Some Important Pear Diseases. 29X 



and they never extend very far into the pulp of the pear. They 

 often penetrate the cells of the epidermis, deriving their nutriment 

 entirely from these outer layers. The erect threads /, figure 169, 

 arise from this mass of threads, break the cuticle, and produce the 

 spores as seen at s. Later, the epidermis is ruptured, and then we 

 find that a definite corky layer has been formed by the cells of the 

 fruit below the diseased area, figure 169, /'. 



Figure 170 shows how these spores germinate. After falhng upon 

 the leaf or the fruit, with suitable conditions, the spore absorbs water, 

 and pushes out a little thread, or germ tube. I'his germ tube has the 

 power of passing in through the cuticle and epidermis, where it 

 branches greatly and develops the thread-like mass of fungous hyphse, 

 and soon again the scab spot is seen on the surface. 



(^. ) How the fungus passes the winter. — It has been seen that pear 

 scab often attacks the twigs of the first year. The fungous threads are 

 extremely resistant, and buried in the bark of the twigs it is generally 

 admitted that the disease may thus pass the winter, producing the 

 following spring a crop of spores to reinfect the young branches and 

 leaves, as well as the fruit clusters. It is also believed that the fungus 

 may pass the winter in the diseased fruit and leaves. In Germany a 

 winter stage of the fungus has been found belonging to the genus 

 Venturia. 



In artificial cultures on bean stems and other nutrient media, I have 

 grown for some time the fungi of apple and pear scab. These cultures 

 have yet given no indication of this other fruiting stage, or winter 

 form of the fungus. 



{c.) Does pear scab differ from apple scab ? — It is well known that 

 pear scab differs from apple scab in some particulars; but some have 

 claimed that these differences are so small as not to denote that the 

 fungi are distinct. However, it is of considerable practical importance 

 to know that some recent work tends to show that these fungi are dis- 

 tinct species ; hence, if this is true, pear scab cannot spread to the 

 apple, and there cause apple scab, or vice versa. 



3. Varieties Affected. 



Scab affects to a greater or less extent a number of the varieties 

 commonly grown in New York. The data upon this subject are lim- 

 ited, but it is generally reported that Le Conte, Kieffer and Barilett 



