- 5 - 



light crop year is very small. 



Since crop size can have considerable influence on the quantity of elements 

 in apple foliage it is very important to know the crop size of any tree from 

 which leaf samples are taken for diagnostic purposes. For example, an analysis 

 which showed 1,60 per cent nitrogen and 1.30 per cent potassium would indicate 

 adequate nitrogen and low potassiimi for a tree with a light crop, whereas for a 

 heavy crop tree nitrogen would be low and potassium would be adequate. 



VJ. D. Weeks 



I I I I I I I I I I I I I I I I 



SILVER LEAF 



The silvering of apple leaves, the color of unpolished silver or the color 

 of lead, is sometimes found on one branch, sometimes involves an entire tree, 

 and, in some orchards, there may be more than one tree with it, 



"True" silver leaf is caused by the fungus Stereum purpureum . But the 

 silver leaf condition does not always have a fungus associated with it. It 

 may be caused by winter injury and other agents which disturb the function of 

 the tree. Not all trees with silver leaf die. Dr. Burrell and Dr. Hilborn 

 report that orchards which had silver leaf in 1934, progressively showed less 

 and less and now show no trouble or an insignificant amount. 



The following treatment deals with the silver leaf caused by the fungus, 

 Stereum purpureimi ; 



Plants Attacked - Silver leaf is most serious on apples and plums but also af- 

 fects peaches, pears, cherries, quinces, gooseberries, currants, and other 

 plants including forest trees and ornamental shrubs. Some varieties of apples 

 appear to be more susceptible than others. 



Distribution and Economic Importance - Silver leaf is not considered of serious 

 importance in the eastern United States. There is more in the Northwest and in 

 Canada and it is serious in New Zealand and, especially on plxmis in European 

 countries. In England, the Silver Leaf Order of 1923 compelled fruit growers to 

 destroy all dead wood of fruit trees killed by Stereum purpureum before July 15 

 every year. 



Symptoms - The first external sign of infection is a silvering of the larger 

 veins of the leaf and later the entire leaf. All leaves of the infected branch 

 have a silvery metallic lustre which distinctly contrasts with the normal green 

 color of healthy leaves. The silvered appearance is the result of air spaces in 

 the leaf tissue and can be produced by injecting healthy branches with extracts 

 of the fungus. Death of trees infected v;ith the fungus takes place a year or so 

 after the silvering appears and may be preceded by some stunting. In some cases 

 of infection, hov/ever, there is no silvering of foliage. 



The fungus invades the heartwood and may turn nearly all of it dark brown 



