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Model
Farming / Wheat
Seed borne diseases of
wheat
Wheat is the most important grain crop, a staple food for more
than one-third of the world population. The major area of the
wheat in Pakistan lies in Punjab followed by the Sindh.
However, the yield per hectare is slightly higher in Sindh as
compared to Punjab. It ranks first as a cereal crop, being
followed by rice only in acreage and production, 5-6
irrigations (21 acre inches) are sufficient for average wheat
crop under optimum soil conditions.
The first irrigation should be given 3-4 weeks after sowing.
Out of all stages, crown root initiation (CRI) is the most
important stage for irrigation, in view of nutrient
availability and root development. Other critical stages are
tillering, heading, milky and dough 21, 50, 80 and 100 days
after sowing (DAS) respectively.
The quality seed of improved wheat varieties is also
considered as most important input for obtaining optimum
production. Only the good seed can give an economic benefit to
the wheat grower. It can maintain the quality of production,
which fetches higher value in the market. Therefore,
availability of healthy and pure seed should be confirmed,
otherwise most of the seedborne diseases of wheat could become
responsible for heavy losses.
Wheat crop is subjected to a number of diseases, which reduces
its overall production to a great extent, because wheat plants
in all stages of growth and in all natural environments are
subject to various mechanical, physiologic and biological
stresses that interfere with their normal growth and
development. Weather, toxicants, pollutants, insects, viruses,
fungi, nematodes, bacteria and weeds are primary hazards to
wheat production.
The actual number of wheat diseases is unknown, but nearly 200
have been reported from the wheat-growing countries in the
world. Over 100 infectious diseases caused by pathogens and
with weeds are parasitic and transmissible from plant to
plant. Amongst these, about 50 are generally seedborne.
Overall, all diseases are injurious in some areas, in some
years and on some plant parts. All parts of plant are subject
to disease and one or more diseases can occur on virtually
every plant and in every field. All draw attention because of
symptoms and generate great concern because of their effects
on the quality and quantity of plants, straw or grain.
In Pakistan, 50 diseases are reported to occur which have
great financial repercussions. The rusts are considered most
destructive, but the problem of seedborne diseases is also of
great importance and could not be neglected.
Smut, bunt, blight and root rot are some important seedborne
diseases, which perpetuates through seeds and causes
considerable loss to crop, under favourable conditions.
However, the causes, symptoms, preventive and curative control
measures of some most important seedborne diseases of wheat,
are being summarized.
Loose smut: This disease is caused by a fungus: Diseased ears
become black and contain black powder of smut spores instead
of grains. Initially, spores are covered by a white or silvery
(thin) membrane, which burst and spores are blown away by wind
leaving behind only naked rachis. Infected ears appear
somewhat earlier than the normal ones. the disease perpetuates
from infected grains look apparently normal.
Control: resistant varieties should be sown. To rogue out and
destroy the diseased ears, use the seed obtained from healthy
crop. In case healthy seed is not available, treat the seed in
the following ways:
Solar energy: Soak the seeds for 4-5 hours in ordinary water
in the month of May-June or July on a hot day, when
temperature becomes high, dry it in thin layer (or about 3-4
inches) from 8 to 12 noon and kept it for planting.
Tapke's method: Direct soaking of seed, in hot water, at 48C
for one hour and fifty minutes. Cool and dry before planting.
Hot water treatment: Initially, soak the seed in ordinary
water for about 4 hours, than dip in hot water at 52 C for 10
minutes. After that, cool and dry the seeds in shade, before
planting.
Anaerobic seed treatment: The seed may soak for 6 hours in
water, drain, place in air tight container for 30 hours at 28
C, dry and use for planting.
Chemical seed treatment: Treat the seed before sowing by using
seed dressing fungicides viz: Vitavax, Benlate, Baytan, Topsin
etc at the rate of 2 g/kg seed.
