Blue mould generally appears during harvesting and storage. Initial symptoms include watersoaked areas on the outer surface of scales. Later, a green to blue green, powdery mould may develop on the surface of the lesions. Infected areas of fleshy scales are tan or gray when cut. In advanced stages, infected bulbs may disintegrate into a watery rot. Many species of Penicillium can cause blue mould. These fungi are common saprophytes on plant debris and senescent plant tissue. Invasion of onion bulbs and garlic is usually through wounds, bruises, or uncured neck tissue. Once inside the bulb, the mycelium grows through the fleshy scales, eventually sporulating profusely on the surface of lesions and wounds. Optimum conditions include moderate temperatures (21° to 25°C) and high relative humidity.