Onion downy mildew

Abstract

, and Riverside counties, onion mildew has been observed in destructive amounts but has attracted less attention than in the other districts mentioned. In California, onion seed is harvested in July and August on plants grown from mature dormant bulbs which were set in the ground the previous fall and winter. The seed-producing plants make most of their vegetative growth in the spring months, which are during the rainy season, and produce their flowers and mature their seed in the semiarid summer weather that follows. This coincidence of the vegetative-growth period with the cool, rainy season, while apparently favorable for onion growth and for high yields (28) in seasons of relative freedom from mildew, may be associated with the destructive onion-mildew epidemics that occasionally occur. NAME, HISTORY, AND RANGE OF THE DISEASE Onion downy mildew has also been called mildew, mold, blight, white blast, and rust, but the name downy mildew is preferred formyeologieal arid symptomatological reasons. Onion downy mildew was first reported in England in 1841 by Berkeley (1), but no information concerning its importance was presented at that time. Since then it has been studied intensively in the Bermuda Islands by Shipley (60) ; in New York by Whetzel (76), Cook (12), and Newhall (50); in Ireland by Murphy and McKay (44), and McKay (36) ; in Russia by Katterfeld (31); and in California by Jones, Porter, and Leach (29). In addition to the regions already mentioned, the disease is present in Asia (10), Africa (73), Australia (51), New Zealand (32), and Argentina in South America (34). With the possible exception of limited localities of unfavorable environment for the disease such as Texas (65), onion downy mildew is now considered general in its distribution. ECONOMIC IMPORTANCE Nature of Losses.-Onion downy mildew is important because of the large reductions in yield caused by the disease in epidemic seasons. With onions for greens, the yield reduction is direct-the injury to and killing of the leaves caused by mildew infection reduces the yield and salability of the product. Occasionally plants are killed but this is apparently rare. With onions for bulbs, the loss is indirect-mildew injury to the leaves reduces the yield and quality of bulbs. According to Murphy and

Effects of atmospheric water potentials, free water, and temperature on production and germination of sporangia inPeronospora parasitica

“…also may require free water for germination (Fried & Stuteville 1977. Patel 1926, Yarwood 1943). The percentage of P. parasitica germinating was highly variable under all test con ditions, a phenomenon noted for other Peronospora spp.…”

Development of detection systems for the sporangia of Peronospora destructor

“…Fungicidal control of onion downy mildew is difficult and fungicides are only effective if they are applied before or immediately after disease first appears in the crop (Kennedy 1998). The environmental requirements for infection and sporulation by P. destructor have been reported (Yarwood 1937(Yarwood , 1943Hildebrand and Sutton 1982). Mathematical models describing climatic effects on sporulation and infection have been described (Gilles et al 2004;Battilani et al New approaches in forecasting diseases of onion crops based on estimation of spore numbers in air samples have been reported (Carisse et al 2005;Berger 1970).…”

Linkage of downy mildew resistance genes Pd 1 and Pd 2from Allium roylei Stearn in progeny of its interspecific hybrid with onion (A. cepa L.)

“…. Bulb yield reductions due to downy mildew infections can be up to 70% in seasons favouring the development of an epidemic ( Van Doorn, 1959 ;Yarwood, 1943), and onion seed production can even fail completely (Hennig et al ., 1990) . Kofoet & Zinkernagel (1990) assumed that no resistance to downy mildew is present in A .…”