''Peronospora farinosa'' is a species name that has been widely applied to

downy mildew Downy mildew refers to any of several types of oomycete microbes that are obligate parasites of plants. Downy mildews exclusively belong to the Peronosporaceae family. In commercial agriculture, they are a particular problem for growers of cruc ...

on leaves of wild and cultivated ''Amaranthaceae'': ''Amaranthus'', ''Atriplex'', ''Bassia'', ''Beta'', ''Chenopodium'', ''Halimione'', ''Salsola'', ''Spinacia'', etc. However, the species name has been taxonomically rejected
see report 20 from the Nomenclature Committee for fungi)
as the original description contained reference to multiple species and could not unequivocally be attributed to a species of Peronospora. In the past, some of the species on important crop plants have been given names as ''formae speciales'', notably '' f.sp. betae'' on sugar

beet The beetroot is the taproot portion of a beet plant, usually known in North America as beets while the vegetable is referred to as beetroot in British English, and also known as the table beet, garden beet, red beet, dinner beet or golden beet ...

(= ''P. schachtii'') and f.sp. ''spinaciae'' on spinach (= ''P. effusa''). However, phylogentic reconstructions have revealed that these "forms" of ''Peronospora'' on different genera and their subdivisions, are distinct species, most of which already have previously published scientific names (see ''Index Fungorum''). Such host specialization possibly also exists with respect to the various wild amaranthaceous species given as hosts of ''P. farinosa''. Downy mildew is a moderately important disease of sugar beet (''Beta vulgaris''). The pathogen persists as

oospore An oospore is a thick-walled sexual spore that develops from a fertilized oosphere in some algae, fungi, and oomycetes. They are believed to have evolved either through the fusion of two species or the chemically-induced stimulation of mycelia ...

s in the soil, or on beet seed crops, or on overwintered volunteer beet plants. Attacks are most important at the seedling stage. The cotyledons are systemically infected, becoming discoloured and distorted. Loss of seedlings causes uneven crop development. Beet leaves are less affected, so a crop can to a substantial effect recover from an attack on seedlings. Control relies on adequate

crop rotation Crop rotation is the practice of growing a series of different types of crops in the same area across a sequence of growing seasons. It reduces reliance on one set of nutrients, pest and weed pressure, and the probability of developing resistant ...

and avoidance of sources of infection (e.g. adequate control of the disease on beet seed crops), as oospores survive only 2–3 years in the soil. Individual infected plants may also be removed. It is not generally necessary to apply fungicides. Downy mildew has more direct importance on spinach (''Spinacia oleracea''), since it affects the harvested part (leaves). Yellow lesions appear on the older leaves. If rotating crops and removing individual infected plants fails, fungicide treatments are effective, and resistant cultivars are available.

References

Further reading