10 



after one or two freezings. It is only in the scabbed leaves 

 that fall to the ground that the scab fungus can continue its 

 development producing the ascospores already described. 

 With this brief statement of the life cycle of the scab fungus 

 in mind we are prepared to consider the causes of its epi- 

 demic appearance and the question of control. , , , 

 demic appearance and the question of control. 



The Ecology of the Disease. ,The conjunctive appear- 

 ance of two primary factors is responsible for the epidemic 

 occurrence of this malad^^ These factors are, (1) an abun- 

 dant production of ascospores in the old leaves on th3 

 ground and (2) the occurrence of extended rain periods 

 during the time of ascospore maturity. The abundant pro- 

 duction of ascospores is primarily dependent upon the gen- 

 eral occurrence of the scab on the foliage the previous sea- 

 son though a severe infestation is not requisite. A single 

 scab spot on a leaf may give rise to hundreds of ascospor-i 

 cases (perithecia) throughout the tissues of the scabbed 

 leaf which falls to the ground. In the spring of 1911 in 

 New York there was an abundance of ascospores in the old 

 leaves on the ground, but no scab epidemic appeared as 

 there was no rain sufficient for spore discharge and infection. 

 In the spring of 1912 the situation was reversed, due' to the 

 almost complete absence of scab during the summer of 1911 

 there were no ascopores in the old leaves. In the spring of 

 1912 weather conditions, however, were ideal for infection 

 but of course no epidemic was possible. Sufficient infection 

 did occur, however, so that reports of a little scab in most 

 sections are recorded. Whether this was sufficient to af- 

 ford the requisite quantity of ascospores for a severe at- 

 tack next spring (1913) remains to be seen. Weather con- 

 ditions are likely to be favorable as we appear now to be 

 going into a series of wet seasons. 



Control. The methods to be employed in the control of 

 any disease of plants involve one of four basic principles, 

 exclusion, eradication, protection from, or immunization to, 

 the causal factor. The very general distribution of the ap- 



