effect of temperature upon infection of potted citrus seedlings; (3) 
a determination by inoculation tests of the stages of development at 
which the leaves and fruits of commercial citrus types reach practical 
immunity ; and (4) the results of experimental tests and of horticul- 
tural practices for melanose control. 
DESCRIPTION OF THE DISEASE 3 
EFFECT ON LEAVES 
Young lesions of citrus melanose occur on tender leaves first as 
minute, almost invisible, dark, round, sunken depressions with yel- 
lowish margins. (PL 1, A.) As the leaf ages the spot becomes flush 
with the surface and later becomes distinctly raised with a somewhat 
irregular margin. It is then rough and mahogany brown in color 
without discolored borders. (PL 2, B.) These lesions range from 
one-fourth to 1 millimeter in diameter. If the infection is severe, 
the leaf partially loses its natural green color and takes on a paler 
green or yellowish shade. The lesions occur for the most part on the 
upper surface of the leaf, but frequently on the lower surface as 
well. These melanose marks appear in various patterns. They may 
be widely separated single minute lesions, or they may appear in 
circular patterns 2 or 3 centimeters in diameter (pi. 2, C), or some- 
times as solid scar tissue of various forms, and this scar tissue persists 
throughout the life of the leaf. 
A careful histological study of citrus melanose made by Floyd 
(10) shows that the host tissue is killed for five or six cells in depth, 
and beneath this area of dead tissue specialized cells develop which 
cause the dead cells to be raised; hence the rough feel of melanose 
spots. (PL 3, A.) 
EFFECT ON FRUIT 
Lesions on fruit (pi. 4, C, D) are similar to those on leaves. They 
are at first dark and sunken without discolored borders, later becom- 
ing brown, distinctly raised, rough to the touch, and usually remain- 
ing rough throughout the life of the fruit. When the infection is 
slight the melanose spots are usually scattered, and they may be 
simply brown, raised dots of about 1 millimeter or less in diameter, 
with or without a grayish border, or they may be very minute specks 
the center of which is brown with a zone of grayish, almost normal, 
tissue and with an outer zone of brownish melanose-scar tissue. This 
type is not very common, but it has been observed particularly in 
cases where infection occurred when the fruit * passing out of its 
early stage was nearing immunity, under which conditions the dis- 
ease usually does not develop in quantity. If the infection is severe, 
melanose spots are much more numerous and may appear in tear- 
streak patterns or as solid scar tissue, raised and rough to the touch, 
that may cover the greater part of the fruit. Shallow cracks of 
rather regular patterns develop in this dead scar tissue. This type 
is usually referred to as " mud-cake " melanose. (PL 4, E, F.) These 
3 Unless otherwise noted, the discussions of the disease in this bulletin relate to the 
orange and the grapefruit. 
