THE FUNGI : ASCOMYCETES AND BASIDIOMYCETES 253 



which is often dark coloured. The sex organs are enclosed within a tissue 

 termed the perithecium, which is without an ostiole. The asci are formed 

 together in a group from a regular layer in the base of the perithecium. 



Many of them cause the White or Powdery Mildews or Blights on flower- 

 ing plants, where they live as parasites on the leaves. They are almost 

 cosmopolitan in distribution, but have been most commonly found in Europe 

 and North America. 



We shall consider two examples of this order, Sphaerotheca humuli and 

 Erysiphe polygoni because of the marked difference in the methods of sexual 

 reproduction. 



Sphaerotheca humuli (The Hop Mildew) 



This Fungus is the cause of Hop Mildew or Mould and is one of the 

 most serious diseases of the Hop. It chiefly affects the leaves and the young 

 strobili, rendering the latter useless. The intensity of the disease varies 

 from season to season and is best controlled by dusting the hop bines with 

 finely powdered sulphur. 



The life-cycle begins with the germination of either the asexual conidio- 

 spore or the sexual ascospore upon the surface of the host plant. A series 

 of hyphae are formed which spread over the surface of the leaf, producing 

 a white web-like coating with uninucleate cells. From these hyphae slender 

 branches penetrate the cuticle and grow down into the epidermal cells, form- 

 ing ramifying branches within the cells by which food is absorbed. These 

 special cells are termed haustoria, and are the only part of the Fungus which 

 enters into the host plant. Penetration by the haustoria is secured in a 

 peculiar manner. The mycelial hyphae send out short branches which 

 apply themselves very closely to the epidermis of the host and spread out 

 into an adhesive disc called an appressorium. From the centre of this 

 disc appears a new and extremely fine outgrowth which pushes through the 

 cuticle like a needle-point. On entering the cell this outgrowth enlarges 

 into a normal-sized hypha and branches freely. 



Asexual Reproduction 



During the summer months reproduction is asexual by means of conidio- 

 spores. The conidiophores are short, upright, unbranched hyphae which 

 abstrict from their upper ends single chains of large oval conidiospores 

 (Fig. 245). 



Sexual Reproduction 



Sexual reproduction in Sphaerotheca humuli occurs in the late summer 

 by the formation of perithecia, which are spherical structures 0-05 to 0-3 mm. 

 in diameter and furnished with simple appendages. The perithecium is 

 formed by a mass of hyphae which aggregate into a tiny ball. It is white at 

 first, but as it matures it turns clear yellow, and finally brown. 



