26 BULLETIN 712, U. S. DEPAKTMENT OF AGRICULTURE. 



in other parts of the country that the combined spray is not only 

 the most practical method of applying insecticides and fungicides 

 when both must be used, but that the combined spray is more effi- 

 cient than separate applications. 



Orchard studies indicate that tender young foliage is most easily 

 infected and that terminal growth is especially susceptible. The 

 striking thing about a badly infected tree is the preponderance of 

 mildewed terminals, which give it a whitened appearance. Most of 

 this infection takes place early in the season, up to the time that 

 vegetative growth is largely completed, but infection continues as 

 long as new foliage is being formed. It frequently happens in 

 the irrigated sections of the Northwest that a secondary vegetative 

 growth takes place in the latter part of the summer, after the re- 

 sumption of irrigation following a period of drought. In infected 

 districts such new growth is usually attacked by mildew after its 

 spread had apparently ceased with the checking of .the earlier vege- 

 tative growth. 



CONCLUSIONS. 



(1) Apple powdery mildew is generally prevalent in the arid 

 regions of the Pacific Northwest. It attacks foliage and twigs, reduc- 

 ing the vitality of the tree, and also attacks the young fruit, causing 

 an objectionable russeting of the skin. It has often caused a reduc- 

 tion of more than 50 per cent of the crop in infected orchards, and 

 it causes a further loss in a reduction of vitality that is impossible 

 to estimate. 



(2) The disease is caused by the fungus Podosphaera leucotricha 

 (E. and E.) Salm., which winters over in dormant buds in mycelial 

 form. Ascospores can not be regarded as essential to the propaga- 

 tion of the fungus. It is spread by conidia disseminated by the wind, 

 and possibly to some extent by insects. Dews furnish sufficient 

 moisture for spore germination. 



(3) Dormant sprays of lime-sulphur solution at a strength of 3^° 

 Baume (a dilution of about 1 to 9, when lime-sulphur testing 34° 

 Baume is used) have had little or no effect on the overwintering 

 fungus and have not hindered the spread of the disease. 



(4) Pruning out diseased shoots can not be depended upon alone 

 to effect control of the disease when it is present in epidemic form. 



(5) The disease is readily controlled by applications of sulphur 

 sprays during the growing season. However, after the advent of 

 burning sunshine, which can be expected in the hot interior districts 

 of the Pacific Northwest after the first of June, the use of sulphur 

 spray materials is certain to cause severe fruit injury. 



(6) Bordeaux mixture is less effective than sulphur sprays against 

 powdery mildew and leaves such a heavy deposit on the fruit that it 



