Black mold: Powdery fungal spores beneath dried skin.

Pathogen: Aspergillus niger

Symptoms Below the dried skin a powdery, black mass of spores forms which is often invisible on the outside. The spores can be present on each of the scales. Infected scales first appear watery, after which white, fluffy mycelium appears on which black spores quickly form. In advanced stages, the bulbs will start to shrivel.

Development and infection Aspergillus is a common fungus mainly found in warm, dry regions. The fungus occurs on many plant-based and animal products. It can survive on damaged and dead plants and animals, fruit and other agricultural produce. Infection usually takes place in the field and requires a leaf wetness period of at least 6 hours No symptoms can be seen on the plants in the field, except maybe a black discolouration of the neck. The fungus penetrates the bulb via the neck and attacks the scales. Wounds and damage on the outer scales and roots also allow the fungus to enter the onion. Problems mainly occur at temperatures above 28ºC. Black mold is often followed by a secondary infection which causes the bulbs to rot. The bulbs can also dry and shrivel.

Prevention and management Prevent damage and diseases on the leaf, which can allow the fungus to penetrate the onion. Avoid damage to the bulbs during harvesting, storage and transport. Keep the temperature (<15ºC) and humidity (<76% Relative Humidity) low during storage and transport.