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 
Fic. 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 Bssig.) 
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 ery out for food. Probably the great 
eause is ‘‘malnutrition,’’ including both food and air. The artificial 
hardpan referred to is a cemented soil, just below the plane of deepest 
