96 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 



shoots in early summer would undoubtedly prevent the spread 

 of the mildew, as hibernation only takes place by means of 

 infected buds. Later on in the summer this method would be 

 of much less value, and might, indeed, lead to increased infection, 

 as the host would be stimulated to produce fresh shoots which 

 are particularly liable to attack. It has been shown that shaded 

 trees remain free from attack, while those exposed to the sun 

 are severely infected ; and it has been suggested that seedlings 

 and young plants of the oak in the nursery could be protected 

 by growing some tall, herbaceous plant between them, and in the 

 plantation by the judicious use of nurses. These would provide 

 the requisite amount of shade in the earlier stages, and would 

 also hinder the free dissemination of conidia by the wind. 



It may be hoped that, as in the case of the majority of 

 epidemics, the oak mildew after attaining its maximum virulence 

 will diminish in intensity. At present, however, there is 

 unfortunately little evidence of any marked diminution of the 

 disease. 



After the completion of the above article, the following notes 

 on the co-operation between insects and fungi in certain kinds 

 of damage to trees were communicated to me by Dr Munro of 

 the Forestry Commission. These are of importance as they 

 describe actual observations made in this country on the 

 relationship between the attacks of oak mildew and certain 

 insects, and it is interesting to note that they agree with the 

 findings expressed above, which were based on observations 

 made by continental investigators. 



I desire to express my thanks to Dr Munro for the use of 

 these notes. 



" Oak mildew as a host of a Cecidomyid fiy. — In 1920, at Kew, 

 my attention was drawn to a Cecidomyid larva on Melampsora 

 on poplars, and in the same year, at Blakeney in the Forest of 

 Dean, I observed a similar larva on the oak mildew, which is 

 severe there. I did not rear the larvae in either case, but I 

 think it is highly probable that they can be referred to 

 Mycodiplosis. 



" Oak mildew complementary to Tortrix viridana attacks. — Oaks 

 are rarely if ever killed or hopelessly injured by the attacks of 

 the oak tortrix {T. viridana), but in 1920 it was apparent in the 

 Forest of Dean that a caterpillar attack followed by an attack of 



