84 PLANT DISEASES 



perennial mycelium, and to the infection of healthy 

 shoots by spores pioduced on the diseased portions. In 

 those years when the disease is especially abundant, it is 

 not unusual to find the trees completely divested of leaves 

 before midsummer. Later in the season a second crop 

 of leaves appear, and these are not attacked by the fungus ; 

 nevertheless a considerable amount of damage is done, 

 the crop of fruit in such cases is practically lost, and in 

 the autumn it is found that the wood is imperfectly 

 matured. 



Preventive Means. — This is a disease which, if sys- 

 tematically and persistently attacked, could be well kept 

 in check. Diseased fallen leaves should be collected 

 and burned ; or better, those branches bearing diseased 

 leaves should be pruned back beyond the point of infec- 

 tion, and thus get rid of the perennial mycelium, which, 

 if allowed to remain, follows the developing shoots year 

 by year. There should be no hesitation about the pruning, 

 as when a branch is once attacked, all beyond the point 

 of infection is diseased, and does not produce fruit, but 

 only fungus spores, which assist in spreading the disease. 

 I have proved by repeated observations that the mycelium 

 does not travel backwards from the point of infection, but 

 forwards ; in other words, it follows the new growth. 



The disease is least abundant, or often entirely absent, 

 in those comparatively rare instances where a genial 

 spring has allowed the foliage to quickly attain its full 

 growth without a check, whereas it is rampant when the 

 leaves have commenced growth under favourable auspices, 

 and are afterwards checked by a sudden fall in tempera- 

 ture. During the temporary cessation of growth of the 

 leaf, the mycelium of the fungus spreads rapidly in its 



