Nonparasitic Diseases - Westcott's Plant Disease - page 257

Fig. 2 Frost Injury on
Holly

formation. Some leaves, including those of rose,
are reddened or crinkled with frost ( Fig. 2 ). Blos-
som buds of fruit trees are critically injured by
frost late in spring. In the South, where plants
come out of dormancy early, orchard heaters,
smudge fires, power fans, and airplanes flying
low to stir up the air are all used to help save the
crop. Many ornamentals are injured when a long,
warm autumn ends in a sudden very cold snap, or
warm weather in February or March is followed
by heavy frosts. Cracks in tree trunks come from
such temperature fluctuations.

Girdling Roots

Unfavorable conditions sometimes deflect roots
from their normal course, and one or two may
grow so closely appressed to a tree as to almost
strangle it. If one side of a tree shows lighter
green leaves with tendency to early defoliation,
dig down on that side to see if a root is choking
the trunk under the soil surface. The root should
be severed and removed, then all cut surfaces
painted.

Gas Toxicity

Grading Injuries

Illuminating gas escaping from aging gas mains
causes slow decline or sudden death, depending
on the plant. Tomatoes are extremely sensitive
and indicate the slightest trace of gas by leaves
and stems bending sharply downward. Plane trees
develop “rosy canker” -long, narrow cankers near
the trunk base with inner bark watermelon-pink
and swollen. With large amounts of gas escaping,
foliage wilts and browns suddenly, followed by
death of twigs and branches; with slow leaks, the
symptoms appear gradually over a year or two.
After the leak is repaired, it is sometimes possible
to save trees by digging a trench to aerate the roots,
applying large quantities of water, burning out
severely injured roots, then replacing soil and feed-
ing to stimulate new growth.
Natural gas is, apparently, not as injurious.

Many shrubs die when they are planted much
deeper than the level at which they were grown
in the nursery. Similarly, many trees die when
they are covered over with fill from house exca-
vations. Roots require oxygen for survival, and
a sudden excess of soil cuts off most of the sup-
ply. A tree expert should be on hand to give
advice before any digging starts. Afterward is
too late. And if grading means filling in soil
around trees, a little well around the trunk is not
enough. There must be radial and circular
trenches laid with tile, and then crushed stone
and gravel, before the top soil goes in place.
Consult Tree Maintenance by P. P. Pirone for
clear descriptions and diagrams for protecting
trees from contractors.