POWDERY MILDEW OF ORNAMENTAL 



PLANTS 



By Freeman Weiss, formerly senior pathologist, Division of Fruit and Vegetable 

 Crops and Diseases, Bureau of Plant Industry 



Introduction 



Toward the end of summer many garden flowers, including annuals- 

 perennials, and flowering shrubs, develop a dirty-white powdery coat- 

 ing on the leaves. This coating is a fungus growth and is known as 

 mildew. Some of the plants that most frequently show this condi- 

 tion are chrysanthemum, perennial aster, peremiial phlox, sunflower, 

 and zinnia. Among flowering shrubs, the rose, lilac, crapemyrtle, 

 and spirea are commonly affected. 



The term "mildew" is not only applied to the disease but is also 

 used to designate the fungus, or group of fungi, that cause the dis- 

 ease. For mildews of this sort the name powdery or surface mildew 

 is more precise, as it distinguishes this group from the downy mildews, 

 another group of fungus parasites more widely known on certain veg- 

 etables and fruits (cucumber, onion, and grape) than on ornamentals. 



The Powdery Mildew Fungus 



In the powdery or surface mildews the white coating forms the 

 vegetative and spore-bearing stages and corresponds to the shoot sys- 

 tem and fruiting of a higher plant. The vegetative stage of these 

 fungi consists of a mat of branching threads known as mycelium, on 

 which are borne two kinds of microscopic reproductive structures or 

 spores: (1) The summer spores or conidia, which are produced 

 throughout most of the growth period in short columns like a row of 

 beads, and (2) the winter or resting spores, which are produced to- 

 ward the end of the growth period within minute capsules. The 

 powdery appearance of the mat is due to the presence of conidia, and 

 the gray or black specks just visible to the naked eye are the resting 

 spore cases. There is also an absorbing system analogous to the 

 roots of higher plants, which consists of microscopic threads or tubes. 

 These penetrate into the leaves of the host plant and extract food 

 therefrom. The mildew is a true parasite and not merely a super- 

 ficial growth. Because of its parasitic action the host plant is weak- 

 ened. Heavily mildewed leaves turn yellow and fall prematurely. 



There are many kinds of surface mildews. One kind is character- 

 istically found only on rose leaves; another occurs on lilacs; still dif- 

 ferent kinds occur on apple, crapemyrtle, hazelnut, and willow. One 

 kind occurs on cereals and many grasses. The mildew occurring 

 most commonly on garden flowers belongs to one or the other of the 

 two species Erysiphe ciehoracearum or E. polygon!, and these species 

 also occur on a wide range of native plants and weeds including aster, 

 clover, goldenrod, knotweed or smartweed, and ragweed. (The com- 



Issued April 1940. 

 207865°— 40 1 



