PROTECTION AGAINST FROST 381 



fruit, particularly with that arising from late bloom. Russet bands, 

 generally extending more or less completely around the middle of the 

 fruit, though sometimes near the calyx end, occur on pears and occasion- 

 ally on apples. Similar russeted areas, frequently somewhat raised, but 

 less regular in location, are found on plums. Apples and pears with this 

 form of injury are said to wilt rather rapidly.^"*"* 



In the apple the outside leaves of a cluster sometimes show a form 

 of injury called "frost-blister."^'*^ As observed in New Hampshire and 

 Missouri, this injury does not appear to reduce the size of the affected 

 leaves which are normally small and it apparently does not extend beyond 

 the first two or three leaves to unfold. The injury evidently may occur 

 when the buds are still verj^ little advanced. The appearance is suffi- 

 ciently described by the name; the "blisters" are caused by the separa- 

 tion of the upper and the lower surfaces. The leaves tend to curl and 

 in many cases drop off. Inasmuch as those most affected are of doubtful 

 importance to the growing spur this type of injury is probably 

 unimportant. 



Another interesting consequence of frost injury is the so-called 

 "secondary bloom." When there is extensive killing of fruit buds the 

 spurs which have bloomed may form new blossoms, which in some cases 

 have been observed to mature fruit, sometimes with and sometimes 

 without seeds. The same phenomenon may occur independently of any 

 frost. It is discussed more fully under Fruiting Habit. 



Summary. — The critical temperature for opening flower buds varies 

 greatly with their stage of development and somewhat with species and 

 variety. Some of the fully expanded flowers of many fruit varieties 

 will withstand an apparent temperature of 25°F. without injury, though 

 some will be killed at or above this point. Unopened flower buds are 

 considerably more frost resistant. Plants in a vigorous condition are 

 apparently no more resistant to frost, but they possess greater recupera- 

 tive ability. Often trees losing a considerable percentage of their blos- 

 soms from frost still have enough good buds to bear a full crop. In 

 many cases danger from frost can be avoided to a great extent by the 

 selection of late blossoming varieties. Relatively greater immunity from 

 frost danger can be secured in this way with those fruits and in those 

 sections showing a considerable range in blossoming. The blossom- 

 ing season of many fruits may be slightly retarded by certain cultural 

 practices, but, except in the case of fruits like the strawberry that can 

 be entirely covered, such methods of frost protection are of secondary 

 importance. In new sections the probable blossoming dates of certain 

 varieties of fruit may be foretold with considerable accuracy by com- 

 parison with the blossoming season of native plants. The probability 

 of frost occurring on any particular night can be foretold fairly accurately 

 by the middle of the preceding afternoon. Several methods are employed, 



