344 GENERAL SCIENCE 



burning the diseased fruit as soon as it appears and by 

 spraying early with Bordeaux mixture and later with 

 lime-sulphur (Figure 305). 



Pear Blight (Fire Blight) . Pear blight attacks pear 

 trees, apple trees, and occasionally plum trees. It is 

 caused by bacteria which live in the cambium layer just 

 under the coarse bark. The first symptom of the disease 

 is the death of the tips of the tender twigs. The leaves 

 turn yellow and then dark brown, and the tree seems to 

 be dying from the top down. In driving along the road 

 one may often see orchards affected by this disease. If 

 neglected, the disease will spread down the tree and to 

 other trees, gradually destroying the whole orchard. 



The diseased limbs should be removed and burned as 

 soon as they are detected, but something more than this 

 is usually necessary to eradicate the disease. The bac- 

 teria move down the tree and form cankers on the large 

 limbs and on the body of the tree. Here they pass the 

 winter. In the spring these cankers exude a sticky 

 liquid containing large numbers of the bacteria which 

 are carried to the flowers and other parts of the tree by 

 insects. The logical point of attack then is the canker 

 during the winter months. Remove all cankers and 

 scrape the diseased parts. Wash the wound with a weak 

 solution of corrosive sublimate, one part to five hundred 

 of water. In a day or two the wound should be painted 

 with lead and oil. 



Mildews. - - The downy mildews comprise a group 

 of fungi which have been very destructive to cultivated 

 crops. Probably the most destructive mildew is that 

 of the potato, sometimes called late blight. The mildew 

 spores are always present in the atmosphere in the summer 

 time, and if they alight on the potato leaves when the 



