60 Diseases of Economic Plants 



The relation between the apple and its parasitic fungi 

 seems to vary, however, within more restricted areas. This 

 may be illustrated as follows: The practice in the eastern 

 United States has been to apply the first spray for apple 

 scab just before the blossoms open, when the flowers in the 

 cluster have separated from each other, and the pink petals 

 are showing. (This is called the "pink," or "cluster-bud," 

 spray.) Wallace, ^^ after studying the period of ascospore dis- 

 charge, stated, in 1913, that there seems to be little danger in 

 New York that infections of apple scab will occur much 

 earlier than blossoming time, and that the cluster-bud spray 

 is sufficiently early. Childs,^"^ as a result of similar study, 

 later showed that, in Oregon, two applications must be made 

 before the bloom, the first coming when the leaves are quite 

 small, followed by the "cluster-bud" spray. Since then 

 various spraying schedules have been worked out experi- 

 mentally in the East. Thus, Blair ''^'^ in Nova Scotia, recom- 

 mends two applications before the bloom, while Morse's 

 work ^^^ in Maine may be taken to show that in the locality 

 from which his data were drawn little or no infection occurs 

 before the blossoming period. 



The relation differs not only with the locality but also with 

 the variety of apple. Varietal susceptibility to or immunity 

 from attack by one fungus is not necessarily correlated with 

 the same relation toward another. This relationship seems 

 to require local determination. The most susceptible va- 

 rieties demand, of course, the most thorough treatment. 

 The subject of varietal susceptibility to apple scab has been 

 summarized by Wallace. "^^ (1) Certain varieties may appear 

 to be immune in one season but may be very susceptible in 

 another season, under different weather conditions. (2) A 

 variety may be resistant in one year and susceptible in an- 

 other year under conditions which for average varieties are as 

 favorable to the disease in the one case as in the other. 



(3) Varietal susceptibility may differ in different localities. 



(4) Only in rare cases is a variety consistently and relatively 

 immune over as short a period as five years. The apparent 



