8 MASS. EXPERIMENT STATION BULLETIN 207. 



5. No injury either from the pure or commercial materials was obtained 

 ^^^th a combination of the lower temperatures and humidities, but traces 

 of it began to appear as these factors became higher. This indicates that 

 one or both of these affect the leaf in some way so that it becomes more 

 sensitive the higher either one goes, and also that medium high tem- 

 peratures and medium high humidities act together. The results of this 

 work show that with reasonably good materials injur\' caused is deter- 

 mined by temperature, humidity and perhaps light. The effects of these 

 are therefore given in greater detail below. 



The Effects of Temperature, Humidity axd Light. 



Pear and Elm. — No case of injury to either of these trees was produced 

 by any of the spraj's, even at the highest combinations of temperature and 

 humidity (hereafter WTitten T and H) obtained in the course of the work. 

 It may be remarked, however, that this was not true with Paris green and 

 calcium arsenate, the results ^^■ith which are not included in this bulletin, 

 both materials seriously injuring the leaves under certain T and H com- 

 binations. Combinations as high as T91 H71, T80 H84, and T84 H82 re- 

 sulted in no injury from lead arsenate, and the conclusion is reached that 

 under any usual combinations of T and H obtainable during the summer 

 months, spraying these trees ^vith any reliable brand of lead arsenate 

 should be entirely safe. 



Apple. — Fig. 1 shows the results obtained by using pure acid lead 

 arsenate paste in clear weather. The dots show the T and H points ob- 

 tained for each test; a circle around the dot indicates that there was no 

 injury; t indicates a trace of injur}^, and s indicates slight or "some" 

 injury. Figures in parentheses give the number of tests at the same T 

 and H. A line AB can be drawn across the chart somewhat below the 

 t spots, which may be termed the safety line under the conditions of this 

 test. Spraying above the TH limits of this line ma}' not result in injury, 

 as four cases on the chart show, but neither can safety be assured above the 

 line. 



Fig. 2 shows the results obtained with the same material applied in 

 cloudy weather. It mil be noticed that the lowest himiidity was 68°, and 

 that only one high temperature (91°) was met Avith under the required con- 

 ditions during the six j^ears the tests were carried on. Apparently, cloudy 

 weather is hardly possible (as is to be expected) with low humidities, and 

 also high temperature tends to dissipate clouds. 



So far as can be judged from these tests, there is little real difference in 

 results between clear and cloudy weather, except perhaps at the high 

 humidity end of the safety lines. It would seem that injury begins a httle 

 sooner in cloudy than in clear weather at medium T and H, but only 

 slightly so \^-ith low T. This is made evident by Fig. 3, where the safety 

 lines only are shoAni together. Here the cloudy weather line diverges 

 from the other toward the high H end of the diagram, though, after all, 

 only by about 5° at H90. 



