DISEASES OF PLANTS 



249 



Apricots and prunes are most commonly affected. The 

 trouble seems to be due to weather conditions, especially to 

 warm weather in early spring followed by cold wet weather. 

 The sudden checking of the activity of the tree seems to 

 produce a fermentation and sour- 

 ing of the sap. The trouble is 

 not caused by fungi. 



Puffy oranges. This affects es- 

 pecially the navel orange in cer- 

 tain seasons. The skin is puffy 

 and spongy. The pulp is worth- 

 less. The cause is not known, 

 but it is not due to a fungus. 



Die back. Trees often begin 

 to die from the top. The smaller 

 branches are usually affected first 

 and then the larger ones. Fruit 

 and nut trees are often affected 

 in this way. The trouble is some- 

 times due to crown gall (page 245) , 

 root rot (page 248) , or the attacks 

 of insects or gophers. But in other cases it is caused by the 

 roots meeting hardpan or alkali or some other unfavorable 

 condition. Unusually warm weather in fall or winter may 

 also cause it. 



Chlorosis. White or pale spots on the leaves, sometimes 

 extending over the whole leaf, are caused by a variety of 

 unfavorable soil conditions, including alkali and excess of 

 lime. They are also produced by other causes. 



Many other diseases, such as the " Anaheim disease " and 

 root rot of grapes, cannot be described for lack of space. 



Directions for making sprays are given on page 412. 



FIG. 149. Club foot of turnip. 



