Source: The New Indian Express [edited]
Due to heavy rainfall and moisture in the air, onion crops in Chitradurga [district] have been hit by purple blotch and bulb rot diseases, leading to massive losses. [The] horticulture department is asking farmers to use pesticides to control the spread. "Until the crops receive ample sunshine, these diseases cannot be controlled. Water-logged areas are suffering a lot," said deputy director G Savitha. About 16 500 hectares [about 40 800 acres] is [cultivated] for onion crops. [A farmer] said once such diseases attack, the bulb starts rotting, and this spreads fast across the crop.
[Byline: Subhash Chandra]
[Purple blotch of onions (_Allium cepa_; some varieties known as shallots) and related crops, such as leek (_A. porrum_) and garlic (_A. sativum_), is caused by the fungus _Alternaria porri_. Crop losses depend on environmental conditions and may range from 25 to 90 percent. The damage may continue to develop post-harvest. Symptoms on onion include small, water-soaked lesions with purple centres on leaves, leaf bases, and flower stalks, which expand and result in necrotic patches; symptoms may be confused with stemphylium blight of onion (_Stemphylium vesicarium_; see previous ProMED-mail posts in the archives). On leek, _A. porri_ leads to white or purple spots appearing on leaves; affected leaves crack and eventually die.
The disease is favoured by high humidity and often occurs as a secondary infection on plants already affected by other pathogens or pests (for example, onion thrips). The pathogen is mostly spread with infected plant material and by mechanical means (including insects and human activities). Disease management may include fungicides, clean planting material, phytosanitation to minimise inoculum and crop rotation.
Bulb rots of onion may be caused by multiple different pathogens, mostly fungi or bacteria. It is not clear which type of bulb rot is referred to above. Management strategies applied for purple blotch may ameliorate some of the other possible pathogens as well, but fungicides will not be effective against bacterial bulb rots. Reliable diagnosis is required for designing effective management strategies for the local situation.
Elsewhere in Karnataka, onion crops are currently being affected by the anthracnose twister complex (ProMED-mail post 20200518.7351461).
India (with states):
Purple blotch symptoms on onions:
Purple blotch photo gallery:
_A. porri_ asexual spores, microscopy:
Information on purple blotch of onion:
http://www.pestnet.org/fact_sheets/onion_purple_blotch_151.htm (with pictures),
Diseases of _Allium_ crops, including purple blotch:
_A. porri_ taxonomy and synonyms:
List of major diseases (including bulb rots) and pathogens of onion: