Mould and mildew are generally considered to mean the same, a fungal growth on various surfaces. In the professional community the term mildew is used to describe fungal plant pathogens. It is caused by many different species of fungi in the order Erysiphales. Most species are specific to a narrow range of hosts. Mildew usually appears on fabric materials when sufficient amount of moisture is present. It manifests itself as thin powdery and white growth which can slowly spread over large areas of material. If mildew is left untreated is can turn into what is generally considered to be mould. Mould often looks dark in colour, fuzzy or slimy depending of the particular species present.
There are two types of mildew – powdery and downy. Powdery mildew can be usually found on roses and other flowering plants, and usually looks like white or gray splotches. Downy mildew is found in agricultural products such as grapes and potatoes. Its appearance varies from plant to plant, with some common indications being leaf spots and distortions and downy patches. In agriculture, downy mildews are a particular problem for growers of potatoes, grapes and tobacco. The presence of obligate plant pathogens, such as powdery mildew, smut, and rust, does not indicate fungal contamination on building materials or furniture. These fungi need living plants to develop and complete their lifecycles and are most likely from an outdoor source. On occasion, powdery mildew may develop on houseplants, such as miniature roses and chrysanthemums, but does not signal water-related damage in the building.
Mildew as a plant pathogen requires plants to complete its life cycle and produce spores and therefore mildew is unlikely to reproduce and multiply in the indoor environment. Moulds on the other hand only require suitable material (plaster, wall paper etc.) to grow and complete their life cycle and therefore they are likely to spread and contaminate indoor air.
The main difference between mould and mildew in a typical indoor situation is that mildew does not produce spores which can contaminate the indoor air but mould is likely to produce significant quantities of spores.
Downy Mildews – Peronosporaceae
Powdery mildews belong to the order of Peronosporales which includes to fungal families, the Peronosporaceae (downy mildews) and the Albuginaceae (white rusts), that are obligate parasites of plants. Examples of downy mildews are Plasmopara viticola, which causes an important disease of the grape vine, and Peronospora parasitica, which attacks members of the Cruciferae (the cabbage family). The white rust Albugo candida also attacks Cruciferae. The downy mildews and white rusts are biotrophs, obtaining their nourishment only from living cells, in contrast to the Pythiaceae, which are usually necrotrophs, killing cells and then obtaining nourishment from them. Although downy mildews and white rusts cannot be grown in pure culture, some species have been grown in association with cultured cells of the relevant host plant. Zoospores remain of importance in some species, but in others, such as P. parasitica, the sporangia function solely as conidia. Finally, the downy mildews and white rusts are obligately biotrophic parasites, usually with a limited host
range and rarely killing host plants.
Powdery Mildews – Erysiphales
Powdery mildews (Erysiphales), whose surface mycelium was conspicuous in late summer, also produce fruiting bodies (cleistothecia) in autumn. Environmental factors also affect spore germination and hence the behaviour of a pathogenic fungus in nature. The water relations of spores vary. Powdery mildews (Erysiphales) are more often seen in dry weather since their asexual spores can germinate and infect leaves without liquid water; indeed wetting reduces the viability of their conidia. On the other hand, sporangia and conidia of downy mildews (Peronosporales) soon decline in viability unless the relative humidity is high. Infection of hops by the hop downy mildew, Pseudoperonospora humuli, is greatly increased if the crop has been soaked by rain.
Moulds can colonise almost any surface in the indoor environment and are typically dark in colour. Mature mould colonies (growths) can represent a serious health hazard if left untreated. Apart from producing typical musty odours the can release potent toxic chemicals into the indoor air called mycotoxins. At the end of their life cycle mould produce large quantities of mould spores which can be highly allergenic.