U. S. DEPARTMENT OF AGRICULTURE 



mon mildews of ornamental plants are listed on p. 4.) It is practi- 

 cally impossible to isolate garden flowers from exposure to these 

 mildews, because the host plants are so numerous and because the 

 vegetative spores, or conidia, of the fungi are so widely distributed by 

 air currents. 



Conditions Favorable to Mildew 



As summer wanes the aging of foliage and the decline in growth 

 vigor induce internal conditions in plants that are especially favor- 

 able for mildew infection. It is not always the older leaves that are 

 attacked; sometimes, particularly in the mildew of roses and crape- 

 myrtle, succulent young shoots are severely injured. A drastic check 

 in growth activity, such as may be caused by drought, intense heat, 

 or depletion of soil fertility^ particularly of nitrogen, also tends to 

 make plants susceptible to mildew. 



Besides the internal changes that influence the development of 

 mildew, the environmental conditions that prevail toward the end 

 of summer — for example, high humidity, cooler nights, and frequent 

 heavy dews and foggy mornings — are especially favorable for mildew 

 development and spread. Shady and damp garden sites are espe- 

 cially conducive to mildew, and the disease may develop even in sunny 

 locations if the air circulation is retarded by a wall or hedge, as in a 

 fence corner or adjacent to a building. 



A growth of weeds promotes mildew by restricting air circulation 

 and by shading the plants and lowering their vitality through reduc- 

 tion of food and water supplies. 



Control 



After mildew has become general enough to be conspicuous, it is 

 usually too late to apply control measures with success, although a 

 change to dry, sunny weather often imposes a natural check. How- 

 ever, ground sulfur, if it is applied in time — that is, at the first appear- 

 ance of mildew — will not only prevent further spread but will suppress 

 the development of mildew that has started. This form of sulfur, 

 known also as dusting sulfur, or sulfur flour, is a product specially pre- 

 pared for dusting plants. Stores that handle plants and spray mate- 

 rials generally carry dusting sulfur, but the drug-store product known 

 as flowers of sulfur and intended for household or medicinal use is 

 not suitable for dusting plants effectively. The value of a sulfur 

 preparation for this purpose depends on the smallness of the particles. 

 If they are small enough to pass a sieve having 300 meshes per inch, 

 the material is suitable for dusting. 



A dust gun capable of applying a diffuse and uniform coating of 

 powder to both sides of the leaves is the best means of application. 

 A small plunger type of dust gun holding 1 or 2 quarts will suffice for 

 a small garden; such guns are inexpensive and fairly efficient if only 

 occasional use is required and only a small area is to be covered. In 

 large gardens and commercial plantings a duster operating with a fan 

 or bellows is preferable both from the standpoint of efficient applica- 

 tion of dust and saving of labor, as the operating motion is easier and 

 there are no idle strokes. A makeshift method of application, such 



