LEAF AND SEEDLING DISEASES 179 



willows and poplars the rust is generally very conspicuous. 

 Microscopically members of the genus are distinguished by 

 the large club-shaped paraphyses which grow among the 

 uredospores. The uredosorus has no peridium such as is 

 found in Melampsoridium. 



Like Melampsoridium, Melampsora causes very little 

 damage to the larch, except where it becomes epidemic in 

 a nursery (see W. G. Smith, 1886). When found it will be 

 difficult to identify, and if the disease is sufficiently harmful 

 to justify special measures, it will be well, as far as possible, 

 to remove poplars and willows from the neighbourhood of 

 the nursery. 



The damping-oft' of larch seedlings. There are two known 

 fungi which cause the damping-off of larch seedlings. It is 

 not easy to distinguish between the two diseases with the 

 unaided eye, but the conidiophores which grow from the 

 diseased portions are readily identified under the micro- 

 scope. Fig. 73 shows the conidia of the two species side 

 by side. 



Phytopkthora omnivora, de Bary (=PJi. fagi, R. Hartig), 

 attacks the seedlings of all conifers and of the beech. It is 

 very closely allied to Phytophthora infestans, the fungus 

 that causes the best known potato disease. The life-history 

 of the fungus is somewhat complex, and will only be briefly 

 described. The conidia shown in fig. 73, A, are borne on 

 conidiophores which either grow through the stomata of 

 attacked portions of the seedling or break through the 

 epidermal wall and cuticle. The conidia fall on to the 

 ground and, instead of germinating in the usual way, the 

 contents divide up into a number of parts, each of which 

 becomes a swarm-cell, which escapes and swims about in 

 the films of water contained in the soil. For a cell which 

 produces these swarm-cells the word * conidium ' is a mis- 

 nomer, and the term sporangium is generally used, and will 

 be employed below. As the swarm-spore is known to 

 botanists as a zoospore or zooconidium, the sporangium is 

 also called a zoosporangium or zooconidangium. If a 

 swarm- spore, while swimming about, comes into contact 



N 2 



