CALIFORNIA CITRUS CULTURE. 



43 



PLANT DISEASES. 



Plant diseases may be considered under three heads : Physiological, 

 or plant sickness, fungous and insect attack. There are several ailments 

 that are wholly physiological. The plants or trees are sick. The most 

 common of these is chlorosis. 



Chlorosis, Yellow Leaf, Variegated Leaf or Mottled Leaf. 



This peculiar color is doubtless simply symptomatic. As pallor in 

 man, so chlorosis in plants, denotes disease something is wrong. As in 



FIG. 22. Die-back on orange: A, showing gum pockets; B, cross-section of a Valencia, 

 showing gum at core ; C, twig showing multiple buds and dead tips. (After Essig.) 



case of all physiological troubles the thing to do is to search for the 

 cause of the ill, and then apply the suggested remedy. In clay soils 

 with poor drainage the earth may be water logged, in which aeration 

 is impossible, and we have a dead soil. This last condition follows from 

 a hard, cemented soil, consequent upon imperfect cultivation. Artificial 

 or natural hardpan will also produce the same condition. Again the 

 plants may be thirsty and suffer from lack of moisture. The soil may 

 be impoverished and the plant cry out for food. Probably the great 

 cause is " malnutrition, " including both food and air. The artificial 

 hardpan referred to is a cemented soil, just below the plane of deepest 