Flag smut or leaf smut: Another fungus is its cause. Long dark
streaks parallel to the veins appear on leaf blades and become
swollen, turn black, rupture and expose black powder with age.
the affected leaves wither, twist and drop with result in
death of whole plant. It may also appear on the sheaths and
occurs sometimes on the stem and very rarely on the ears.
Every shoot of the plant becomes infected very frequently. the
ears are replaced by a twisted mass of leaves, do not bear any
grain, but if the grain is formed, it is always much
shrivelled and useless. The disease is perpetuated through
seedborne or soil-borne spores, which can survive in the soil
upto a period of three years.
Control: Resistant varieties should be sown and planted early,
Using the wet method of sowing i.e. irrigating just after
sowing. Crop rotation Use healthy seed obtained form healthy
crop and sowing in healthy soil. Green manuring with guar
reduce the incidence. Rogus out and burn the infected plants.
Chemical seed treatment as recommended previously.
Old or complete bunt or stinking smut: The disease is also
caused by the fungi. Diseased ears are darker in colour and
infected grains, which are found to be filled with black
powder that of spores, on crushing give out an offensive smell
like that of rotten fish. The diseased plants are sometimes
stunted in growth. All the grains of an ear become infected
and get very much reduced in size. It is primarily seed-borne,
but is also soil-borne.
Control: Resistant varieties should be sown and use of healthy
seed in healthy soils, prefer early planting using the wet
method of sowing (irrigation just after sowing). Rogue out and
burn the infected ears before harvesting and/or threshing.
Treat the seed with seed dressing fungicides as recommended
for loose smut of wheat.
New or partial or karnal bunt: A fungus called Tilletia is
reported as its cause. When the grains ripen, the diseased
spikelets are more open, the outer glumes spread out giving
enough space and bunted grains become visible.
Only individual grains are infected and are also partially
affected, generally at the tips of grains. The prepared floor
is of dark colour and gives out fowl smell. Soil-borne spores
cause infection through wind at flowering stage.
Control: Cultivation of wheat varieties resistant to disease
or to lodging. Collection and burning of infected ear heads.
Avoiding threshing a diseased crop in the field. Time of
sowing and long range crop rotation can reduce incidence.
Avoiding heavy manuring and heavy irrigation.
Ear cockle or tundo: A nematode is reported as cause but
sometimes, a bacterium also found associated with nematode and
the disease also named as yellowing rot. Leaves become rolled,
wrinkled, turn yellow and die.
Stems are twisted, heads are distorted, yellowish, gummy and
sticky, spikelets are rotting, grains are replaced by hard,
small light brown to dark coloured nematode galls. Affected
plants are shorter and grains are very much lighter in weight.
Disease causing nematode can remain dormant in the grains
having galls over a period of more than 10 years and is
capable to cause.
Control: Use of healthy seed (free from galls). Separate the
galls by winnowing and sieving. The seeds can be freed from
galls by floating in ordinary water or 20 per cent common salt
solution, but salt should be removed / washed out with water,
and seed dried before sowing.
Foot rot, leaf spot or blight and black complex: Foot rot
disease makes its appearance in seedling stage, either the
seeds rot in the soil or seedlings show rotting of roots and
brown spots develop on the lower parts of the stem. The
affected seedlings ultimately die and result in thinning of
the crop.
In the adult crop the disease known as leaf spots because it
appears on the lower leaves forming oval to oblong spots.
These spots enlarge and ultimately affected leaves turn brown.
If the plants either do not develop grains or the grains
produced are shrivelled and sometimes have black tips, it
termed as black point complex. The disease causing fungus
perpetuates through seed borne or soil borne infection present
in the diseased seeds or plant debris lying in the field.
Control: Sowing healthy seed obtained from healthy crop.
Application of nitrogenous and phosphatic fertilizer or
farmyard manure in places where severe infections occur. When
the disease makes its appearance, apply immediately to the
crop. Carry out late sowing. Chemical seed treatment as
mentioned before.
Courtesy Daily Dawn, 10
December 2001 |
