APPLE DISEASES 



487 



vicinity of stands of infected cedars and 

 native pome fruits. 



The examination of the orchard men- 

 tioned by Mr. Zepp in his letter as quoted 

 revealed a very interesting situation. Not 

 over 100 feet from the nearest pear tree 

 stand half a dozen incense cedar trees of 

 various ages, from those but a few years 

 old to those probably 50 years old or 

 more. A careful examination of these 

 trees showed that practically the entire 

 foliage was covered with the telial stage. 

 The readiness with which the nearby cul- 

 tivated pome fruits, as well as the cedars, 

 were infected might easily be understood, 

 since the heavy wind currents, moving 

 up and down the small canyon in which 

 the trees are growing, easily carried the 

 spores. 



During the course of the writer's 

 studies inoculation experiments were 

 taken up and it was shown that practical- 

 ly all pome fruits could be infected by 

 this rust. 



Hosts 



The hosts upon which the fruiting stage 

 of this rust have been produced by inocu- 

 lation from the incense cedar, are as fol- 

 lows: 



Apple, flowering crab, pear, mountain 



ash. native crab apple, quince, Japan 



quince, serviceberry, thorn apple or haw. 



P. J. O'Gara 



Scab 



Venturia pomi 



H. S. Jackson 



Apple scab is the most serious and most 

 generally distributed fungous disease of 

 the apple known, and in the Northwest 

 during favorable seasons, west of the Cas- 

 cade mountains, is particularly severe. 



Symptoms 



Apple scab attacks both foliage and 

 fruit. On the foliage the spots are at first 

 more or less circular in outline, olive 

 green or brown in color, becoming darker 

 and more irregular in shape as they be- 

 come mature. The leaves are frequently 

 more or less curled or wrinkled. When 

 the spots are abundant, the leaves fall 

 prematurely and considerable defoliation 

 may thus take place when infestation is 



abundant. This may result in a failure 

 of the fruit buds to develop normally and 

 so affect the amount of the crop the fol- 

 lowing year. 



On the fruit the fungus produces more 

 or less circular spots of a greenish-black 

 color. The vegetative stage of the fungus 

 causing the disease develops under the 

 cuticle of the apple fruit, finally ruptur- 

 ing it by the elongation of the threads 

 which bear the spores. The ruptured 

 cuticle may frequently be seen clinging as 

 whitish membranous shreds about the 

 edge of recently developed spots. As the 

 spots become older, all trace of the fungus 

 may become obliterated and the only evi- 

 dence of the former spot is seen in a large 

 or small, rough, russeted spot. Fre- 

 quently the fruit is distorted when mature 

 as the result of early scab infections. 

 Where scab spots are abundant the fruit 

 may become cracked. Scab in an.v degree 

 of severity on the fruit renders it un- 

 sightly and unmarketable as fancy fruit. 



In the spring of 1912 at Corvallis, apple 

 scab was observed to develop abundantly 

 while the trees were in blossom on the 

 sepals, petals and ovaries, as shown in 

 Fig: 1. 



Cause 



As noted above, apple scab is caused by 

 a parasitic fungus. The technical name 

 of this fungus is Venturia pomi. Two 

 distinct phases are known, the conidial or 

 summer spore stage and the sexual or 

 ascus spore stage. The summer spore 

 stage develops on both foliage and fruit 



Fig. 1. 



Apple Scab on Blossoms. Note spots 

 on petals, sepals and ovaries. 



