Black mould is ubiquitous in soil and as a saprophyte on plant waste. It can cause problems in onions and garlic, especially if temperatures are extremely high when the crops are at a mature stage or in storage.
This disease is particularly damaging to onion and garlic bulbs in tropical and subtropical regions, but can occur worldwide.
Symptoms and diagnosis
The most important characteristic is the presence of powdery mould spores on the surface, or between the scales of the bulb. Affected bulbs can display varying degrees of soft rot, but this could also be caused by other fungi or bacteria. In growing crops, several symptoms may become visible in areas where the developing bulb is damaged or split.
Conditions for disease development
A. niger is known to be seed-borne, but soil is another important source of inoculation. A. niger colonises old onion foliage during the growing season and can spread to the neck of the bulb and subsequently to the bulb itself. The disease can be dispersed within the crop and to adjacent crops via airborne spores carried by the wind during harvesting. Problems caused by this disease arise at extremely high temperatures (>30°C) and during periods of high humidity. The pathogen is relatively inactive at temperatures below 15°C. In the UK, problems with post-harvest onions only occur if the storage temperatures and humidity are too high. While black mould can certainly cause problems in moderate regions, most problems occur in tropical and subtropical regions where the natural conditions are favourable for the development of this disease.
Impact and importance
In Europe, it is known to incidentally cause black mould problems. The effects of seed treatment can be limited, as the pathogen is commonly found in soil. The impact of this disease can be minimised by means of careful crop management during harvesting and storage.