Snapdragon downy mildew

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Snapdragon downy mildew, caused by Peronospora antirrhini Schroet., has been recorded only on Antirrhinum orontium, a wild species of Europe, and on Antirrhinum majus, the cultivated snapdragon. Kenneth F. Baker3 found this mildew on Antirrhinum nuttallianum at Otay Lake, San Diego County, California, in 1941. All varieties of snapdragon inoculated by the writer, or observed under conditions of heavy natural infection, have been susceptible. Murphy ⁽¹³⁾4 indicates that dark-colored varieties are the most susceptible. In a heavy natural infection at Guadalupe, California, in 1940, breeding lines with dark, waxy foliage had fewer infected plants than light-colored ones.

Peronospora antirrhini was first reported on Antirrhinum orontium in 1874 ⁽¹⁵⁾ from Germany, on this same species in Switzerland in 1907 ⁽¹⁰⁾, and in Denmark in 1913 ⁽¹²⁾. It is not recorded to have caused commercial damage until 1936, when Murphy ⁽¹³⁾ in Ireland reported it to be severe on nursery seedlings of cultivated snapdragons. It has since been found in England ⁽¹⁾, ⁽⁷⁾, ⁽⁸⁾, New South Wales ⁽²⁾, California ⁽⁹⁾, Pennsylvania ⁽¹¹⁾, and Oklahoma ⁽¹⁴⁾. McWhorter5 found the disease at Portland, Oregon, in the spring of 1944. Snapdragon mildew was first observed by the writer on May 25, 1938, on specimens brought in by a nurseryman near Hayward. By 1940 it not only was severe throughout the San Francisco Bay district, but also had appeared in southern California.

Importance of the Disease

Snapdragon downy mildew has been principally a seedling disease of nursery plants. Systemically infected plants are unsalable. They are unsuitable for planting because they generally fail to grow and finally die. Losses have varied from none to all plants in a given planting. The standard practice in the San Francisco Bay area is to plant about 1,000 snapdragon seeds per flat, and later to transplant the resulting seedlings at the rate of 100 to 120 seedlings per flat. The plants are sold in the second flats within a few days to a few weeks after transplanting. If more than about 10 plants per flat show systemic infection, the entire flat is usually discarded. One nursery in Oakland discarded its entire stock of 600 flats of snapdragons at one time because of downy mildew infection. In 1940-1942 the infection was so generally destructive in southern California that it was impossible to obtain healthy seedlings. At a seed farm at Guadalupe, 90 to 95 per cent of the plants in 1,400 flats were lost because of mildew in January, 1940, and at a nursery in Los Angeles about 1,000 flats were discarded because of mildew in 1942. Murphy ⁽¹³⁾ and Green ⁽⁷⁾, ⁽⁸⁾ also indicate extensive nursery losses in Ireland and England.

Losses may also occur on greenhouse plants grown for cut flowers, but the writer has little information on how severe this may be.