358 MANUAL OF TREE DISEASES 



plied to the foliage in a finely divided form in order to cover the 

 leaves as completely as possible. Then when the fungous spores 

 lodge on the leaf, they are either killed outright or soon after 

 germination by contact with the^copper or sulfur. Thus it is 

 seen that the use of spray and dust mixtures will prevent leaf- 

 spot diseases only when the mixture is applied before infection 

 has taken place. In addition it should be remembered that 

 while the leaves are growing, the increase in leaf surface exposes 

 unprotected tissue and therefore applications must be renewed 

 frequently. Applications should be made before rain periods, 

 since the spores are disseminated and produce infection while 

 the leaves are wet. The fungicides will not wash off materially. 

 After a time they weather and are no longer efficient. In gen- 

 eral, applications should be made at intervals of ten or fourteen 

 days. 



So far as is known, bordeaux mixture, lime-sulfur or sulfur 

 dust may be used at the ordinary concentrations without injury 

 to trees. The first two are liquid mixtiu-es and the latter is 

 applied dry. For large trees the spraying machines for liquid 

 fungicides must develop a high pressure. Such machines are 

 expensive. Blowers to apply sulfur dust are less expensive. 

 Lime-sulfur is obtained on the market in concentrated form. It 

 usually may be used with safety at a dilution of one part of 

 the concentrate to fifty parts of water. Dry sulfur for dusting 

 must be finely ground. Ten parts of dry arsenate of lead to 

 ninety parts of sulfur-flour is recommended. The arsenate of 

 lead makes the distribution of the sulfur more uniform and 

 gives it greater adhesive qualities. Bordeaux mixture in various 

 forms and strengths may be bought ready for diluting. Home- 

 made bordeaux mixture, however, is more efficient and is not 

 difficult to prepare. It is generally used at the strength 

 designated as 5—5—50, that is, 5 pounds of quick-lime, 5 pounds 

 of copper sulfate and 50 gallons of water. 



Bordeaux mixture is made as follows : Prepare a stock solu- 



