PREHARVEST DROP CONTROL 



At this time (mid- June) it appears that we shall have a larger crop of 

 Mcintosh in most orchards than we had a year ago. It is generally thought, 

 and our results agree, that trees with a Ijeavy crop are generally more 

 susceptible to preharvest drop than similar trees bearing a light to moderate 

 crop. Therefore, one should be aware of the increased chances of serious drop 

 losses in heavy crop years. 



The control material situation has stabilized and no new materials will 

 be available for drop control this fall. As you know, we've been recommending 

 two sprays of NAA (20 ppm each time) about 10 days apart for drop control of 

 Mcintosh for those who wish to minimize drop for 3 weeks. The major weakness 

 with this old recommendation is that 2 applications are required. Last year 

 we combined NAA (10 ppm) and 2,4,5-TA (20ppm) and obtained as good drop control 

 as occurred with 2 sprays of NAA. Consequently, we are suggesting this com- 

 bination or 2 sprays of NAA for Mcintosh only, in our 1959 recommendations 

 (Special Circular No. 254 - Preharvest Drop Control of Apples). Since 2,4,5-TA 

 occasionally fails to work, one should watch the extent of drop after the first 

 10 days (when using a NAA plus 2,4,5-TA combination) and if for some reason the 

 2,4,5-TA is not adequately controlling drop, a second NAA spray (20 ppm) may 

 have to be used. 2,4,5-TA is very slow in becoming effective which is why NAA 

 is needed in addition to it. 



Another point that might be stressed more in controlling drop of Mcintosh 

 with NAA or a NAA plus 2,4,5-TA combination is that Mcintosh typically commence 

 to drop at a very slow rate (often less than 1% a day) for the first 5 to 10 

 days after sound apples start falling. Then the rate of drop often rises rather 

 abruptly to 2 or 3 per cent a day. Since NAA becomes effective for drop control 

 within 2 days after an application it seems most logical to delay the stop drop 

 application as long as possible so that its effective period is certain to pre- 

 vail during the period when fruit drop is generally most serious. Since hot 

 weather is a prime factor in increasing the rate of drop, exceptions to this 

 general situation are possible. One should keep in mind the weather that is 

 predicted for several days ahead so that stop drop sprays precede unseasonably 

 hot weather. 



Another point of interest is the influence of spot picking on drop. Since 

 trees with a moderate crop drop less than those bearing heavily, one might expect 

 that spot picking might reduce the overall drop. Last year, however, this did 

 not seem to be the case in one block where we obtained drop records. Spot-picked 

 trees dropped very heavily during the warm period of September 24 to 26 even 

 where 1/3 to 1/2 of the crop had already been harvested. It appears that weather 

 conditions are much more important in determining the rate of drop than whether 

 or not spot picking has been done. 



As usual, it is highly desirable to keep the leaves in a good healthy con- 

 dition, free of mite injury and nutritional deficiencies of potassium and 

 magnesium, in order to keep drop losses within reasonable bounds. 



2,4,5-TP (20ppm) is still our recommended treatment for late varieties. 

 Delay its application as late as possible but be sure to apply it before the 

 foliage is injured by frost. 



F. W. Southwick 



