Bulletin 229, Diseases of Citrus in Florida 177

diameter. They are usually characterized by longitudinal splits
through the star-like extensions of the markings. This is a
minor injury of no economic importance, the cause of which has
never been demonstrated. It was observed by Stevenson in
Porto Rico to be associated with bordeaux spraying and is held
by Winston, Bowman, and Bach in Florida to be an injury asso-
ciated with the use of bordeaux mixture or bordeaux-oil emul-
sion.
 MISCELLANEOUS LEAF-SPOTS
 A number of different types of spots and discolorations other
than those caused by the more common and familiar diseases
described and illustrated in this bulletin are sometimes observed
on citrus foliage. They frequently attract the attention of grow-
ers as being something new or unusual and may often be mis-
taken for some of the more serious diseases. Different unim-
portant fungi appear to be associated with a few of these spots;
insect or other mechanical injuries are responsible for others,
while the cause of most of them is unknown or merely suspected.
In some cases the spots take the form of hard, glazed, more or
less raised areas. One form of this type of spot closely resembles
citrus canker but does not exhibit the spongy interior character-
istic of canker spots. Since virtually nothing is known concern-
ing these miscellaneous spots and they are of such infrequent
occurrence and little economic importance, no special considera-
tion will be given them.

 INJURIES DUE TO METEOROLOGICAL CAUSES
 COLD INJURY
 Few, if indeed any, of the regions of the United States in which
citrus trees are grown commercially, with the exception of the
Florida Keys, are safe from cold waves of sufficient intensity to
injure the fruit or trees, or both. As far back as historical
records extend, the citrus-growing sections of Florida have been
visited from time to time by cold waves of sufficient intensity
to cause more or less serious loss to the industry (Fig. 82).
 Types of Freezes.-Frosts or freezes vary greatly in the man-
 ner of occurrence and severity, depending largely upon whether
 they are the result of local conditions or of widespread atmos-
 pheric disturbances. Accordingly, they may be considered to be
 of two general types. The first type is commonly designated as