Table of Contents
Introdcution
Rapeseed-mustard is India’s most important and predominant source of edible oil and raw materials for industry. It pertains to the family Brassicaceae (Cruciferae) and genus Brassica.
A total of 43% of India’s mustard is produced in Rajasthan alone. Rapeseed and mustard are temperate climate crops, though they can also be cultivated in the tropics at higher elevations.
Mustard productivity is lowered by a number of biotic and abiotic factors.
There are five major abiotic constraints: drought, salt, heat, frost, and heavy metals stresses.
Additionally, biotic stresses such as Alternaria blight, white rust, downy mildew, powdery mildew, and Sclerotinia stem rot are also considered destructive to mustard crop production.
There are many fungi that cause yield loss in plants at every stage of growth from seedlings to harvest.
Mustard diseases identification and management
Alternaria Leaf Spot / Blight
Identification of Alternaria Leaf Spot or blight
This disease is caused by two species of Alternaria i.e. Alternaria brassicae and Alternaria brassicola.
The disease is first identified by the appearance of spots on leaves, stems and siliquae.
Alternaria brassicola produces dark sooty velour spots that are large and black in color, while A brassicae produces light grey spots.
Both pathogens cause symptoms in concentric rings, which are known as target effects. The black or brown coloured spots have circular, concentric rings visible inside them.
There is rapid spread of these lesions to other plant parts such as stems and siliques.
In many cases, the circular spots combine to form large patches, causing leaf blight.
Symptoms of severe infection include damping off of seedlings and wilting and rotting at the foot and root.
This infection not only damages fresh crops but also has the capability of damaging any stored crops.
Seeds with spores or mycelium underneath or on the seed coat are the main vectors/propagators/transporters of these pathogens.
Additionally, the spread of spores is also caused by water, wind, animals, and infected tools.
As a result of left over infected crops on the ground after harvest, the majority of infections occur.
Additionally, this phytopathogen can be found dormant on perennial plants, vulnerable weeds, and crop debris.
Management of Alternaria Leaf Spot or blight
Combining agricultural management practices with chemical protection is needed to manage this disease.
Agricultural practices include :
- Only use clean, healthy and certified seeds.
- Sow the seeds at the appropriate time.
- Nutrition for the plants should be maintained. Read about fertilizers here
- Making sure that the left over debris and infected weed to be completly removed from the field.
- Proper field sanitation should be practiced.
- Irrigation at the flowering and pod formation stages should be avoided.
- Crop rotation should not be practised with cruciferous vegetables. (Cruciferous vegetables are vegetables of the family Brassicaceae with many genera, species, and cultivars being raised for food production such as cauliflower, cabbage, kale, garden cress, bok choy, broccoli, Brussels sprouts, mustard plant and similar green leaf vegetables )
- The adoption of deep summer sowing and early sowing are also helpful in managing the crop.
Chemical protection :
Treat seeds with 2.5 grams of Mancozeb per kilogram of seed. Spray the plant with 2.5 grams of Mancozeb or Zineb per liter of water every 15 days.
White Rust
Identification of white rust
White rust is also called white blisters or stagheads. This disease is caused by the obligate pathogen Albugo candida.
In this disease, both local and systemic infections occur.
In cases of local infection, white or yellow pustules appear on the underside of leaves, and they later coalesce into larger patches.
Whereas, malformations and distortions of floral parts are caused by systemic infection causing hypertrophy and hyperplasia.
Inflorescences are completely replaced by a swollen sterile structure known as a stag head.
Pathogens are mainly transmitted through contaminated seeds and soil.
A cool and humid climate (12-14°C temperature & 60-80% relative humidity) makes this disease more likely to develop.
Soil or seeds contain the oospores that are responsible for primary infection.
Management of white rust
Management practices to combat this disease include using healthy, clean, and certified seeds, collecting and disposing of infected plant debris, following a long crop rotation with non-host crops, and seeding early in the season.
Dressing the seed with Metalaxyl (Apron 35SD)@6g/kg followed by a single spray with Metalaxyl 8% + Mancozeb 64% WP @1000g in 400 liters of water/acre can also be practiced to fight this disease.
*A Must read : weed herbicide resistance
Downy Mildew
Identification of downy mildew
This disease is caused by the fungus Peronospora parasitica, and it can damage plants at any stage of development, including seedlings, cotyledons, and harvest.
The symptoms appear on all aerial parts but are most prominent on leaves and inflorescences.
An irregular greyish white necrotic spot appears on the lower surface of the leaves.
A stag head formation is the most obvious symptom, as it is an infection of the inflorescence that causes hypertrophy of the peduncle or inflorescence.
There are no seeds or siliqua produced by the infected inflorescence.
In this case, the fungus is an obligate parasite that survives by forming oospores inside the host tissues and on its host plants.
Wind borne sporangia are responsible for secondary spread.
Management of Downy Mildew
The disease can be managed to some extent by practices like collection and destruction of infected plant debris, crop rotation with non-host crops and early sowing of seeds.
This disease can be managed chemically by treating seeds with Metalaxyl (Apron 35SD) @6g/kg seed followed by spraying with Metalaxyl (Ridomyl MZ) @0.2%.
Powdery Mildew
Identification of Powdery Mildew
Erysiphe cruciferarum is responsible for this disease. The disease affects all parts of the plant above the ground.
Both sides of the leaf surface develop a powdery white residue. This results in discolouring of the leaf or even premature falling of the leaf.
When environmental conditions are favorable for this fungus, a plant’s leaves, stems, and siliques are affected. Shriveled seeds are released by the disease affected siliques.
The fungus is ectophytic in nature, and it spreads on the leaf’s surface, sending haustoria into the epidermal cells.
Wind borne conidia spread the disease. This disease is favored by dry climate and gets severe in inundated conditions.
Management of Powdery Mildew
Agricultural practices like collection and destroying of infected plant debris is helpful in management of this disease.
For chemical control of this disease, spray the crop with wettable sulphur@0.2% or Dinocap@0.1% or tridemorph@0.1%.
Sclerotinia Stem Rot
Identification of Sclerotinia Stem Rot
Sclerotinia stem rot is also referred to as white blight, stem disease, or watery soft rot. Sclerotinia sclerotiorum is responsible for the disease.
The pathogen is necrotrophic and globally distributed. Infections are caused by airborne spores or sclerotia.
In the soil, sclerotia are spores that can survive for many years as dormant spores.
Germination of the spores and infection are enhanced by moist, humid conditions. Mycelium is formed by the germination of sclerotia in soil, which then infects the plants.
Two to three weeks after infection, the disease symptoms can be seen.
On branches, stems, and pods, the pathogen causes light brown faded patches. The patches gradually become grayish-white as they expand.
The infected stems split and open exposing black hard bodies called sclerotia. This form is the resting form of the fungus.
Management of Sclerotinia Stem Rot
Good agricultural practices such as sowing certified healthy disease-free seeds, and crop rotation can help prevent the occurrence of this disease.
Making sure that the field is sanitized properly and the infected left over debris should be removed from the field before sowing.
