THE MONTHLY BULLETIN. 479 



that the disease known as apple mildew occurs in nearly all parts of the 

 United States, and that it is more common on nursery stock than on 

 orchard trees. In the west it attacks only the foliage and young twigs, 

 and not the fruit. The climate is said to be very favorable to the 

 development of this disease ; the Yellow Newtown and Yellow Bellflower 

 varieties being most susceptible. As 80 per cent of the apple trees grown 

 in the Pajaro Valley are of these varieties, the trouble has become very 

 serious. Probably in a third of the orchards of the valley, 50 per cent 

 of the terminals are diseased and show the grayish mildew covering. 

 The following description is given of the disease : 



Under the microscope the fungus presents the appearance of a much-branched and 

 loosely interwoven tangle of verj- fine threads lying on the surface of the leaf or shoot. 

 Scattered all through this tangle and forming a powdery layer on its surface are 

 enormous numbers of minute reproductive bodies called conidia. or summer spores. 

 Collectively the fine fungous threads are termed the mycelium. The mycelial threads 

 branch and rebranch as they grow over the surface of a leaf or shoot, and at intervals 

 short, saclike processes, called haustoria, penetrate the outer, or epidermal, layer of 

 the leaf or twig and by means of these haustoria the fungus absorbs its nourishment. 

 Thus the mildew derives its food materially from that particular portion of the leaf 

 or twig surface upon which it is growing. 



The very characteristic powdery appearance of the mildew is due, as stated above, 

 to enormous numbers of summer spores. These are produced in chains from certain 

 branches of the mycelium, and their function is to start new mildew infections during 

 the summer. The chains readily break up into individual spores, which are very light 

 and are easily carried about by the wind. When a spore lodges in a suitable place, 

 such as the under side of the young leaf, it quickly germinates, if the moisture con- 

 ditions are suitable, and sends out a small, threadlike germ tube, which is the begin- 

 ning of a new mycelium, and by this means a new infection is established. 



It is shown that there are two sources of infection in the spring, 

 when the foliage begins to come out. First, there are formed in 

 the month of July irregular dark smoky-like patches, which contain 

 the winter spores, or ascospores, which remain on the twigs until early 

 spring, where they may germinate and start infection. This source of 

 infection is said to be of much less importance in the Pajaro Valley than 

 what the authors term "dormant bud infection." The latter is due to 

 the fact that the mildew remains dormant under bud scales during the 

 winter season, becoming active in the spring and producing injury 

 almost immediately upon the unfolding of the buds. 



After six years of investigational Avork, during which time from 250 

 to 300 spraying experiments were conducted, and over 100 spraying 

 materials tested, the conclusion has been reached that sulfur, in some 

 very finely divided form, is the most satisfactory remedy, an iron-sulphid 

 mixture being recommended very strongly, the first application to be 

 made with the calyx spray for codling moth, the second ten days later ; 

 the third three weeks after the second, and the fourth three weeks after 

 the third. 



It is urged that the trees be kept in a thrifty condition, as when very 

 healthy they are not so subject to mildew attack. Therefore, anything 

 applied to them that will stimulate the growth is desirable. In this con- 

 nection, the winter spray of crude oil emulsion is recommended. Prun- 

 ing is found to be very effective in checking the disease, the idea being 

 to remove all mildewed tips during the winter season, and again when- 

 ever they appear in the summer time. — Gr. P. W. 



