344 Apple hihI Pear Scab Fuiuji 



exactly similar iiiannor as when attackinij; niatiiiv 1(Mvos as above 

 described. 



Although great ditforences occur between tlu' attacks on certain 

 varieties in successive seasons, yet there are some varieties which 

 always remain highly resistant, if not completely immune, to this 

 disease. It was therefore interesting to determin(> the fate of the 

 fungus on the young leaves of resistant varieties. For this. Hrandey's 

 Seedling and Newton Wonder apples were selected as resistant varieties 

 for ]'. inacquaJix. with Cap of Liberty, a well-kiunvn cider apple, as the 

 susceptible variety for comparison: also the pear William's Bon 

 Chretien, a variety susceptible to 1'. pirina, was infected, being regarded 

 as resistant to V. lnacqu<tJi'<. All were inoculated with conidia of 

 r. iuaequali^ obtained from the leaves of the "Cap of Liberty,'' and 

 the results were very similar. Appressoria were formed as usual, 

 infection hyphae sent out into the cuticle, but in resistant varieties the 

 adverse elYect of the host was quickly felt (Figs. 23, 24, 25) although 

 sometimes the fungus succeeded in penetrating completely through the 

 cuticle and forming a subcuticular pad. This usually appeared quite 

 unhealthy, the cells being smaller and narrower. The cuticle being so 

 thin no horizontal development of the mycelium in it could be detected 

 except in a few cases. Inoculations of V. pirina also were made on 

 the fruit of the apple. King of the Pippins, w^hich is susceptible to 

 T'. iuaequalis, but in no case was the infecting hyphae seen not to have 

 penetrated into the cuticle. The apple fungus easily penetrated into 

 the cuticle of the pear and the pear fungus that of the apple. On 

 Brandey's Seedling the fungus exhibited a preference for that portion 

 of the cuticle above the junctures of the epidermal cells already referred 

 to (Fig. 25). Eventually in the resistant types the mycelium ceased 

 to grow and practically died ollt. With the susceptible types on the 

 other hand a healthy subcuticular mycelium was quickly formed and 

 developed in the normal way described above until the production of 

 conidia resulted. 



General Considerations, 



The problems of immunity are particularly dithcult because in 

 dealing with them we have to deal with fundaniental life processes 

 which do not lend themselves to experimental conditions. All the 

 work which has been done towards the elucidation of the limiting 

 factors of immunity has been done with obligate parasites such as the 

 Mildews and the Rusts, but unfortunately none of these fungi will 



