HARVEST SPRAYS FOR THE CONTROL OF FRUIT DROP 11 
~ Rarn AFTER APPLICATION 
| In experiments at Beltsville little or no decrease in effectiveness of 
harvest sprays has been noted in cases where heavy rains have occurred 
- several hours after spray applications. By simulating rains with 
_ heavy sprayings with water after harvest-spray applications, Over- 
~ holser, Overley, and Allmendinger (29) found that the only serious 
_ decrease in effectiveness occurred when the water spray was applied 
' soon (2 hours) after the harvest spray. ‘Trees receiving such treat- 
ment dropped 18.4 percent of the fruit as contrasted with 25.4 percent 
drop by trees not receiving a harvest spray and 7.9 percent by trees 
- receiving no water spray after the harvest spray was applied. Rate 
of fruit drop was not affected by water spray applied 8 and 24 hours 
after harvest spray. 
"TEMPERATURE 
_ The comparative ineffectiveness of some harvest-spray applications 
- on apples at Beltsville in 1940 under relatively cool weather conditions 
suggested that the temperature prevailing during the drying period 
of the spray might have been an important factor. Tests with Wil- 
' liams, Delicious, and Winesap in 1942 (/) clearly indicated that 
- a-naphthaleneacetic acid sprays applied in midday at relatively high 
temperatures (80° to 85° F'.) were more effective at certain concentra- 
tions than when applied in the early morning of the same day under 
cooler conditions (55° to 60°). With the Williams variety, on which 
hormone sprays are extremely effective, these differences were ob- 
tained only when the concentration was below 5 p. p.m. However, 
with both Delicious and Winesap, both 2.5 and 5 p. p. m. sprays 
were generally more effective when applied under conditions prevail- 
ing at the higher temperatures. Whether or not 10 p. p. m. sprays 
would have behaved similarly on the later varieties is not known. 
Overholser, Overley, and Allmendinger (79) have reported similar 
results with the Delicious variety in Washington. Applications dur- 
ing the warm part of the day were more effective in reducing drop 
than were those made under cooler conditions of early morning or late 
afternoon of the same day. These results, together with those men- 
- tioned previously, suggest an interrelationship between temperature 
and effectiveness and indicate the value of using higher concentrations 
on days or parts of days when the temperature is relatively low. Ifa 
limited amount of spraying is to be done, it would seem feasible to 
make applications during the warmer part of the day for greatest 
_ effectiveness. 
Temperature plays some part also in the time required for the 
spray to show worth-while effects after application. With high tem- 
mM peratures 24 to 48 hours may be sufficient; but with relatively low 
temperatures 3 to 4 days may elapse before the effect becomes ap- 
parent. It is believed, however, that low temperature after harvest 
spraying does not seriously affect the net results, since the rate of 
normal drop is also slowed up under cool conditions. 
CoMPATIBILITY OF Harvest Sprays Wir OTHER SPRAY MATERIALS 
___ The possibility of increasing the effectiveness of harvest sprays by 
the addition of supplementary materials has been rather widely 
