Department of plant pathology

1. COLLEGE OF AGRICULTURAL TECHNOLOGY
(Affiliated to Tamil Nadu Agricultural University, Coimbatore-3)
Kullapuram (Po),ViaVaigai Dam, Theni-625 562
Onion Purple blotch
Student Course teacher
A. Sowbejan Dr. Parthasarathy. S
2015021122 Assistant Professor (Plant Pathology)

2. Name of disease
Common name: Purple blotch of onion
Pathogen name: Alternaria porri

3. Occurrence
• The disease was first time reported from Bombay (Ajrekar,
1921).
• Since then has been reported from many states of the
country like Maharashtra, Gujarat,Tamilnadu, Rajasthan,
Haryana and Punjab (Pandotra, 1964).
Inventor:
Alternaria porri (Ellis)
Hosts:
Onion, shallot, leek

4. KINGDOM Fungi
PHYLUM Ascomycota
CLASS Dothideomycetes
ORDER Pleosporales
FAMILY Pleosporaceae
GENUS Alternaria
SPECIES A . porri

5. Economic Importance
• These pathogens reportedly cause storage yield.
• The disease causes 20-25 per cent losses to the onion
seed crop.

6. Symptoms
• The characteristic symptoms of the disease appear as
a small, light coloured, sunken lesions, which become
zonate with purplish center.
• These lesions rapidly enlarge and eventually girdle
the leaf or inflorescence stalk.
• In moist weather the surface of the spot is covered
with the brown or almost black sporulation of the
fungus.
• Usually the affected leaf or stem falls down and dies
within 3 or 4 weeks under favorable environmental
conditions.

7. ©http://https://www.apsnet.org/publications/imageresources/Pages/FI000318.aspx

8. Disease cycle
• The fungus survives from one season to other in
infected plants debris as dormant mycelium.
• the fungus can also survive in diseased onion leaves
and seed stalk debris for 12 month buried at 5 to7.5
cm depth.
• wherever the chlamydospores of the fungus are
formed they can also serve as source of penetration,
invasion takes place either through stomata or directly
through the cuticle , to form an intercellular mycelium,
which in turn forms conidiospores and conidia.
• Conidia produced on the primary infection lesions
serve as the source of secondary inoculum.

9. Favourable condition:
• The fungus requires rain or persistent dews for
reproduction and penetration.
• The optimum temperature for disease development is
21 – 30 degree C and relative humidity above 90%.

10. • Various cultural practices like use of healthy seed/planting
materials, crop rotation with non-related crops, collection
and destruction of infected debris, good drainage, summer
ploughing (Srivashtava et al. 1996)
• Use of recommended doses of fertilizers
• Hot water (50 ºC for 20 min.) soak was found to be highly
effective in reducing seed borne inoculum (Aveling et. al.,
1993)
• Use of disease resistance varieties viz. Pusa red, IIHR-56-
1, hybrid PVM-7 etc.
• Spraying crop with Copper oxychloride, mancozeb,
Metalaxyl+ mancozeb, Fosetyl-al, Difenconazole etc.
Management