Bulletin 229, Diseases of Citrus in Florida


as to constitute a dense covering over the surface of older ones.
These hairs are of two kinds-sterile hairs which are universally
present on mature plants, and fertile hairs which develop during
rainy periods. The latter bear sporangia, from which motile
swarm spores are liberated.
 On the twigs and smaller branches, this alga bears irregular
patches which tend to encircle the attacked parts and the fusion
of several lesions may result in the de-
velopment of a rusty-colored, velvety
coating involving a few inches of the
twigs and branches. As a result of the
alga being imbedded under the epi-
dermis, the bark becomes swollen and
the twigs or branches become enlarged
at the places attacked. The attacked
bark eventually becomes somewhat t:
scaly and the twigs may be stunted
from the partial girdling effect of the
alga, which appears to be dependent
upon the host plant only for water and
dissolved nutrients.
 As a rule, the injury to citrus trees ,
in Florida caused by this parasitic alga
is usually considered to be of minor
importance. Occasionally, however,
in the coastal sections of southern Fig. 95.-Alga spots on
 citrus lea.f.
Florida, it may cause considerable in-
jury to the tree. A high degree of atmospheric humidity appears
to be the principal condition favoring the development of this
alga. Where this parasitic alga tends to become troublesome,
it can be controlled by an application of bordeaax mixture or
bordeaux-oil emulsion. This, together with maintaining the
trees in a vigorous condition, will reduce the chances of the new
wood becoming infected.
 LICHENS
 Lichens frequently occur on the trunks, branches, twigs and
 leaves of citrus trees and are sometimes mistaken for injurious
 pests by those unfamiliar with them. Lichens belong to one of
 the lower forms of plant life, in which a fungus and an alga
 live together with mutual benefit to each other. As a rule, they
 are not at all parasitic but live entirely on the surface and ob-