Diseases of Vegetable, Ornamental and Spice Crops

FUNGAL DISEASES (PART – I)

1. CLUB ROOT

This disease is also known as finger and toe disease and was first discovered by Woronin in Russia. This is a very serious disease of cauliflower, cabbage, broccoli and Chinese cabbage and in India it is common only in Darjeeling.

Symptoms:

• The disease initiates from the roots and first symptom on the above ground plant parts appear as epinasty on hot sunny days followed by yellowing of leaves and reduction in the vigour of the plants.
• The underground roots of such plants are hypertrophied forming clubs of different shape and sizes depending upon the infection sites.
• The malformation generally occurs in primary, secondary and tertiary roots and sometimes extends to hypocotyls.
• Infected plants wilt in direct sunlight.
• The clubs are easily invaded by the secondary saprophytic organisms, causing rotting of the roots.

Pathogen:

• The fungus responsible for this disease is Plasmodiophora brassicae Wor.
• The incitant is biotroph, endocellular and plasmodial fungus.
• The life cycle involves production of two different plasmodial phases i.e. sporangial plasmodium which gives rise to thin walled sporangia and sporogenic plasmodium which produces thick walled, hyaline and spherical resting spores.
• The planospores are anteriorly biflagellated and uninucleated.
• Plasmodia and secondary sporangia may be seen in root hairs and epidermal cells.
• The resting sporangia are spherical, uninucleate and minutely spiny.

Disease cycle and epidemiology:

• The fungus can survive in soil for several years in the form of resting spores which are disseminated by water and other cultural operations.
• Seedlings raised in infested soil also help in spreading of the pathogen to uninfested fields.
• The fungus can also perennates on wild cruciferous plants.
• The pathogen enters the host through the root hairs and injured roots.
• After entry, the invaded host cells enlarge and the fungus multiplies to develop into plasmodium.
• As a result of cell division, the fungal spores are distributed to the newly formed cells which give the club shaped appearance to the root tissues.
• Such infected roots decay and release the spores in the soil.
• Soil temperature ranging from 9-30^o C, moisture and pH play an important role in disease development and its progress.
• Cool and wet climate along with acidic soils (pH 5.0- 6.3) are most favourable for development of the disease.

Management:

• Destruction of diseased debris and wild hosts from in and around the field helps in reduction of primary inoculum.
• Amend the infected soil with lime to increase the soil pH to 7.2 and immediate irrigation is must.
• Use disease free seedlings for planting in healthy areas.
• Soil solarization has also been found effective in reducing the inoculum.
• Use resistant cultivars if available.
• Soil drenching or dipping of the seedlings in benomyl (0.05%) or carbendazim (0.05%) solution for 15-20 minutes before transplanting helps in reducing the disease incidence.
• Besides these, combination of calcium cyanamide with nitrogen and boron completely inhibits clubbing and root hair invasion by suppressing the resting spore germination.
• Biocontrol agents like Pseudomonas fluorescens and Streptomyces graminofaciens helps in reducing size and growth of club as well as root infection.