INSECTICIDES AND FUNGICIDES. 27 



further to sulfur dioxide, which in water forms sulfurous, and on further 

 oxidation, sulfuric acid. Both sulfurous and sulfuric acid are toxic to 

 fungi. There is probably some chemical reaction between the acid and the 

 protoplast of the fungus which results in the death of the latter. 



Lime-sulfur Injury. — Lime-sulfur solutions are superior to Bordeaux 

 mixture in that they cause less injury to foliage and rarelj' any fruit injury. 

 Under certain conditions, however, which have not been very well defined, 

 injury has resulted. Wallace (39) finds that this injury differs from that 

 produced l)y Bordeaux in that it appears within a very short time after 

 the spraj^ is applied, and infers from this fact that it is due to the burning 

 effect of the soluble polysulfides before the solution has dried on the leaves. 

 It most often appears as irregular dead areas on the margins and tips of 

 leaves where the liquid collects in larger drops and becomes more con- 

 centrated as it dries. Hence he warns against drenching the leaves. 

 Addition of lime seems to have no effect on this injury. Injury is worse 

 where the leaves have been previously wounded by insects, scab or other 

 agencies, and the solution has direct access to the interior tissues. Dif- 

 ferent crops show different degrees of susceptibility to injury. Peach trees 

 are often entirely defoliated by lime-sulfur of a strength that is entirely 

 safe on apples. Pears show varietal differences in this respect, the Duchess 

 being very easily injured. The orchardists of Nova Scotia have within the 

 last few years almost abandoned the use of lime-sulfur spray because it 

 causes a serious dropping of the fruit. Such damage has not been noted in 

 this State. 



(2) Self-boiled Lime-sulfur. 



The use of mechanical mixtures of sulfur and lime dates far back into 

 the history of plant-disease control. Freshly slaked lime provided a cheap 

 base for a paste suitable for applying and distributing the flowers of sulfur. 

 The only sources of heat in these early mixtures were the hot water some- 

 times recommended for mixing, and the reaction of the lime in slaking. But 

 sulfur fungicides were almost forgotten during the quarter century which 

 followed the introduction of Bordeaux. The use of the self-boiled mixture 

 in its present form was revived by Scott (40) of the United States Depart- 

 ment of Agriculture in 1907 for the control of brown rot and scab of 

 peaches. Bordeaux mixture and sulfur fungicides which contain sulfur in 

 solution were found to be highly injurious to peach foliage when applied 

 at a concentration sufficient to control these diseases. Scott found that 

 this mixture, which contains but a verj- small percentage of solulile sulfur 

 at most, gave good control of the diseases and caused no burning of the 

 foUage. Within a few years it became the most extensively used and suc- 

 cessful fungicide for peaches and plums throughout the country. Objec- 

 tions to the use of self-boiled lime-sulfur are : — 



1. The poor suspension of ingredients necessitates constant strong agitation and 

 frequent cleaning of nozzles. 



2. Especially in dry seasons, it leaves deposits on the fiuit if applied within a 

 few weeks of ripening. 



3. The labor costs of preparation are exceedingly heavy. 



