Mustard

Disease Management

Disease Management

• The mustard crop is very often attacked by many diseases which results in serious losses in the yield.
• Diseases like Alternaria Blight and White rust cause about 10-70% damage.
• The major diseases which attack the cruciferae family are as follows:

Disease	Casual Organism
Alternaria Blight	Alternaria brassicae
Rusts	Cystopus candidus
Downy mildew	Peronospera brassicae
Powdery mildew	Erysiphe poligoni

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Alternaria Blight Alternaria brassicae

• Several spevies of Alternaria attack crucifers but Alternaria brassicae in particular attack mustard and rapeseed .
• When too many spots appear on the leaf the leaves dies prematurely, indirectly effecting the yield.

Symptoms

• Leaf spots appear as small,dark coloured areas which spread rapidly to form circular lesions upto 1 cm. In diameter.
• In humid weather, there may appear a bluish growth in the center of these lesions.
• Concentric rings may also form in the lesions.
• On the petioles, stems and seedpods the spots are linear.
• The conidia are cark,obclavate,muriform and are borne singely or in chains.
• In temperate countries these fungi are seed borne and cause shriveling of the seeds.

• Spores and mycellium in diseased plant debria serve as the means of perination.
• Conidia are abundantly formed in moist atmosphere.
• Conidia are disseminated by wind.
• In tropical conditions the seed borne innoculum is devitalized soon after the harvest of the seeds in summer.

Control

• The crop should be sprayed 3-4 times at 10 - 15 days interval with any of the following:
  1. Dithane M-45 (0.2%)
  2. Difolatan (0.2%)
  3. Daconil (0.2%)
• The spraying should be started at the flowering time of the crop.

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Black Rot of Crucifers Xanthomonas campestris

• t is verty widely distributed.
• It is genrally found in cabbage but also occurs in mustard and indian mustard.

Symptoms

• The first sign of the disease appears as chlorosis near the leaf margin.
• The chlorosis progresses towards the centre of the leaf blade.
• In the affected portions the viens and vienlets turn brown and finally black.
• In cases of severe infections, the leaves finally wither.
• The blackening may extend to the stems.
• Bacterial ooze may be seen from the plants that are infected.

Causal Organism:

• The pathogen responsible for black rot in crucifers is Xanthomonas campestris.
• It is an aerobic gram-negative bacterium.
• The colonies are yellow, convex, shinning and of a mucoid consistency.
• The pathogen lives on the plant residue in the soil or in the seed.
• The secondary spread of the disease is through irrigation water, wind, implements, tools etc.

Control:

• Seed treatment with mercuric chloride for half an hour is effective in eradicating seed borne infection.
• Hot water treatment of the seed at 50°C for 30 minutes is also effective.
• A three year rotation of the crop is desirable.
• Strict sanitary precautions should be maintained.

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Clubroot of Crucifers

• This is potentially the most dangerous diseases of the mustard family.
• Although the disease is not prevalent in India, it has no restrictions imposed by temperature and is widespread in as far south as the central part of Sri Lanka,
• Once the disease appears in the area, it becomes difficult to raise a profitable crop of crucifers in that area.
• Affected plants are usually a total loss.

Symptoms

• The disease may progress to a considerable extent without showing any above-ground symptoms.
• The earliest above-ground symptoms are unthrifty development of the plants and flagging of the leaves in hot sunny days, as if the plant is suffering from lack of water.
• When such plants are unrooted the hypertrophy of the root system can be seen.
• The infected roots enlarge relatively rapidly to form "clubs".


• Depending on the host species, the clubbing may be of several types, such as.

  a) Clubbing is complete on main and lateral roots as in cabbage.
  b) Clubs are present only on main roots, laterals are free.
  c) Clubs are present on laterals but main roots are free.
  d) Rootlets are free from clubs
  e) Clubs appear as tumors on the root as in radish, and dark decomposing spots are present on the root.

• These variations in the area of root systems affected are due to specific roots infected by the pathogen.
• There is no normal development of the cork cambium at the surface and, therefore, the tissues are easily invaded by secondary saprophytic organisms such as soft rot bacteria.
• This results in conversion of the root system into a black rooting mass of tissue.

The casual organism

• Plasmodiophora brassicae Wor. Was first fully described by Woronin in 1877.
• The life cycle of the pathogens starts from its resting spores lying in soil or diseases plant debris.
• The walls of the spores contain chitin but no cellulose.
• The spore is spherical and up to 4 microns in diameter.
• Germination of the spore can occur without any resting period.
• The spore swells to several times its original size and releases a single anteriorly biflagellate primary zoospore.
• Before infection of the host the zoospore becomes amoeboid.
• In contact with root hair or epidermal cells of the host roots the zoospore discards its flagella and bodily enters the root through a hole dissolved in the cell-wall.
• The hole is then closed by host action.
• The naked spore then forms a thallus in the lumen of the cell.
• The thallus (plasmodium) then cleaves into spherical zoosporangia or gametangia lying packed together in the host cell.
• This stage can reach within four days of infection.
• The fungus can penetrate young roots readily but normally it cannot invade roots with secondary thickening which are infected only through wounds.
• The resting spores lie free in the cell.
• With decay of the root tissues these are released into the soil.
• Although prolonged resting period is not required for germination of the resting spores, many of them remain viable for many years (upto 10 years).

