GREY MILDEW OF COTTON

Diseases of cotton

1. COLLEGE OF AGRICULTURAL TECHNOLOGY , THENI (Affiliated to Tamil Nadu Agricultural University, Coimbatore-3) Kullapuram (Po),Via Vaigai Dam, Theni-625 562 GREY MILDEW OF COTTON STUDENT COURSE TEACHER Miss. SNEHA R Dr. PARTHASARATHY S ID. NO. 2015021120. Asst. Professor (Plant Pathology)

2. GREY MILDEW OF COTTON Causal organism: Anamorph - Ramularia areola (Atkinson) . Teleomorph - Mycosphaerella areola (Ehrlich And Wolf).

3. IMPORTANCE Among several fungal disease is predominant causing major yield losses. Grey mildew is said to be polycyclic disease as it can infect crop repeatedly in single cropping season. Grey mildew is an important fungal disease, which has been affecting cotton yield in India.

4. The disease increased by 10-30 per cent this year when compared to the prevalence in last four years. It has become a major disease and needs effective control during early days only. Low temperature and humidity prevailing during the winter season also contribute to the disease intensity.

5. HISTORY This disease is common in many parts of India . The grey mildew disease was reported for the first time on upland cotton in aubum, alabama, UAS in the year 1890 and was records as ‘areolate mildew’ of cotton ( Atkinson, 1890) .

6. The disease was recorded under various symptom descriptive names such as areolate mildew, false mildew, frosty blight, white mold and grey mildew. In maharastra state, the grey mildew is commonly referred to as ‘dahiya’ or ‘dahya’ disease because of symptoms resembling sprinkled curd on foliage ( Gokhale and Moghe,1965 ).

7. ECONOMICAL LOSSES In India, seed cotton yield losses ranging between 26 to 66 per cent have reported under grey mildew epiphytotic conditions in the states of Tamil Nadu, Andhra Pradesh and Maharashtra, under exclusive monoculture of ‘diploid’ cotton, the losses extending even upto 90 percent has been recorded ( Sangitrao et al ., 1993).

8. DISTRIBUTION This disease is common in many parts of India. It is also reported from the U.S.A., parts of Africa, Middle East, Brazil and other countries.

9. SYMPTOMS In grey mildew infection, both surfaces of the leaves get uniformly covered by white powdery growth of the fungus. This further leads to curling and drying of the leaves and the disease leaves defoliate eventually .

10. Initial infection appears as triangular, square or irregularly circular whitish spots of 3 to 4 mm size on leaves. As the disease severity increase, the smaller spots merge together and form bigger spots.

11. The whitish powdery growth visible on under surface of the leaves and visible later on the upper surface. Appearance of white/ grey spots because of the raised aerial growth of the fungus resemble sprinkled curd or Dahi and hence the disease is referred to as vemacularly ‘Dahiya’ and ‘Dahya’.

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17. Grey mildew of cotton

18. CAUSAL ORGANISM The asexual form of the grey mildew fungus i.e., the imperfect conidial stage is known as Ramularia areola Atk. S exual stage i.e., the perfect state of the fungus is known as Mycosphaerella areola .

19. SYSTEMATIC POSITION Kingdom : Fungai Phylum : Asgomycota Class : Dothideomycetes Sub Class : Dothideomycidae Order : Capnodiels Family : Mycosphaerellasea Genus : Mycosphaerella Speices : M. Areola Mycosphaerella areola Ehrlich & F.A. Wolf, (1932).

20. LIFE CYCLE OF PATHOGEN The asexual form of the grey mildew fungus i.e., the imperfect conidial stage is known as Ramularia areola Atk. S exual stage i.e. the perfect state of the fungus is known as Mycosphaerella areola .

21. The pathogen survives during the summer in the infected crop residues. The perennial cotton plants and self-sown cotton plants also harbour the pathogen during summer months . The primary infection is through conidia from infected plant debris and secondary spread is through wind, rain splash, irrigation water and implements.

22. FAVOURABLE CONDITIONS Wet humid conditions during winter cotton season. Intermittent rains during North-East monsoon season. Low temperature (20-30˚C) during October-January. Close planting, excessive application of nitrogenous fertilizers. Very early sowing or very late sowing of cotton. Cultivating susceptible cotton varieties.

23. MODE OF SPREAD The disease spread mainly through air borne conidia. The air borne conidia of grey mildew fungus were trapped during the month of November to January in July/August sown crop ageing between 116 to 123 days. The highest daily mean of conidia numbering between 11 to 28/cu. m were observed in the month of November.

24. The air born conidia of R. areola showed afternoon pattem of circadian periodicity with peak occurring at 16.00h. The conidia were observed in the air when the temperatures ranged between 27-31 ° C (Maximum) and 18-23 ° C (Minimum), and the percentage Relative humidity (RH) between 70-90 at 7.0 h and 14.00 h.

25. MANAGEMENT   CULTURAL CONTROL Deep ploughing. Destroy crop residues. Crop should be rotated with cereals, and preference should be given to tolerant varieties.  Crop residues should be removed and the fields must be deeply ploughed in order to bury and destroy the remaining plant tissues. 

26. CHEMICAL CONTROL First foliar spray of 3 gm wet table sulphur per one litre of water in the initial stages of the disease to be done. Dusting of 8-10 kg of Sulphur powder effectively controls the disease. Also about one gram of Carbendazim or Benomyl per litre of water is effective. If the disease intensity is more, new fungicides like one litre Hexaconazole or 300 gm Nativo-75 WG per hectare is required to control the grey mildew disease.

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