DISEASES OF TREES 



to the death and disappearance of the whole of the seedlings 



at certain places, blanks four inches and more in length may 



be formed in the seed-drills. 



The infectious character of the disease may be gathered 



from the peculiar way in which it is distributed. A diseased 



plant soon becomes surrounded 

 by diseased neighbours, and thus 

 the epidemic spreads centrifu- 

 gally in beds that have been sown 

 broadcast, and in two directions 

 where the seed has been drilled. 

 Should a frequented footpath lead 

 through a beech wood that is being 

 regenerated by seed, all the plants 

 growing on the path and along 

 the sides contract the disease and 

 die in a short time. It has also 

 been observed that if the disease 

 has once appeared in seed-beds it 

 usually recurs in succeeding years 

 in a much-accentuated form. The 

 disease is known to be greatly 

 favoured by rainy weather espe- 

 cially if accompanied by heat and 

 by any kind of shading, whether 

 produced by standard trees or by 

 artificial covering. The first ap- 







pearance of the disease in any year 

 ca " % * due to the oospores 

 of the parasite, which lie dormant 



. ,, .. . .' 



ln the ^ ll during winter, and 

 infect the germinating seedlings in 

 spring. The mycelium spreads in 

 the tissues of the seedling, and, 



in the case of the beech, both in the stem and in the cotyledons, 

 the latter being probably attacked as they are being pushed up 

 through the ground. In the tissues of the cotyledons the mycelium 

 is almost entirely intercellular (Fig. 15, <), withdrawing the 

 nourishment from the interior of the cells by means of small 



FIG. 15 -Cellular tissue from the 



cotyledon of a diseased beech. 



The starch-grains have been ab- 



cell -walls, a ; the mycelial fila- 

 ments, which are of varying 

 thickness, b b, grow intercellu- 

 larly, and are provided with 

 minute haustoria ; each fertilized 

 oogonium contains an oospore, c c. 



