence and spikelets and transforming the ovary into smut sorus of dark green colour
with masses of chlamydospores. The diseased plants mature earlier
and
the spikelets are affected. (Plate 8)
Pathogen
Reticulate, Teliospores globuse and smooth walled. No resting period.
Germinate to produce primary spordia which unite to
form
‘H’ shaped structure.
Favourable conditions
Temperature of 18-20˚C. High soil moisture.
Disease Cycle
Bunt balls that break during harvest contaminate
seed and
soil. When infested seeds are sown bunt spores germinate, and the
fungus infects the growing
point of the wheat seedling. Following infection of the young seedling the
fungus
grows within the plant, generally without
producing symptoms, until the head
develops.
When new seed begins to develop in the head, the fungus replaces the tissues of the developing seed with its own spores. Bunt can survive in soil for at least a year
and
for many years on seed. The fungus is externally seed borne.
Management
ü Treat the seeds with carboxin or
carbendazim @ 2g/kg.
ü Grow the crop during high temperature period.
ü Adopt shallow sowing.
ü Grow resistant varieties like HS 507.
Powdery mildew
Causal Organism: Erysiphe graminis
var. tritici
Symptoms
Greyish white powdery growth appears on the leaf, sheath, stem and floral parts. Powdery growth later
become black
and
cause drying of leaves and other
parts. (Plate 9)
Pathogen
Fungus
produces
septate, superficial, hyaline mycelium on leaf surface with
short conidiophores. The conidia are elliptical, hyaline, single celled, thin walled and
produced in chains. Dark globose cleistothecia containing 9-30 asci develop with oblong, hyaline and thinwalled ascospores.
Favourable conditions
The optimum temperature ranging from 15 to 200C, relative humidity 24 to
75% favours infection.
Disease Cycle
The primary
inoculum is spores on volunteer wheat or spores within cleistothecia. The pathogen perpetuate in the soil or in the disease debris in the form of perithecia. The large number of conidia produced on the diseased leaves
serve
as secondary
inoculum that
causes the
infection on
the
healthy leaves.
Primary infection is by the ascospores and
secondary spread through airborne
conidia.
Management
ü Resistance source WH 896, WH 912, WH 542;
ü Balanced fertility (avoid high nitrogen).
ü
Burning of crop debris
and
deep summer ploughing helps in eliminating the inoculum.
ü Crop rotation helps in minimizing the disease incidence.
ü
Foliar spray of sulfex (0.2%) at the appearance of the disease and repeat after
10-15 days.
ü One spray of propiconazole (Tilt 25 EC@
0.1
%) on disease appearance (which
usually occurs
during early March
in
northern plains)
is highly
effective.
Tundu
Causal Organism:
Anguina tritici (Nematode)
+ Clavibacter tritici
Symptoms
The tundu disease is characterized by the twisting of the stem, distortion of the ear head and rotting of the spikelets with a profuse oozing of yellow
liquid from the affected tissues. The ooze contains masses of bacterial
cells. The nematode
alone causes wrinkling, twisting
and
various other distortion of the leaves, stem and produce small round galls on the leaves. The infected plants are shorter and thicker
than
healthy plants. In the distorted earheads dark galls are found in place of kernels.
When the
bacterium
is
associated with
the nematode,
the disease symptoms are intensified at the flowering stage and yellow ear rot sets in due to
combined
action
of
the nematode and bacterium.
The earhead becomes chaffy and the kernels are replaced by dark nematode galls which also contain the bacterium.
The infected
plants
produce more tillers than the
healthy ones. Another interesting feature is the early emergence of ears in the nematode infected plants which is
about 30 to 40 days earlier
than
the healthy ones. (Plate 10)
Favourable condtions
These nematodes
are spread through seed galls in the seed lots during
planting and harvesting. Wet weather favours larval movement and infestation. The
nematode invades the
crown and
basal
stem area,
finally penetrating floral primordia. This leads to formation of nematode galls in ear
heads.
Disease Cycle
The disease starts from the seeds contaminated with the nematode galls.
When such contaminated seeds are
sown in the field, they absorb moisture from the soil and the larvae (juveniles) escape from the galls and climb upon the young
wheat plants. At the time of flowering, the nematodes enter the floral parts and form galls in the ovaries. When once the
nematode is inside the tissues of the ovary, the bacterium becomes active and causes rotting. The yellow
ooze coming out of the
rotting earhead provides the inoculum for the secondary spread of the disease which is favoured by wind and rain. The nematode probably
functions as a vector
transporting the bacterium to otherwise inaccessible meristematic regions of the host.
The
nematodes secrete
some
substances
in
the presence of
the host
bacterium which can remain viable for
atleast 5 years in the galls of A. tritici. The
nematode galls are reported to remain in the soil for 20 years or more and the
bacterium can also survive for
the
same period inside the nematode gall.
Management
ü Sow gall free seeds. Separate the galls from the seed by floating in 2 to 5 per cent brine (NaCl) solution.
The galls, which float on the surface, can be easily
separated and destroyed away from the fields. The seed thus
cleaned should be washed with fresh water and used for sowing.
ü Wheat, barley or oat should not be sown in the infested soil.
ü Spray the crop with streptocycline @
1g
in 10 liters of water.
ü Apply Temik 10G @ 20kg/ha or Aldicarb sulphon @ 2kg ai or Carbofuron
Black Point
Causal Organism:
Alternaria alternata, Alternaria spp
Symptoms
Disease causes blackening of embryonic region of the seed (black point), discoloration of area beyond the embryonic
region (black discoloration). (Plate 11)
Favourable conditions
The warm and humid weather at grain filling or near maturity favors this disease.
Management
ü
This disease is of minor importance. Only
when
the disease percentage is
high, it
causes
concern to the trader
and
the consumer. The discolored seeds are
mostly shrivelled and they are separated out during processing.
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