Aug. 7> 1916 Mottle-Leaf of Citrus Trees 725 



obstacle in irrigation and gives rise to a droughty condition in the areas 

 affected. The association of mottling with an inadequate soil-moisture 

 supply appears in some instances to be clearly indicated. The relation- 

 ship, so far as plowsole is concerned, is, however, complicated by the fact 

 that plowsole is often, though not necessarily, associated with a low 

 humus content. 



RELATION OF MOTTLING TO YIELD 



Badly mottled trees produce smaller fruits and a smaller number of 

 fruits per tree than trees not mottled, and severely mottled branches pro- 

 duce less fruit buds. A slight mottling of the leaves does not appear to 

 have any serious effect on the yield of fruit. The results of the field 

 observations indicate that if less than 20 per cent of the leaves show mot- 

 tling, the yield is not measurably decreased. The yields of oranges and 

 lemons in the groves studied were obtained in most cases as far back as 

 1907, but the freezes of 1912 and 1913 proved such a disturbing factor 

 both as regards yield and tree condition that it was not found possible 

 to establish any relation between the yield and the mottling as deter- 

 mined in 1914. 



SUSCEPTIBILITY OF DIFFERENT CITRUS TREES TO LEAF MOTTLING 



Mixed groves of lemons and oranges were not found in the field survey, 

 so that the relative mottling of the two species under the same cultural 

 conditions could not be directly determined. From indirect compari- 

 son there appears to be no great difference in this respect. Grape- 

 fruit and tangerine mottle readily, but no opportunity was presented for 

 a direct comparison with lemon or orange trees. There are few tanger- 

 ines produced in the areas studied. 



There seemed to be no difference between the Washington Navel and 

 the Thompson Improved Navel so far as susceptibility to mottling was 

 concerned. Where differences in mottling were found, it was also found 

 that the two varieties were on different stocks. One mixed grove of 

 Washington Navels and Valencias was studied, in which the two varie- 

 ties were alternated in the same row, so that the conditions were the same 

 for each. In this case both varieties were equally mottled. 



RESULTS OF THE SOIL ANALYSES 

 METHODS OF SOIL ANALYSIS EMPLOYED 



The total carbon was determined by boiling 20 gm. of soil with 50 or 

 75 c. c. of a mixture of sulphuric acid and potassium bichromate, using 

 the larger amount with soils containing more than the average amount of 

 organic matter. The acid mixture was made up in the proportion of 

 120 gm. of the bichromate to 1,000 c. c. of concentrated sulphuric acid. 

 The carbon dioxid was absorbed in N% sodium hydrate in a bead tower 

 and the whole of the hydrate solution removed and titrated. 



