Downy Mildews (Order Peronosporales)
almost all the members of this order are obligate parasites of dicotyledonous plants. They have a mycelium which grows between the cells of the host plant. Members of Peronospora, are among the fungi which produce downy mildew diseases.

Powdery mildews (Order Erysiphales)
The order Erysiphale contains a single family, the Erysiphaceae. There are at least 90 species in this group and all of them are obligate parasites, causing the diseases known as powdery mildews. Common powdery mildews include the species Erysiphe which occurs on grass and cereal plants. In addition to producing ascospores as the teleomorph anamorph conidiospores produce conidia. The spores of the powdery mildews vary in shape from tubular to flank-like and are hyaline (colourless). The spores are relatively large, exceeding 50 micrometers (microns) in many cases. Technically these spores should be within the Fungi Imperfecti class because they are produced as a result of asexual reproduction. The scars typical of this type of reproduction can be seen at each end of the spore.

Xylariales
The order Xylariales contains at least 1,200 species including Xylaria within the family Xylariaceae. This group of fungi lives on dead wood, but also on isolated from leaves and other cellulosic plant material; a few species are pathogenic. Although the spores vary in shape from species to species, they are quite distinctive, partly because of their very dark brown colouration. Their spore size ranges 10 from to 25 micrometres (microns) in length.

One septate ascospores
Spores within this group are collected together because of their visual appearance. These spores which all have the similar morphological feature of a single transverse septum range in colour from hyaline to dark brown in colour. Spores within this group usually range from 10 to 20 micrometres (microns) in length.

Didymella
One particular 'one-septate' hyaline ascospore is Didymella. This fungi is both parasitic and saprophytic. Spores fitting the description of Didymella have been reported above the fields of wheat and barley, because it is capable of infecting both crops. The highest concentrations occur in England during the months of July and August. A peak of 90,000 m-3 was reported during one six hour period, (Allitt, 1986). An increase in cases of acute asthma in Birmingham in 1983 was thought to be due to high levels of Didymella ascospores, (Packe and Ayres, 1985). Because it relies upon a crop as a substrate, its numbers drop dramatically after the crops have been harvested. The spores require a high relative humidity for their release so that they very often occur after a period of rain.