igii BETTER FRUIT Page 39 



INJURY CAUSED BY THE APPLE POWDERY MILDEW 



THE injury caused by this fungus 

 disease is at first not very serious, 

 only a few twigs on the tree may 

 be attacked, and then only partly covered 

 by the growth of the parasite. Under 

 these conditions the tree remains strong 

 and vigorous with undiminished produc- 

 tive power. Such is the condition for 

 perhaps the first three years of the infec- 

 tion, but a careful observer will notice 

 a gradual increase in the number of twigs 

 infected from year to year. The number 

 of leaves partly dwarfed by the growth 

 of the fungus on the under surface also 

 increases. This gradual increase in the 

 amount of the infection appears at first 

 to be due entirely to the greater amount 

 of wintering over fungus, but there comes 

 a time when the tree is weakened by the 

 disease, and the parasite then flourishes 

 more abundantly as a result of reduced 

 resisting power of its host. 



Trees which have become thoroughly 

 infected with the mildew are character- 

 ized by the very small amount of wood 

 growth produced during a season, and 

 the continuation of such a condition 

 results in dwarfing. Some of the younger 

 orchards in the badly infected areas have 

 been so much dwarfed as to practically 

 stand still, a condition which will result 

 in total loss if not remedied. Those trees 

 which had reached the full bearing size 

 before the advent of the mildew have not 

 been so much affected, but nevertheless 

 retarded and injured to an appreciable 

 extent. 



BY W. H. VOLCK, WATSONVILLE, CALIFORNIA 



The reduction in wood growth is also 

 accompanied by even greater injury to 

 the foliage. Mildew-attacked trees show 

 a dwarfed and crinkeled condition which 

 is quite characteristic, and must, even 



APPLE TREE ANTHRACN0S1-; 



more than the reduction of the wood 

 growth, contribute to the decline in vigor 

 of the tree. Such a condition is doubt- 

 ■less responsible for a vast reduction in 

 foliage area, greatly affects the assimi- 

 lating power of the tree, and we are 

 therefore led to the conclusion that the 

 quantity and quality of the fruit must 

 suffer to a marked degree. Just what 

 the effect on the fruit has been is not 

 fully determined. It is a matter of rec- 

 ord that size, quality and yield have dete- 



riorated in orchards now badly infected 

 with the mildew, but such deterioration 

 has also been noted where the mildew 

 could hardy be regarded as being serious 

 enough to cause the efifect. 



In the setting of the fruit the weak- 

 ening of the buds caused by the mildew 

 of the past season may reasonably be 

 assumed to have a material influence. 

 The fungus also frequently starts out 

 during the blooming period, often infest- 

 ing the stems of the young fruit. Such 

 infection may cause failure to set. After 

 setting has been assured the presence of 

 mildew on the trees must still affect the 

 fruit by reducing the assimilating or feed- 

 ing power of the tree. Reduction in 

 assimilating power may in itself be suffi- 

 cient to explain the entire effect of the 

 mildew, but it is quite probable that the 

 trees are also poisoned by substances 

 excreted by fungus. Mildewed orchards 

 are not necessarily failures from the crop 

 producing standpoint, but the condition 

 seems to be one of gradually diminishing 

 quality, which, in time, means unpro- 

 ductiveness. 



The following account is not intended 

 to be a technical description, but one 

 that will enable the reader to recognize 

 the disease and understand its methods 

 of propagation. 



Mildews are in general fungus para- 

 sites that grow over the surface of the 

 host plant, but do not penetrate the 

 tissue to a very great extent. Such pene- 

 tration as does take place consists in a 



NEWTOWN FOLIAGE, UNSPRAYED 



NEWTOWN FOLIAGE SPRAYED WITH IRON SULPHIDE 



