BETTER FRUIT 



STATE ASSOCIATE EDITORS 



OREGON— C. I. Lewis. Horticulturist. Conallis. 



WASHINGTON — Dr. A. L. Melander. Entomoloslst ; 

 O. M. Morris. Horticulturist: W. S. Thornber. Horticul- 

 turist. Pullman. 



COLOR.-iJ>0— C. P. Gillette. Director and EntomolOBist ; 

 E. B. House. Chief of Depariment of Civil and Irrigation 

 Engineering. State Agricultural College. Fort Collins. 



ARIZONA— E. P. Taylor. Horticulturist. Tu^-son. 



WISCONSIN— Dr. E. D. Ball. Director and Entomologist. 

 Madison. 



MONTANA— O. B. Whipple. Horticulturist. Bozeman. 



CALIFORNIA— C. W. Woodworth. Entomologist. Berlte- 

 ley; W. H, Volck. Entomologist. Watsonville: Leon D. 

 Batchelor. Horticulturist. Riverside. 



INDIANA— H. S. Jacltson. Pathologist. Lafayette. 



An Illustrated Magazine Devoted to the Interests 



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Entered as second-class matter April 22, 1918, 



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Volume XIII 



Portland, Oregon, April 1, 1919 



Number 10 



Apple Powdery Mildew a Serious Menace to Orchards 



By D. F. Fisher, Assistant Plant Pathologist, Fruit Disease Investigation, U. S. Department of Agriculture 



APPLE powdery mildew is now 

 commonly found in all of the 

 ma,ior apple-growing districts of 

 the Pacific Northwest. In the hot in- 

 terior irrigated districts, where it was 

 believed fungous diseases could never 

 thrive, powdery mildew has established 

 itself and is now very much at home. 

 It is hard to estimate the loss which is 

 due to powdery mildew, since its most 

 serious result is a general devitalization 

 of the tree, spreading the loss over more 

 than one season. In cases of severe 

 infection, however, it frequently hap- 

 pens that more than 75 per cent of the 

 foliage is attacked, the terminals 

 stunted or killed, and the crop reduced 

 more than 50 per cent. Thus far, how- 

 ever, its serious outbreaks have been 

 .sporadic and more or less localized. 

 But careful observers have noted a 

 steady spread of the disease, so that 

 now few orchards can be fouml free 

 from mildew, and it appears to be only 

 a question of a short time before the 



l'ir.fn[. ]--.\ late stage of foliage Intcctjon on 

 watersprouls. Note that many of the leaves 

 has'e been shed and that those remaining ai'e 

 covered with spores, and are somewhat folded. 

 The twigs are covered with mycelium, em- 

 bedded in which may be seen the dark patches 

 of "winter spores." 



disease will be of universal occurrence 

 all over the Northwest. Orchaidists 

 would do well to consider the danger 

 of this situation, for even if they find a 

 mildewed twig only here and there 

 these innocent appearing infections 

 only await favorable climatic or sea- 

 sonal conditions to assume a serious 

 aspect. Climatic conditions cannot be 

 foreseen — a serious outbreak of pow- 

 dery mildew can never be exactly fore- 

 told — hence it is the part of wisdom to 

 take no chance, but to anticipate the 

 outbreak and combat the disease when 

 conditions are most favorable for suc- 

 cess. Unfortunately, growers are prone 

 to wait until it is plain that serious 

 damage has already been done before 

 they become alarmed and try to "cure" 

 the disease. But to await this condition 

 is to invite a hard fight. By this time 

 not only has great damage been accom- 

 plished but the fungus has become 

 established, and instead of a compara- 

 tively easy one-year fight, the grower 

 will find it a battle of two or more sea- 

 sons to again bring his orchard to a 

 position where mildew will not worry 

 him. In this connection the grower 

 should realize that the most efficient 

 way to combat mildew is to prevent in- 

 fection rather than attempt to "clean il 

 up" after it becomes established. 



Before proceeding further it would be 

 well to consider the nature of apple 

 powdery mildew. While it has occa- 

 sionally been found on pears in the 

 Pacific Northwest, it is most important 

 as a disease of the apple. The disease 

 is caused by a fungus, technically 

 known as Podosphaera leucotricha, 

 which grows upon the surface of leaves, 

 twigs, blossoms and fruit. At frequent 

 intervals the fungus mycelium (or 

 "spawn") sends "suckers" into the sur- 

 face cells of the infected parts, whereby 

 it obtains its nourishment and devi- 

 talizes its host. It reproduces itself by 

 means of the whitish-colored spores 

 which are produced in such abundance 

 on infected parts, and which give these 

 parts their characteristic powdery ap- 

 pearance. These spores are produced 

 very rapidly. They are wafted about 

 by the wind and when lodged on tender, 

 uninfected leaves, and the necessary 

 moisture is present, they will start a 

 new infection. It is in this manner that 



the disease is spread. The natural con- 

 trolling factor is seen to be the presence 

 or absence of moisture, since spores 

 require moisture to germinate just the 

 same as true seeds. Hence in rainy 

 springs the disease is very liable to 

 spread rapidly and assume serious pro- 

 portions. In the interior irrigated dis- 

 tricts, where natural lainfall occurs 

 but seldom during the growing season, 

 the dews which are of almost nightly 

 occurrence suffice as a source of moist- 

 ure supply. 



Because the fungus prefers very 

 tender growth, it is frequently found 

 on watersprouts and terminals, and is 

 most serious early in the season. How- 

 ever, if unchecked, it will persist and 

 spread all summer. In bat^lly-infected 

 orchards the whitened appearance of 

 the trees, due to the general presence of 

 mildew on the terminals, is a character- 

 istic manifestation of the disease. When 

 the infection is very severe the termi- 

 nals are frequently killed. On the 

 foliage, infection usually takes place on 

 the under side of the leaves, where 

 early stages are exhibited as small, 

 white, felty patches of mycelium. The 



FiGiHE 2 — Mildew russcling of a Black Ben 

 Davis apple. 



