SPOTS on leaves and flower shoots, oval, at first pale green, later yellow, with violet-grey regions where conidiophores with conidia occur, finally blackish, because of the development of dark-coloured fungal saprotrophs (species of the genera Alternaria, Cladosporium, Epicoccum, etc.).
CONIDIOPHORES WITH CONIDIA in delicate, violet-grey conglomerations, formed on leaves and flower shoots.
CONIDIOPHORES (cp) 200-570 x 10-17 µm, thickened, up to 20 µm, at the base, branched monopodially 5-7 times.
Branches (b) straight or slightly curved, erected; final branches beaklike or forked curved, steadily tapering, blunted at the top, slanted at a more or less acute angle, sometimes straight, up to 23 µm long, 2.5-5 µm wide at the base.
CONIDIA (c) ellipsoidal, narrowed at the base, sometimes with a large wart, rounded or slightly sharpened at the top, yellow brown, 42-60 x 20-32 µm.
OOGONIA 37-55 µm, with a thin, colourless wall.
OOSPORES 30-45 µm diam, with an irregularly plicate episporium, up to 6 µm.
PLANT HOST AND DISTRIBUTION. Peronospora destructor affects different plant species of the genus Allium (Kochman and Majewski 1970; Smith et al. 1988).
The fungus has a worldwide distribution (Kochman and Majewski 1970; Smith et al. 1988).
NOTES. Peronospora destructor is the causal agent of downy mildew of onions (Kochman and Majewski 1970; Smith et al. 1988). This higly destructive disease frequently transforms into epidemics, especially in wet summers (Brooks 1953).
In plant host tissues, P. destructor forms intercellular mycelium with simple or forked haustoria. The infection may be systemic or local.
The fungus most abundantly sporulates and germinates at about 10-13oC and a high (ca. 100%) relative humidity. This obligate parasite infects plant leaves through their stomata. The penetration is favoured at prolonged wetness on the plant surface.
Peronospora destructor overwinters in the form of mycelium in higher parts of bulbs and in seeds, as well as in the form of oospores in plant debris and in the soil. In the soil, the oospores can retain their viability for at least four years.
Kochman J., Majewski T. 1970. Grzyby (Mycota) IV. Glonowce (Phycomycetes), Wroslikowce (Peronosporales). Warszawa, 308 pp.
Brooks F. T. 1953. Plant diseases. Geoffrey Cumberlege. Oxford University Press. London, New York, Toronto.
Smith I. M., Dunez J., Lelliott R. A., Phillips D. H., Archer S. A. 1988. European handbook of plant diseases. Blackwell Scientific Publications.