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Powdery Mildew of Apple 



The powdery mildew of apple is due 

 to the parasitir fungus, Podosphiera 

 leucotricha, the vegetative body or 

 mycelium of which develops as a coat- 

 ing of minute interlacing whitish fila- 

 ments on the parts of the plants at- 

 tacked. The fungus produces two spore 

 stages in its life history: the conidial or 

 summer spores, which are produced 

 throughout the growing season and give 

 to affected parts a whitish, powdery 

 appearance; the ascigerous stage, which 



gives rise to the ascospores. The latter 

 is produced only upon the twigs, and 

 the bodies bearing the ascospores may 

 be found buried in the dark felted 

 mycelial mass towards the end of the 

 growing season. The conidia serve to 

 spread the fungus during llic growing 

 season. It is apparent that I he fungus 

 is carried over the winter 1)\ mycelium 

 which hibernates in the buds and also 

 by the ascospores. The part which the 

 latter play in the life history of the 

 fungus is somewhat problematical. 



May 



The mildew confines its attacks in the 

 main to young shoots and blossom clus- 

 ters. Both stem and leaves of shoots 

 may be alTected and either killed, de- 

 formed or reduced in size and vigor. 

 Blo.ssom clusters may be blighted and 

 young fruits may be affected later than 

 at the blossoming period. The mildew 

 is known on the fruits of the pear also. 

 The amount of blighting of blossoms 

 varies in different localities. Secondary 

 infections may occur on mature leaves 

 to a limite dextent. 



The control of the disease calls for 

 the employment of two methods, (1) 

 pruning and (2) application of fungi- 

 cides. In light attacks of mildew it 

 seems probable that pruning alone will 

 suffice, while in orchards where the 

 disease has gained considerable head- 

 way spraying must be resorted to in 

 addition to the pruning. 



1. Pruning. If is known that infested 

 buds on badly mildewed shoots produce 

 seriously diseased shoots the following 

 spring. Spraying will not prevent these 

 infections, so the affected shoots should 

 be removed and destroyed by burning. 

 This may be done at any time consistent 

 with horticultural practice and if not 

 done earlier should be made a part of 

 the regular dormant pruning operations. 

 If mildew is serious it will be advisable 

 to prune out more brush than ordinary 

 to sfimulate the growth the following 

 season. In general, the pruning prac- 

 tice should aim to eliminate close inter- 

 lacing of branches and vigorous shoots 

 of the current year's growth should be 

 cut back one-third to one-half. 



2. Spraying. The time of application 

 of the spray may be given first consid- 

 eration. It has been demonstrated that 

 winter spraying is without efTect on 

 mildew in California. It has not yet 

 been determined whether this holds for 

 Washington conditions or not, but it is 

 probable that such will be the case. 

 The times of spraying to be recom- 

 mended are as follows: (1) Just after 

 the petals fall. (2) In connection with 

 the second spraying for codling moth or 

 earlier if the mildew is serious. (3) 

 Three or four weeks after the second 

 spraying. It may be necessary to spray 

 a fourth time after a like interval if 

 mildew is serious and conditions con- 

 tinue favorable. 



The selection of the fungicide is a 

 matter of considerable importance and 

 should depend in part at least upon 

 what other diseases are present in an 

 orchard. In some sections of Washing- 

 ton powdery mildew is the only fun- 

 gous disease of the apple that is present, 

 which in others the orchard must be 

 protected from scab also. In case scab 

 is present the regular lime-sulphur 

 treatment (1-30) for this disease should 

 I)rove of value in the control of mildew. 

 The number of spraxings for scab will 

 vary according to ccmditions and the 

 severity of the disease. Those most 

 generally recommended are as follows: 

 (1) Just as the blossom buds separate 

 and show pink. (2) Just after the petals 

 fall. (3) Ten days to two weeks later. 

 Experience will show whether the first 



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