POWDERY MILDEW OF ORNAMENTAL PLANTS 3 



as shaking the dust from a cheesecloth bag, is wasteful of dust and in- 

 efficient because it fails to coat the under side of the leaves. 



The best time to apply sulfur dust is in the late afternoon when 

 the air is still and the temperature is falling. The foliage should be 

 dry in order to obtain uniform coverage. It is not necessary to 

 apply the dust so heavily that a conspicuous coating is left. Sul- 

 fur should not be applied to plants when the temperature is near or 

 above 90° F. because of risk of sulfur-burn injury. 



Sulfur dust alone does not flow easily through a duster; it should 

 be mixed with some other material to act as a flux. Hydrated 

 lime, talc, wheat flour, crude kaolin, or arsenate of lead thoroughly 

 mixed with sulfur in the proportion of 1 part to 10 causes the sulfur 

 to flow more easily, producing a lighter, more uniform coating than 

 can be obtained with sulfur alone. 



A sulfur spray may also be used. Sulfur dust will not mix with 

 water to form a spray, but specially prepared dry forms of sulfur 

 known as "wettable sulfur" spray materials are available for this 

 purpose. The widely used fungicide lime-sulfur, which is obtainable 

 in both dry and liquid concentrated form, may also be employed, 

 provided it is ^ sufficiently diluted; but its odor is objectionable, it is 

 ruinous to painted surfaces, and it is more likely to cause chemical 

 injury to the foliage than wettable sulfur spray or sulfur dust. The 

 manufacturer's specifications for summer or foliage applications 

 should be followed with each of these materials. Spraying is likely 

 to give better results than dusting on tall shrubs or trees such as 

 lilacs and crapemyrtles, because the air movement may prevent 

 thorough coverage of tall plants from a hand dust gun. The spray 

 coating lasts longer but causes a more conspicuous residue. Dusting 

 gives an effective coating in less time than spraying and is less messy, 

 but it must be repeated more frequently. 



Applications of either spray or dust should be repeated at suf- 

 ficiently close intervals, depending on the frequency of rain and the 

 growth rate of the plants, to keep the foliage coated with fungicide. 

 Ordinarily this will require one or two applications of dust weekly, 

 or one application of spray every 2 weeks. 



Elimination of Overwintering Sources 



The surface mildew fungi live over winter mostly in the form of 

 resting spores that mature on the fallen leaves. In a few instances, 

 for example mildews of rose and of crapemyrtle, the fungus also sur- 

 vives on shoots and in buds of the previous year's growth. The 

 collection and burning of infected plants and fallen leaves at the end 

 of the growing season is an indispensable measure in preventing the 

 recurrence of mildew. In the case of the shrubs mentioned, where 

 the fungus also overwinters on the host, pruning of infected parts 

 and spraying during the dormant season with lime-sulfur at the manu- 

 facturer's prescribed rate for winter spraying may also be necessary 

 to attain effective control. 



