NON-PARASITIC, OR PHYSIOLOGIC PLANT DISEASES 573 



ating growth may be regarded as a preventive or remedial measure. 

 Drainage may prove to be remedial to exanthema which is only of one 

 kind while there may be several kinds of die-back. 



Mottle-leaf. — Mottle-leaf of Citrus trees is marked by the loss of 

 chlorophyll from parts of the leaf, the portions farthest removed from 

 the midrib and larger veins being first affected. As the disturbance 

 progresses, the yellowish spots increase in size until the remaining 

 chlorophyll is found in narrow areas along the midrib and larger veins. 

 The advanced stages are distinguished by a marked decrease in the 

 size, quality and yield of fruit. No organism has yet been proved 

 to be associated with mottle-leaf which is common in the groves 

 of southern California. Orchards fertilized with organic materials, 

 such as stable manure, usually showed less mottling than groves the 

 soils of which were treated with commercial fertilizers. The results 

 of soil analyses show in the case of oranges a marked inverse correla- 

 tion between the humous content of the soil and the percentage of 

 mottling, the latter tending to diminish as the humous content increases 

 and experiments show that this humus should be well decomposed. 

 It would seem, therefore, that the mottling of orange leaves in the areas 

 studied is definitely correlated with the low humous content of the 

 soil, the mottling diminishing as the humus increases.^ 



Curly-top of Sugar Beets. '^ — The curly-top of sugar beets seems to 

 have attracted the attention of growers in California about 1898. It 

 is distinguished by the following symptoms. An inward curHng of the 

 leaves, a distortion of the veins of the affected leaves, having roots and 

 checked growth. It has caused great financial loss in the beet dis- 

 tricts of the western United States. Experimental study of the disease 

 shows that the leaves of the curly-top plants have an oxidase content 

 two or three times as great as the healthy and normally developed 

 ones. It appears that an abnormal retardation of growth in sugar 

 beet plants is accompanied by an increase in the concentration of 

 oxidases in the leaves or a change in the juice of the latter by which 

 the pyrogallol oxidizing oxidase becomes more active. 



Peach Yellows. — This disease which according to the early records 



1 Briggs, Lymax J., Jensen, C. A. and McLane, J. W.: Mottle-leaf of Citrus 

 Trees in Relation to Soil Conditions. Journ. Agric. Res. 6: 721-739, pis. 3, 1916. 



2 BuNZEL, Herbert H.: A Biochemical Study of the Curly-top of Sugar Beets, 

 Bull. 277, U. S. Bureau of Plant Industry, 1913. 



