434 



JOURNAL OF THE EOYAL HORTICULTURAL SOCIETY. 



ON A FUNGUS DISEASE OF EUONYMUS JAPONICUS 



LINN. F. 



By Eknest S. Salmon, F.L.S., F.R.H.S. 



Within the last five or six years a fungus disease has appeared on 

 Euonymus japonicus Linn, f ., a shrub extensively planted in the South 

 of England, especially on the sea-coast. In the last-named situation it 

 is an especially valuable shrub, on account of the peculiarity it possesses 

 of withstanding the salt in the air, and on this account it is used 

 extensively either for the making of borders or hedges in public gardens, 

 &c, or trained against walls like a creeper. 



The effects of the disease caused by the fungus soon become apparent, 

 with the result that the use of the shrub as an ornamental plant is 

 destroyed. The fungus causing the disease is a white mildew, known as 

 Oidium Euonymi-japoniccB (Arc.) Sacc. In the early stages of the 

 disease, the leaves of the affected plants bear small isolated, rounded, or 

 irregularly shaped white patches (see Fig. 139). The patches are composed 

 of interwoven branched thread-like hyphce, which collectively form the 

 mycelium, or vegetative part of the fungus. Towards the centre the 

 hypha? are often so numerous and so densely interwoven that the patch 

 presents a sub-crustaceous appearance ; towards the edge of each patch 

 the mycelium is very thin and filmy, and is composed of young, very fine, 

 branched hyphae radiating outwards. From the under surface of the 

 hyptue a great number of minute suckers are produced. These suckers, 

 or haustoria, are formed at short intervals by each hypha from minute 

 lateral swellings, the appressoiia, which have a lobed outline (see 

 Fig. 140, 6 and 5a). From each appressorium a minute tubular projection 

 grows downwards, pierces the cuticle of the epidermis, and swells out 

 inside the epidermal cell, forming a rounded vesicle, the haustorium 

 proper (Fig. 140, 5h). It is by means of these haustoria that the fungus 

 maintains its parasitic life, the whole of its food being absorbed by the 

 haustoria from the cell-contents of its host-plant. 



As soon as suitable climatic conditions prevail, the mildew begins to 

 form its spores. Short upright branches, the conidiophores, are produced 

 on the upper surface of the hyphae. Each conidiophore, when mature, 

 cuts off from its apex one or more spores, or conidia. The conidia are 

 white in colour and somewhat variable in shape ; they are usually 

 narrowly elliptic to cylindric, with rounded ends, and measure 

 30-38 x 13 14 // ; occasionally they are oblong, and about 30 x 14-15 ^ ; 

 or rarely they are oval, and measure 27 x 13 /u (see Fig. 140, 4). The conidio- 

 phores of the fungus gathered in the open show usually only one or two 

 conidia at the apex ; but if the fungus is cultivated on leaves surrounded 

 by a damp atmosphere, a chain of conidia, six or more in number, is 

 produced (Fig. 140, 3). The conidiophores are very quickly formed, so that 

 at the end of a few weeks an actively growing patch of mycelium bears 



