Diseases of fruit plantation medicinal and aromatic crops (2+1)

Powdery mildew

Casual Organism – Sphaerotheca fragariae
Symptoms

 
• Whitish powdery growth appear on upper surface of the leaf and young stem. Defoliation fruit cracking reduction in fruit size and fruit yield are the common effects of the disease.
• Occasionally a powdery or surface mildew causes some damage to the plants.
  

Etiology

• Mycelium of S. fragariae is white, septate, ectophytic and sends globose haustoria into the epidermal cells of the host.
• Conidiophores are short and erect. Conidia are one celled, oblong, minutely verrucose with many large fat globules.
• Cleistothecia are formed towards the end of the season on leaves, petals, stems and thorns.
• Cleistothecia are characterized with simple myceloid appendages.
• Each ascus contains 8 ascospores in it.
Mode of spread and survival

• The fungus overwinters in the form of mycelium in dormant buds and shoots which are not entirely killed.
• Either conidia or ascospores serve as primary inoculum. Secondary spread is through wind borne conidia.
Epidemiology

• Infection occurs when air is saturated with moisture and the atmospheric temperature is around 200C. Optimum conidial germination occurs at 90 - 97 per cent relative humidity and at a temperature range between 17 to24⁰c.
• Spore production is maximum at 24 to 28⁰c.

Disease cycle

• Infected leaves, buds and twigs bear clestothecia, in favourable conditions which produce ascus (sexual fruiting body) and in which ascospores are produced.
• Ascus liberates ascospores which are blown by the air current, lands on the air currents, the host surface and cause powdery mildew infection.
• White powdery growth comprises of numerous oidia.
• Oidia releases barrel shaped conidia which cause infection and continue asexually life cycle.
• Under adverse climatic conditions the fungus enters the sexual stage and produces antheridium and ascogonium.
• In gametangial contact type of reproduction the ascogonium and antheridium come in close contact with each other fallowed by plasmogamy, karyogamy, mitosis, meiosis and the ascospore formation take place.

Management

• The diseased and fallen leaves should be collected and burnt.
• Four sprays at 10 day interval with wettable Sulphur (0.3 per cent) or Dinocap (0.07 per cent) or Carbendazim (0.1 per cent) controls the disease effectively.
• Spraying with Phalton (0.3 per cent) + Carbendzim (0.025 per cent) or Benomyl (0.1 per cent) or triadimeton bayleton( 0.1 per cent) at 3 days interval also controls the disease.
• Some of the resistant varieties like Aawliver, Abhisarika, Adolf morstman, African star, Barbara etc should be known.
• Excess soil fertilization, especially with nitrogenous fertilizers and crowding of plants should be avoided.