Important diseases of cole crops and organic way of managing it

Dr Anita Puyam *

'Black rot of crucifers' 'Club root of crucifers' 'White rust' 'Downy mildew' 'Black leg'
All the pictures are downloaded from the internet and are used only for demonstration purpose.

Cole crops refer to several vegetables in the mustard family, including brussels sprouts, broccoli, cauliflower, cabbage, collards, kohlrabi and kale. All cole crops are cultivated varieties of the species Brassica oleracea. It requires cool and moist climate and prefer 60°-70°F temperatures for optimal growth and can withstand light frosts without injury. The prevailing cool-season in Manipur is quite favourable for these crops and it is grown all over the during October-January.

The main hurdles in production of these crops is mainly biotic stresses. Among them black rot of crucifers, white rust, black leg, club root of crucifers and downy mildew are important. Recently, the idea of organic farming has stormed the state and under such scenario, to manage these diseases without using chemical fungicides is a great challenge. However, this challenge needs to be undertaken considering the beneficial impact of organic farming.

The following article described about the symptoms and management of the disease using organic strategy.

Black rot of crucifers:

It is a bacterial disease caused by Xanthomonas campestris. On the leaves, the most distinct symptom is the V-shaped lesions that appears on the leaves starting from the edges towards the veins. Close inspection of infected leaves and stems may reveal black veins running through the infected tissue from which the disease gets its name.

The bacteria produce a sticky polysaccharide called xanthan that eventually plugs the vascular tissue inside the veins causing them to collapse and turn black, eventually turns yellow, wilts and dies. Systemically infected plants may produce chlorotic areas anywhere on the leaf. Severely infected plants shed their leaves and the roots may also be affected leading to ultimate death of the plant. The curds of infected cauliflower heads often become blackened.

Club root of crucifers

The soil borne fungus Plasmodiophora brassicae is the causal organism of the disease. Clubbing together of roots after infection by the pathogen is a peculiar symptom produced by the plant. It spreads extensively on the plant roots later the plants wilts and yellowing of leaves is noticed aboveground. If infection occurs in an early stage of growth, young plants will remain stunted and ultimately dies, and if infection occurs in a later stage, the plants fail to develop marketable heads or growth.

Below ground, the roots showed swollen and remains distorted. Due to distorted root structures, the plant's ability to take up water is highly reduced and besides, it fails to develop a protective outer layer rendering susceptible to invasion by soft rotting bacteria. Although clubroot has been found in soils exhibiting a wide pH range from 4.5-8.1, the disease is primarily associated with acid soils.

White rust:

It is fungal disease caused by Albugo candida. Milky-white or creamish-colored concentrically arranged pustules appears on the lower portion of the leaf. It is closed at first but later rupture and turns powdery-white. The pustules may appear on any above-ground part of the plant however more common on the underside of the leaves. The leaf area, opposite the white rust pustule, often bulges out slightly and appears mildly chlorotic but shows chlorophyll retention in the form of a green island as the leaf ages.

It also results in systemic infection resulting to hypertrophy and hyperplasia of the host tissue. It is more conspicuous in inflorescence producing a peculiar symptom called stagheads where these structures are vaguely reminiscent of the heads of stags with antlers, the antlers being represented by pedicels of flowers and siliques. Flowers and siliques become malformed and seeds abort in malformed siliques.

Downy mildew

All parts of the plants are affected however leaf symptoms are commonly observed. Irregular and dark black specks appear on the underside of the young leaves and on the upper side yellow spots similar in shape may be accompanied. Later the smaller lesions coalesce resulting in larger spots with sunken, paper tan-colored spots. Early infection on young plants can lead to stunting. On the underside of the leaf spots, a white-grey, downy growth can be observed.

On mature cabbage, dark sunken spots on the head or wrapper leaves can be seen or may develop a purplish tinge. It may also predispose the plant to soft rot bacteria which makes the disease further more severe causing rotting of tissue in the field or after harvest. In cauliflower and broccoli, curds/heads developed blackened areas. It also affects roots however it predominates on the upper part of the root and it may be distorted. Flowers and seed stalks especially mustard, are also infected.

Black leg:

The disease is a fungal disease caused by a fungal called Phoma lingam. Gray-green to ash grey lesions appear on the lower leaves. Ultimately, the infected tissue may dry out, crack and finally dies off. Under severe conditions, the lesions spreads to the stem commonly seen in cabbage and cauliflowers and pods in case of rapeseed and mustard. Basal stem lesions are most destructive causing lodging of the plant.

Organic management strategies: Monitoring and managing the diseases during the critical stage viz. transplanting, curd formation and at pod formation stage is necessary. Some management strategies are as follows:

Cultural practices:

Sanitation of the field by proper stubble management is recommended as the pathogen can survive in the crop debris for a long period of time. Specially for soil borne disease like downy mildew or debris borne diseases like black leg and black rot, two/three years of crop rotation with non-host crops like any other solanaceous crops will help to avoid the pathogen inoculum.

To inhibit the inoculum, soil drenching with 10-12 kg/ha bleaching powder (permitted for organic farming) is also recommended. But crop rotation is not effective for club root disease management as this pathogen can survive in the soil for several years

Seed treatment: Considering the nature of disease to be seed borne in nature (black rot, black leg), the first strategy is to free the seeds, from the bacterium before sowing. Pathogen-free seed should be used for nursery development. Certified seeds must be procured from the certified seed agency.

Or else to inhibit the seed borne inoculum, seed treatment with hot water at 50 0C for 15 min is recommended. This will increase seed vigour and germination, besides, inactivating pathogen inoculum. The inoculum may also remain in the soil as long as the disease debris is in the field.

Botanical extract: Anti-microbial plant extracts like neem, onion, garlic, turmeric etc. can be used to manage the disease. For example, onion extract 1% is found to be effective against black leg and white rust disease. Garlic 3% is found to be effective against downy mildew and white rust. Acacia arabica extract is found to be effective against black rot disease. All these extract can be used as a spray concentration during the critical stages of the crop.

Biological control agent: Especially for soil borne diseases like club root, white rust, downy mildew bio-control agent like Trichoderma harzianum @ 10g/litre and Pseudomonas fluorescenes @10g/litre can be used for soil treatment. Bio-control agent Bacillus subtilis and Penicillium citrinum is used for control of white rust and club root.

Growing of decoy crop: If radish is grown in advance of the susceptible crop, may be rapeseed/ mustard etc. will be helpful to control club root disease.

Raising soil pH of the field soil to 7 or above by adding lime for pH 5 @ 2.5t/acre and for pH 6.7-7.9 @10-20t/ha will also help in controlling club root disease as the disease is more severe in acidic soil.