FRUIT THINNING WITH CHEMICAL SPRAYS 13 



set is likely to occur, than in seasons when generally warm, dry con- 

 ditions prevail (as in 1947). The data presented in table 5 illustrates 

 this point. In the Bond and Birchmount orchards Delicious and 

 Winesap trees were within one tree space of a pollinizer (Golden 

 Delicious and Jonathan) and fruit set on unsprayed trees was ap- 

 proximately the same for the 2 years in both orchards. However, it 

 is evident from the data in table 5 that the Elgetol sprays were much 

 more effective in reducing fruit set in 1948 than in 1947. In 1948 the 

 sprays prevented many of the flowers from setting fruit even though 

 pollination and fertilization had apparently taken place prior to 

 treatment. 



This greater indirect action of the Elgetol when applied during a 

 cool, humid bloom season may be a result of a greater amount of the 

 toxicant being absorbed. An alternate possibility is that many of the 

 fruits may have been "weak," due to imperfect fertilization or low 

 seed content per fruit. It would seem reasonable to expect that such 

 fruits would be more susceptible to the indirect action of the toxicant. 

 Extensive observations indicate that the tendency toward the "in- 

 direct'' type of thinning is greater with weak trees or parts of trees 

 with weak fruiting wood. When a combination of both weak trees 

 and cool, humid weather occurs, thinning sprays used at conven- 

 tional concentrations may result in almost complete defruiting. 



It should be emphasized that in orchards where pollination is a 

 limiting factor in years of adverse weather, dinitro sprays usually 

 result in serious overthinning (table 5, Ox Team orchard. 1948). In 

 such instances a combination of both a light set and a greater than 

 average killing power of the spray frequently results in a greater set 

 reduction than desired. Since dinitro sprays are applied at full bloom 

 or later, it is fortunate that weather conditions influencing fruit set 

 from the earlier opening blossoms are largely known before it be- 

 comes necessary to determine the desirability of using these sprays. 



Failure to obtain sufficient thinning is often associated with weather 

 conditions that are conducive to heavy fruit set. During bloom 

 periods of moderately high temperature, fertilization processes pro- 

 ceed at a very rapid rate. Under such conditions most of the thinning 

 with dinitro chemicals is of the direct type, and consequently the 

 sprays should be accurately timed to coincide with full bloom. Even 

 with the best timing possible, less thinning than desired may be 

 realized. During the 1950 season approximately 20,000 acres of 

 apples received dinitro sprays in central Washington without a single 

 instance of overthinning. Practically all growers using the sprays 

 obtained substantial benefits, but many were disappointed because of 

 insufficient thinning. The 1950 season was characterized by warm, 

 dry weather with a daily accumulation during the bloom period of 

 230 degree-hours above 50° F., i.e., the sum of the hourly degrees 

 above 50° during which such temperatures prevailed. During this 

 type of season the possibility of using higher concentrations of 

 Elgetol to obtain greater thinning has not been sufficiently explored. 

 By doubling the conventional concentration in a favorable bloom 

 period it may be possible to obtain a substantial amount of thinning 

 of the indirect type. 



