132 STATE BOARD OF HORTICULTURE. 



add soluble iron compounds, where the soil is deficient in these 

 ingredients, or where the iron is not in a form for root-absorp- 

 tion. Some soils are certainly deficient in soluble iron, and no 

 amount of nitrates, ammonia salts, etc., will replace this 

 ingredient. The farmer must add soluble iron salts to his 

 land if he desires to obtain a full yield with the least expense. 

 Dr. E. WolfF, the German authority, found that when soluble 

 iron was omitted in certain culture experiments, the young 

 plants became yellow and sickly, but they quickly became 

 green and assumed a luxuriant growth when a small i|uantity 

 of iron solution was added.' (Griffith on Manures, p. 268, etc.)" 



*"Tlie 'Die-Back' Trouble. — This serious trouble is exan- 

 thema or ' die-back.' The name is given to a weakness affecting 

 orange, lemon, and other orchard trees. There are several 

 especially bad cases in the San Gabriel Valley, where solid 

 blocks of citrus trees are now utterly worthless. Trees seven 

 years old and in a frostless location have not attained a height 

 of over four feet, in some instances, and bear little or no fruit, 

 while adjoining trees of the same age and seemingly under 

 similar conditions are of large size and bear heavy crops. 



"Orange trees affected with 'die-back' make an apparently 

 healthy growth in the spring and early summer, but the young 

 shoots soon turn yellow, the leaves drop off, and the twigs die 

 back to the older wood, from -which a brown granular sub- 

 stance exudes. In a season or two, this older w^ood also dies. 

 Adventitious buds keep developing at the axils of the leaves, 

 until at the end of the season there are small knots, where 

 there should be healthy lateral branches. Experiments with 

 Bordeaux mixture and carbonate of copper have been made 

 in a badly affected grove near Pomona. The work so far has 

 showni no appreciable results, but it has not yet been carried 

 through one season. 



"[In almost all cases of 'die-back,' examination has shown 

 some fault in the subsoil, which puts the roots under stress. 

 Such fault may be an underlying hardpan or impervious clay, 

 pure and simple ; or it may be excessive wetness or drvness of 

 the substrata surrounding the deeper roots ; or the rise of 

 bottom water from below, as in cases of over-irrigation. The 



*J. W. Miller, in University of California Bulletin No. 138. 



