476 



ENCYCLOPEDIA OF PRACTICAL HORTICULTURE 



der certain climatic conditions the dis- 

 ease is not uncommon on full grown trees 

 in the orchard. It usually attacks only 

 the tips of actively growing branches but 

 may occur in spots on the underside of 

 otherwise unaffected mature leaves. 



This disease is caused by a fungus 

 known technically as Sphaerothera mali. 



O.e 



Fig. 1. ISi'auch of Apple Attacked by Powd- 

 ei-.v .Mildew. 



the mycelium of which develops mainly 

 as a covering on the surface of the grow- 

 ing twigs and leaves and sends feeding 

 threads only into the external cells. The 

 twigs present a white, mouldy appearance, 



due to the growth of the threads of the 

 fungus in considerable quantity on the 

 surface. (See Pig. 1.) Sometimes this 

 is so copious as to resemble felt. Spores 

 are produced in great abundance imder 

 favorable conditions and give the affected 

 twigs and leaves a frosty appearance. 

 Such twigs are usually more or less 

 thickened and shortened, and frequently 

 distorted. The leaves present a wilted 

 appearance and are smaller than normal. 



When the disease is abundant, the nor- 

 mal functions of the tree are interfered 

 with on account of the reduction in the 

 foliage. Affected trees may fail to form 

 blossom-buds, or the fruit may not be of 

 good size and quality. 



The fungus hibernates as mycelium on 

 the affected twigs. This is the principal 

 method by which it is carried over the 

 winter. Sexual spores may also be formed 

 in protected fruit bodies partly buried in 

 the felted mass of mycelium. 



Treatment 



It has been found that this fungus, un- 

 like most powdery mildews, is a difficult 

 one to control. Since the fungus winters 

 on the twigs, it might be expected that a 

 dormant spray applied late in the spring 

 would control the disease; but this has 

 not been found sufficient where the dis- 

 ease is abundant. Investigations carried 

 on in California by Volck indicate that a 

 special spray, the iron sulphide, will hold 

 the disease in check so that it will not 

 cause serious damage, if the spray is ap- 

 plied as soon as the foliage buds open 

 and is repeated at frequent and regular 

 intervals. Where lime-sulphur can be 

 safely used as a remedy for apple scab, 

 it should not be necessary to use the iron 

 sulphide. 



When the disease is present only in 

 slight amount, pruning out the diseased 

 twigs, removing them from the orchard 

 and burning them, will usually be found 

 sufficient to hold the disease in check. 



Riblinsmpliy 



1909. B. M. Duggar, Fungus Diseases of 

 Plants. 



1910. New York Experiment Station, 

 Bulletin No. 328. 



