Florida Agricultural Experiment Station


off. Twigs and branches attacked by scaly bark frequently ex-
hibit more or less formation and exudation of gum.
 On the fruit, where characteristic dark-colored, sunken spots
 develop (Fig. 36), the disease is quite distinctive and is com-
 monly termed "nail-head rust". The disease starts as round,
 or occasionally ring-like, spots which usually begin to appear in
 July and August and may continue to appear until the fruit
 ripens. As the fruit approaches maturity, and while it is still
 green, the spots become sunken and brown, later becoming dark
 brown and hard. Secondary organisms, such as the withertip
 fungus, frequently appear to invade the spots and intensify the
 injury. The spots on the fruit usually are round and vary in
 diameter from one-fiftieth to one-half inch. Fruits spotted by
 scaly bark color prematurely and many of them drop before the
 picking season. Occasionally there is a slight exudation of gum
 from the spots on the fruit. The disease affects only the rind
 of the fruit but produces an unsightly and unsalable fruit.
 On the leaves:-Spots also occur occasionally on the leaves but
 are of no economic importance. They appear as brown blotches
 showing on both sides of the leaf. The tissue of the spot be-
 comes hard, glazed, and brittle and the spots eventually become
 surrounded by a yellowish border.

 CAUSE OF SCALY BARK
 Fawcett's investigations indicate that a fungus similar to
 Cladosporium herbarum Lk., which has been known as C. herb-
 arum, var. citricolum, may be responsible for the initial spots
 and that Colletotrichum gloeosporioides, which causes wither-
 tip, and other fungi may be important secondary agents in the
 damage done by the disease in the later stages. Phomopsis citri
 and Diplodia natalensis, two fungi which cause stem-end rot, are
 also frequently found in the dead twigs and branches that ac-
 company this disease under Florida conditions. Considerable
 work has been done in Florida recently by West in isolating and
 studying the organisms associated with scaly bark lesions. A
 number of fungi and bacteria have been obtained but none of
 them have reproduced the disease when they were inoculated
 into citrus twigs of various ages.
 Scaly bark is quite slow in its growth, developing so gradu-
ally into its destructive form that the growers are often unaware
of its importance until it has been present for several years. It