'hop-mould' 751 



branches grow up from them, each of which gives rise to a 

 chain of oval conidia (Fig. 254). The latter become free from 

 each other and give the mould-spot a mealy appearance. 



Many of the conidia are carried by the wind to neighbouring 

 plants where they germinate in a few hours and produce germ- 

 tubes which penetrate into the epidermal cells of the leaves, and 

 subsequently give rise to new mycelia thereon. As thousands 

 of conidia are produced by a single mycelium, and each of 

 them is capable of producing a new mould-spot when climatic 

 conditions are favourable for their distribution and germination, 

 it will readily be understood how quickly and silently the disease 

 can extend through a hop-garden. It is in this manner that 

 the parasite is propagated during summer. 



The mycelium and its conidia are short-lived and cannot 

 exist through winter. In late summer and autumn or earlier, 

 if the affected leaves are weak, the fungus produces on its 

 mycelium small round closed ascocarps, designated perithecia, 

 which are often visible to the naked eye as minute black points 

 where a mould-spot has been. These are dark brown in colour 

 and formed of a network of cells, some of which grow out into 

 long unbranched and hair-like appendages (h, Fig. 255). 



According to some authorities they are the result of a fertilisa- 

 tion process between two crossing hyphae. 



Within and protected by the outer brown wall of the peri- 

 thecium is a single transparent oval sporangium or ascus 

 containing eight ascospores. The latter are oval and similar 

 in size to the conidia, but instead of germinating as soon as 

 they are produced, they rest during winter in their asci and 

 perithecia, after falling to the ground with the dead leaves and 

 affected hop-strobiles. In bad attacks where the bines have lain 

 or where diseased leaves and strobiles have fallen, the surface 

 of the soil is strewn with large numbers of ripe perithecia. 

 In the following spring the asci absorb water and burst their 

 walls and those of the enclosing perithecium, the ascospores 



