Diseases of Vegetable, Ornamental and Spice Crops (2+1)

FUNGAL DISEASES - II

3. BLACK SCURF

Black scurf causes qualitative damage as it decreases the quality and market value of the tubers both for table as well as seed purpose.

Symptoms:

• Stem canker and blight phase and the scurf phase are the two distinct phases of the disease.
• In stem canker phase, the growing tips of sprouts show browning.
• Sunken, circular or elongated brown necrotic spots may also be observed on the sprouts.
• Severely affected sprouts are killed.
• Later, when shoots emerge, similar necrotic lesions are observed on the stem which may extend downwards and may completely girdle the stem.
• The most prominent symptom of black scurf is the presence of black crust on tubers due to the formation of sclerotia of the fungus (Plate 4).
• The pathogen produces a large number of sclerotia superficially on the surface of growing tubers.
• These sclerotia may be hard or spongy forming a black crust of scurf on the tuber surface.
• These are normally seated on the skin and do not cause any damage to the tuber inside.
• Black scurf phase is more common than stem canker in India.

Pathogen:

• The disease is caused by Rhizoctonia solani Kuhn.
• The perfect stage of the fungus is Thanatephorus cucumeris (Frank) Donk.
• The fungus is described in detail in bean diseases.

Disease cycle and epidemiology:

• The fungus perpetuates in the form of sclerotia on the seed tubers or in the soil.
• On germination in the presence of moisture, the sclerotia produce mycelium which may cause infection on the sprouts, stem or young tubers.
• The fungus can survive in soil for many years.
• The fungus although has a wide host range but shows some level of specificity depending on the host crops.
• The strains infecting members of graminicolous crops do not generally cause infections on solanaceous crops including potato and vice verse.
• Soil moisture and temperature are the two important factors affecting scurf development.
• Low temperature and high soil moisture are conducive for infection on sprouts and stems.
• This phase is not common on early autumn crop in North Indian plains as the temperature remains high.
• The more common phase is black scurf where the sclerotia formation is favoured at temperature 28^oC.
• The number of sclerotia on tubers varies with soil temperature and moisture.
• Black scurf on tubers is more common in crops grown in sandy to sandy loam and moderately wet soils.

Management:

• Planting of healthy and disease free tubers is helpful in reducing the incidence of the disease.
• Two to four years rotation with cereals, brassicas and legumes is helpful for the management of this disease.
• The increase in organic matter content of the soil helps in reducing the population of the fungus through enhanced activity of the antagonist microorganisms.
• The normal unsprouted tubers should be dipped in carbendazim (0.1%) or Monceren 25 SC (0.25%) for 10 minutes.
• The seed after treatment should be dried under shade by spreading on the floor of the cold store.

4. POTATO WART

The disease is quite serious in temperate regions of the world. In India, the disease was first reported by Ganguli and Paul (1953) from Darjeeling and is presently restricted to that area only due to restrict domestic quarantine on this disease.

Symptoms:

• All the underground parts except roots are attacked.
• The plant cells multiply rapidly at the infection site and produce hypertrophied tissue masses resulting into wart type symptoms on the tubers and beads like projections on the stems or stolons varying in size from small protuberances to large intricately branched structures (Plate-5 a &b ).
• Warts are normally soft, pulpy, spherical and similar in colour to tubers.
• The colour may change to green on exposure to sunlight.
• Sometimes whole tuber may be covered with warts.
• Sometimes secondary microorganisms invade the wart tissues causing their decomposition.

Pathogen:

• The disease is caused by fungus Synchytrium endobioticum (Schill.) Perc. which is holocarpic and endobiotic in nature.
• The fungus produces sporangia which release a large number of uniflagellate planospores and originate from different sori, these behave as planogametes and copulate to form zygospores which act as resting sporangia of the pathogen.

Disease cycle and epidemiology:

• The resting sporangia are released into the soil on disintegration of warty structures where they can remain dormant for several years.
• When soil becomes wet after irrigation or rainfall in the next spring, these resting sporangia germinate and cause fresh infections.
• The longevity and viability of resting spores are affected by soil type and crop rotation.
• Other factors like soil moisture and pH also affect the process of infection.
• For germination of resting spores and sporangia, a free film of water is necessary.
• When moisture conditions are available, infection can occur at temperatures ranging from 10-24o C and at soil pH ranging from 4 - 8.6.
• The optimum temperature for infection is 21o C and slightly acidic or neutral pH is preferred.

Management:

• Since the pathogen is soil borne, its eradication is very difficult.
• Strict domestic quarantine measures enforced in India have helped to restrict the disease to Darjeeling district only.
• Follow crop rotation with non-host crops for 8-10 years.
• Some potato lines viz., Rondo, Edina, Mira from European countries have shown considerable resistance to wart.
• Varieties developed by CPRI, Shimla like Kufri Jyoti, Kufri Sherpa, Kufri Jeevan and Kufri Muthun have demonstrated resistance to late blight as well as wart.