— 34 — 



nourishment which exists within the cells of the host, the mycelium gives off spe- 

 cialized branches known as haustoria. These penetrate the cells and function as 

 absorbing organs. There are two types of haustoria, in some genera they are 

 button shaped, in others they are branched and filamentous. 



The conidiophores are borne by the mycelium in the neighbourhood of the 

 stomata. They pass out through the stomata singly or in small groups, giving a 

 characteristic downy appearance to the infected areas, from which the popular 

 name of these fungi is derived. The conidiophores are branched, the method of 

 branching serving as a convenient means of generic separation. Conidia or sum- 

 mer spores are produced abundantly on the ends of the branches of the conidio- 

 phores. In shape they are typically ovate to oval and their length averages about 

 I-I200 of an inch. They are easily disseminated by wind or water, and it is by 

 these means that the fungi are able to spread so rapidly during the summer 

 months. 



Oospores are produced by most species within the tissues of the host plant. 

 These resting spores are enclosed within two thick walls. The inner one is smooth 

 and light coloured and is known as the endosporium. The outer coat or exosporium 

 is darker in colour and rough on the outer surface. The oospores result from sex- 

 ual fusion and are useful in tiding the fungus over the winter, or other unfavour- 

 able conditions. 



Germination of Spores. — When a conidium falls on the surface of a leaf, 

 if conditions are favourable it may germinate in one of two ways, depending on 

 the genus to which it belongs. The first method of germination is by sending out 

 a germ tube directly. This tube enters the tissue of the host through a stoma, 

 and develops into a new fungus-plant. In some genera, instead of germinating 

 directly, the conidia give rise to minute biciliate zoospores or swarm spores. These 

 swim around in a drop of water for a few hours, then become stationary and 

 send out a germ tube which, entering the tissue of the host through the stoma, 

 develops into the intercellular mycelium. In Phytophthora (Late blight of Potato) 

 both methods of germination have been observed. 



Effect on the Host. — These fungi attack the leaf, stem, fruit and even the 

 underground parts of a large number of widely separate plants. On the leaf they 

 are characterized by brownish or yellowish discolorations, more or less irregular 

 in outline bearing the downy conidiophores on the underside. In moist weather 

 these spots usually spread rapidly until the entire leaf is affected. Except in a 

 very bad attack, the host is not killed. The leaves however may die and fall off, 

 thus retarding the normal development of the plant. In cases where the host is 

 wild, the adaptation between ii and the parasite in such that little or no injury 

 results from the attack. 



Conditions favouring or retarding the development of the fungus. — The 

 climatal conditions favouring ^he development of the parasites are high tempera- 

 ture and humidity; and conversely, a dry cool climate is unfavourable. Berlese, 

 in his monograph on the Peronosparaceae, states that there is little development 

 below 200C. (68<*F.) and that the optimum temperature lies between 20°C. and 

 300C. (68«F. and 86^F.) 



General Methods of Control. — Wherever practicable, spraying with Bor- 

 deaux mixture will effectively control the downy mildews. It is interesting to 

 note that the fungicidal value of copper sulphate was first discovered in connec- 

 tion with the downy mildew of the grape. The vine growers of Bordeaux, France, 



