7 o DISEASES OF TREES 



The numerous species of mildews have recently been arranged 

 in several genera according to the number of asci in the 

 cleistocarp, or according to the number of spores in the ascus, 

 or, finally, according to the structure of the so-called appendiculae, 

 which are peculiar filiform radiating processes of certain cells 

 of the wall of the perithecium. Here we need only allude to 

 a few species. 



Erysiphe (Phyllactinia] guttata forms the mildew of Fagus, 

 Carpinus y Corylus, Quercus, Betula, Alnus, Fraxinus, Lonicera^ 

 Pyrus communis, and Cratcegus. The cleistocarps are fur- 

 nished with appendiculae which are straight, unbranched, and 

 thickened in a bulbous manner at the base, and internally 

 produce several asci, each containing two spores. In beech woods 

 this parasite sometimes causes premature withering of the leaves. 



Erysiphe bicornis (Uncinula A certs) very often injures the 

 leaves and young shoots of Acer. I have encountered this 

 species most frequently on Acer platanoides and A. campestre. 

 It covers the whole of the leaf, or forms large greyish white 

 blotches on one or both sides (the black patches on these leaves 

 are due to Rhytisma acerinum). The cleistocarps possess several 

 asci holding eight spores, and the appendiculae are simply forked 

 at the apex. The gonidia are elliptical in shape. Even so early 

 as August the leaves of the maple are often completely covered 

 with these white patches. 



Erysiphe Tulasnei is closely related to the former species, 

 but occurs only on the upper side of the leaves of the Nor- 

 way maple. The gonidia are globular. Erysiphe (Uncinula\ 

 adunca produces the mildew of the leaves of willows and 

 poplars. 



Erysiphe (Sph&rotheca) pannosa forms the well - known 

 mildew on the shoots and leaves of the rose. In wet years 

 especially it is necessary promptly to pluck and burn the 

 diseased leaves. 



Oidium Tuckeri, the fungus which causes the disease of the 

 grape, was observed in England for the first time in 1845, but 

 has since spread throughout all the vine-growing countries of 

 Europe. The mycelium grows on the leaves, shoots, and fruit. 

 When .the last is attacked the epidermis dies and loses the 

 power of expansion, so that as the berry grows the epidermis is- 



