Three Rust Diseases of the Apple 



An interesting observation was made on the natural infection of 

 Mcintosh, and Wealthy foliage in an orchard near Hudson, New York. 

 On adjacent trees, the number of spots per infected leaf, on June 27, 

 1928, was 3.7 on Wealthy and 8.7 on Mcintosh (table 5). On the 



♦Youngest leaf. 



Mcintosh foliage, however, many of the infections had produced only 

 small, pale areas, with a minute necrotic spot in the center. There was 

 practically no defoliation on this variety, and yet only an occasional spot 

 could be clearly distinguished on September 11. None of the spots had 

 developed sufficiently to produce aecia; indeed, very few spots had pro- 

 gressed sufficiently to produce pycnia. At that time the Wealthy foli- 

 age presented the symptoms of a rather severe infection. From a 

 comparison of the symptoms found here with those produced by artificial 

 inoculations (described later), it seems probable that both the apple and 

 the hawthorn rust were involved on Mcintosh foliage, with the apple 

 rust predominating. Here is apparently a case of resistance or hyper- 

 sensitiveness similar to that encountered among the cereal rusts (Stak- 

 man, 1915) and elsewhere. 



SYMPTOMS AND DIAGNOSTIC SIGNS 



Considerable attention has been given by earlier workers to the symp- 

 toms produced by all these diseases on cedars (Weimer, 1917), and by 

 apple rust on apples. It is not possible to give a complete treatment of 

 the symptoms of quince and hawthorn rusts on the apple until observa- 

 tions can be made over a wider range of varieties and conditions. We 

 shall consider here more particularly the characters which will be of 

 service in diagnosing the diseases. 



