56 



Downy Mildews, induced by the fungus Plasmopora viticola. 

 Both these fungi have been recorded for Britain, but the latter 

 is exceedingly rare and is not dealt with in this leaflet. 



The Powdery Mildew fungus is now believed to be a native 

 of Eastern Asia. It was first observed in Europe in 1845; 

 appearing in England near Margate that year, and very soon 

 after in the vineyards of France, Spain and Italy, where it 

 spread rapidly and caused enormous devastation. Since then it 

 has spread to practically all the large vine-growing countries 

 and has caused much damage to cultivated vines both in the 

 Old and New Worlds. 



Powdery Mildew is the common vine mildew familiar to 

 every vine grower in England as giving trouble both under 

 glass and in the open. It caD now be completely controlled 

 under glass, and with due care and attention can be very 

 largely held in check on outdoor vines. 



Description. The appearance of the disease being so 

 well known hardly requires description. The fungus attacks 

 the young shoots forming white patches of fungus threads 

 (mycelium) on young leaves and shoots. As in the case of 

 other Powdery Mildews the mycelium is superficial, and merely 

 sends down minute suckers in order to extract nourishment. 

 In spite of this, however, great damage is done, and the foliage 

 is often crippled and sometimes killed. On this same mycelium 

 spores are produced in enormous numbers. They are exceed- 

 ingly minute and are readily blown about. They occur in such 

 profusion that the masses of spores are visible on the leaf as a. 

 white mealy powder. 



In addition to the foliage the fungus also attacks the flowers 

 and fruits. This causes the fall of the flowers and also of the 

 fruits if still quite young. If the fruits are attacked at a 

 later stage the fungus may cause distortion and cracking of 

 the berries, thus rendering them useless. No new infection 

 takes place after the grapes have begun to ripen. 



The mildew usually commences about the time the grapes 

 are in flower, and continues to spread through the whole 

 growing season. In America and certain other countries it 

 forms resting spores in autumn which provide for the fresh 

 outbreak of the Mildew in spring, but in Europe this form 

 of over-wintering is practically non-existent and the fungus 

 hibernates by means of mycelium in the buds. 



Measures of Control 1. Outside. Where vines are grown 



extensively in the open the selection of a well-drained site and 

 of one in which the vines have abundance of sun and air will do 

 much to minimise mildew attacks. Another point of great im- 

 portance is to plant the vines well apart and to pinch the 

 laterals back carefully in summer to prevent crowding. 



Both on the Continent and in America Powdery Mildew 

 in the open is largely controlled by the use of flowers of 



