MILDEW ON GRAPES : CAUSES OF 



103 



This mildew appears to the naked eye like a little white powder 

 only, resting on the leaves, etc., but by the aid of the magnifying 

 glass it is seen to be a true vegetable parasitical growth, as we see it 

 represented by the accompanying figure. It is a most insidious 

 enemy and requires extreme watchfulness, so as to observe its very 

 earliest appearance in order to check its progress. It vegetates very 

 rapidly ; from a small speck it will, in the course of a few days, spread 

 over an entire house, and if not arrested in its growth, its threads will 

 have penetrated so deeply into the tissues of the affected parts as to 

 completely destroy them. The mildew itself may be arrested and 

 killed, but its effects remain, the skin or cuticle of the berry being 

 blackened and injured beyond recovery. It seems to render the berry 

 incapable of distending further, so that it soon splits open, and is, of 

 course, ruined. The tissues of the leaves are also injured in much 

 the same way. 



Fig. 40. a, MILDEW OF GRAPES ; b, OIDIUM TUCKERI WITH CONIDIA 

 GERMINATING ( x 200 dia.). 



As to the causes of the Vine mildew, they are, like those of most 

 other diseases, very difficult to trace. It is sufficient that it does 

 exist. Certain atmospherical conditions are favourable to its develop- 

 ment, as to that of all fungoid growth. There is no more fertile 

 source than cold, damp, sunless weather, with a stagnant atmosphere, 

 and especially if this is succeeded by bright sunshine. Of Vines 

 grown in the open air, there is seldom a season in which they are 

 not affected to some extent, but frequently it occurs so late in the 

 season as practically to do but little harm. 



The prevention of mildew ought, if possible, to be the chief 

 endeavour of all Vine-growers ; and in houses or vineries its inroads 

 may almost be prevented. In the open air, it is much more difficult 



