No. 7. DEPARTMENT OF AGRICULTURE. 581 



spring months, the reddish or orange spores are produced for a sin- 

 gle year in the case of G. macropus, for several years in the case 

 of the other two. With moderately warm weather and continuous 

 moisture for 12 to 48 hours, these spores will germinate in the 

 gelatinous masses, and give rise to a limited amount of growth, and 

 produce within this period a number of smaller secondary spores. 

 As the mass dries, these last become detached, and being exceedingly 

 small and light, are easily carried long distances by the wind, as well 

 as by insects that feed on the masses. The gelatinous masses may 

 swell and dry several times in April and May, as the showers come 

 and go, giving rise to successive crops of secondary spores through- 

 out a period of three or four, or perhaps six or eight weeks. 



These secondary spores are short-lived, and to reproduce the fungus 

 they must meet with suitable conditions of moisture, temperature 

 and food supply within a very few days at most. The food supply 

 must be leaves or the fruit, or possibly the young shoots, of some 

 pomaceous plant; for G'. macropus, it must be ordinarily apple, 

 wild crab-apple or hawthorn; for G. glohosiun, apple, quince, moun- 

 tain ash, or hawthorn; and for G. clavipes, apple, pear, quince, or 

 juneberr3^ 



The fungus threads, on entering apple tissues, develop slowly, 

 and it is usually two weeks before the yellow spots are perceptible, 

 and four to eight weeks or even more before they are completely 

 developed. They are distinctly yellowilsh, early develop minute 

 black dots on the upper side, later produce swelling of leaves toward 

 the underside, and develop on the cushion thus formed a number 

 of little cups with delicately fringed borders. Attack on fruit is 

 near the flower end, spots growing to be as large as a silver quarter ; 

 there is no swelling but the fungus threads grow deeply into the 

 flesh producing a conical, firm, slightly discolored mass. Super- 

 ficially the black dots occur near the center and cups may be formed 

 at the margin. This stage on pomaceous plants is known as the 

 "cluster-cup" stage, which is closely similar but not identical for 

 the several rust species. A third type of spore is produced abundant- 

 ly in these cups, and serves to carry infection back to the cedars in 

 midsummer or later, to produce in turn galls that may not mature 

 until as late as the spring after the next, thus bringing us back to 

 the starting point. Unlike the grain rusts, these parasites seem to 

 have no means of spreading directly from cedar to cedar or from 

 apple to apple. 



During 1910, in southern Pennsylvania, Maryland, Virginia and 

 West Virginia, there was an abnormally severe outbreak of the cedar 

 rust on apple. The explanation for this is to be found, no doubt, 

 in moist spring weather conditions that greatly favored apple in- 

 fection. In Center county. Pa., at least, the cedar apples and pus- 

 tules matured and formed secondary infection spores several weeks 

 earlier than usual, and their production extended over about twice 

 the usual period. Furthermore, moist weather prevailed several 

 days at a time when young apple leaves and fruit were developing, 

 a condition that })romotes the formation of secondary spores, and 

 their germination after chancing to reach the apple. The rusts on 

 cedars we have always with us; what the eft'ect on apples will be 

 in any season depends. In so far as we know, largely upon weather 

 conditions. 