Control

• The fungus is one of the most persistent soil invaders and crop rotation fails to control disease.
• The spore germinated poorly or not in alkaline media and hence liming of the soil has been considered as one of the methods for reducing disease intensity.
• However, this method has not proved effective in all types of soil.
• Infection can occur at soil temperatures from 9° to 30° C. Qpt. Temperature for infection is 17° to 25°C.
• Infection is limited by low soil moisture (below 45% of the water holding capacity).
• However, even a temporary rise in moisture, as just after irrigation, can facilitate infection.
• The disease is known to be reduced by low potassium and low phosphorus.
• The organism is disseminated locally by drainage water, farm implements, wind-borne soil, moving animals and seedlings raised in infested soil.
• The last is the most important source of dissemination of the fungus in an area.


• Emphasis should be placed on such preventive measures as,

  a) Eradication of cruciferous weeds such as wild mustard,
  b) Use of well drained disease free plots,
  c) Use of seedlings raised in disease free soil, and
  d) Very long rotations avoiding any type of cruciferous crop.

• In areas where this organism has established itself there are two factors to be considered,
• First, seedbeds are serving as source of dissemination of the pathogen and, secondly the fields where the crop is planted already had diseases crop in the past.
• Therefore, the first step is to ensure production of disease free seedlings by treating the seedbeds and then reducing the loss in the fields by adopting suitable cultural practices.
• The organism can be destroyed by soil fumigation with Vapam, mythyl bromide, etc.
• The seedbeds can be selected in locations where there was no disease and where crop refuse has not been dumped.
• If seedbed site does not strictly meet these conditions, then the beds should be fumigated.
• Raising the soil pH to 7 by adding lime (hydrated lime, not air slaked lime) to the soil can be tried and may prove helpful in some areas.
• A soil with pH 5 may require about 2.5 tons of lime.
• This application is done 6 weeks before planting.
• It needs repetition every time the crop is planted.
• Pentachloronitrobenzene (PCNB) formulations such as Terraclor, of a formulation having 75% active ingredient should be applied to one hectare.
• This soil appplication should be donea week before planting.
• To economise on the cost the fungicide can be used as water suspension for giving first irrigation to the transplanted seedlings.
• The suspension should contain 1.5 kg fungicide per 200 litres of water.
• In this method the disadvantage is that while infection of the main roots is checked the laterals which spread out to the soil zone where the fungicide has not reached may get infected if the fields is heavily infested.

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Downy Mildew Peronospora brassicae

• Downy mildew is very common among young plants about 2 - 6 inches high.
• It does not progress after plants have grown up.
• The damage to the plants is through general weakening of the growth in the early stages.
• In mustard, the disease attacks the inflorescence too.

Symptoms

The disease is characterized by the appearance of purplish brown spots on the underside of the leaves. These spots may remain small or may enlarge considerably. The upper surfaces of the lesions are tan to yellow.

• Downy growth usually appears on the lower surface of the lesions.
• The greatest deformities appear on the stem.
• The stem swellings may be small to several inches long.
• The stalks are abruptly bent.
• The young ovary may be very much elongated into a twisted body.
• The floral parts are atrophied.

Casual Organism

• Peronospora brassicae cause the downy mildew in crucifers.
• It is an obligate parasite.
• It is strictly intercellular with large finger shaped branched haustoria, which fill the cell cavity.
• Numerous erect branched conidiophores emerge thruogh the stomata on the under surface of the leaf.
• In floral organs only oospores are formed.
• The fungus has physiologic races.
• It penerates in the soil through oospores.
• Seeds with contaminated thrash may also carry the disease.
• Wild host plants also serve as source of primary inoculum.
• Conidias are responsible for the secondary spred of the disease.

Control

• Sanitation of the soil and crop rotation help in reducing the occurance of the disease.
• Weed host plants should be searched and destroyed.
• Spray fungicides at regular intervals.

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Powdery mildew

Erysiphe crucifearum

• In Haryana and Rajasthan, the disease appears in severe proportions causing considerate yield loss.

Symptoms

Symptoms appear as dirty white circular floury patches on either side of leaves. Under favourable environmental conditions entire leaves, stems and silique are affected. The fungus over summers through cleistothecia as ascospores or as mycelium on volunteer host plants. The disease is favoured by dry weather. It becomes severe under irrigated conditions.

Control

• The disease can be managed by application of sulphur dust at 30kg/ha or by spraying 2g wettable sulphur or 1ml dinocap or 1ml tridemorph per litre of water.

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White Blister Albugo candida

• This disease is unimportant in proportion to its widespread occurrence.
• Seed crops suffer heavy damage due to distortion of the floral parts.
• Its occurrence with downy mildew warrants prompt control measures.

Symptoms

The infection can be either local or systemic. All parts of the plant except the root may show the symptoms. In the case of local infection, isolated pustules develop on the leaves and stems. The pustules are raised, shiny white areas, measuring 1 to 2 mm. They appear together as large patches. When the young stems and inflorescence are infected the fungus becomes systemic in the tissues and deformities like hypertrophy occur.

• Blisters are also formed on the inflorescence and floral parts.
• The floral parts show swellings and distortions.
• The axis of the inflorescence and the floral parts may be enormously thickened.
• In cases where the systemic infection has taken place early, the entire plant may remain dwarfed and only small leaves may appear.
• The stem often becomes twisted giving a zig zag appearance.

Causal Organism:

• Albugo candida is also known as Cystopus candidus.
• The mycellium is intercellular.
• The mycellium produces knobed shaped haustaria in the host cells.
• A large number of haustaria are seen in a single cell.
• They loose their viability after some time.
• They germinate readily when immersed in water.
• The germination of the sporangium is maximum at 25° C.
• The pathogen perpetuates through the oospores lying in the diseased plant debris.
• Cool,moist weather favours the development of the disease.

Control

• The disease can be controlled by crop rotation and clean cultivation including use of clean seeds and destruction of weeds.

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