864 Bulletin 181. 



sterility. Properly speaking, a self-sterile tree is one which is self- 

 unfruitful; it must have other varieties near it in order to bear well. 

 But it appears that self-sterility in orchard fruits is often confused 

 with the unfruitf ulness resulting from other causes. It would there- 

 fore be well to clear away this confusion at the outset, in order that the 

 discussion of self-sterility may be better understood. The influences 

 which sometimes make trees unfruitful, which are often confused 

 with the unfruitfulness resulting from self -sterility, are (1) heavy 

 wood growth, (2) the attack of fungi on tlie blossoms, (3) frosts, 

 (4) unfavorable weather during the blooming season. It should also 

 be said that a tree is not self-sterile when it does not blossom. This 

 bulletin does not a,ttempt to tell why trees do not bloom, except 

 that it is generally due to poor management. The only thing which 

 concerns us now is, why trees which blossom full do not set a rea- 

 sonable amount of fruit. 



Blossoms May Drop Because of Heavy Wood Grow^th. 



Young trees generally set little or no fruit the first few years, 

 when they are growing fast, although they may blossom full. With 

 most varieties this early dropping of the blossoms occurs only two 

 or three seasons, but Northern Spy and a few other varieties of apples 

 are often unfruitful ten to thirteen years fi-om this cause Older 

 trees jnay show the same results if stimulated too highly with nitro- 

 genous fertilizers. The logical i-emedy is to check this excessive 

 growth of wood hy withholding nitrogen or by putting the orchard 

 into sod for a few years. 



The direct cause of this unfruitfulness is not known. The sta- 

 mens and pistils are usually well developed and pollen may be pro- 

 duced in abundance. Since young trees drop their blossoms as badly 

 in a mixed orchard where other pollen is available, as when alone, 

 the troul)le probably lies more with the pistils than with the pollen. 



Up to this limit of excessive growth, there is a fairly constant 

 relation between the vigor of a tree and its productiveness. Lack of 

 vigor causes much more unfruitfulness than excessive vigor. If a 

 tree is unhealthy or dying because of poor nourishment, few of its 

 blossoms are strong enough to set fruit. The same results may fol- 

 low if the tree is exhausted by over-bearing. 



