Bulletin 229, Diseases of Citrus in Florida


 SYMPTOMS OF COLD INJURY
 The effect of freezing upon citrus trees varies greatly, depend-
ing upon many factors and conditions aside from the severity
and duration of the cold.



















Fig. 83.-Trees in low portion of grove killed as a result of the cold
 settling therein. The remainder of the grove in the background was
 uninjured.
 On the Foliage.-The first noticeable effect of frost on the
leaves is the dark, greasy appearance that develops. This may
develop on parts or all of a leaf. The same discoloration may also
be noted on young twigs and green fruit. If not chilled too far,
the leaves may regain their normal appearance and remain on
the tree, apparently uninjured.
 When the degree of cold is sufficiently severe to injure the
tissues a portion of the leaf blades may be killed while other
portions may remain alive. When the tips of the blades or other
localized parts are killed the dead areas are commonly invaded
by the withertip fungus and other organisms and are often mis-
taken for diseases due to organisms. In other cases the effect
of frost may be so slight as to kill merely some of the surface
cells. The surfaces of leaves injured in this way may become
brown and corky or dark and hard from the infiltration of gum-
like by-products of decomposition. Such injuries often result
from a light frost on the surfaces of the leaves in the early
morning, followed quickly by bright sunshine.