14 BULLETIN 1474, U. S. DEPARTMENT OF AGRICULTURE 
THE CAUSAL ORGANISM 
The inability to recover Phomopsis citri Fawcett from either natur- 
ally or artificially induced melanose markings makes it impossible to 
follow strictly Koch's postulates for proof of pathogenicity; but, 
on the other hand, cumulative evidence, including extensive inocula- 
tion experiments, goes to prove beyond a reasonable doubt that citrus 
melanose as it occurs in Florida is caused by this fungus, which 
also causes a stem-end decay of a wide range of citrus fruits. In the 
latter case the four postulates of Koch are easily followed. 
The ascogenous form of this fungus has not been found. 5 P. citri 
frequently produces two types of " spores," the one a short, broad, 
functional conidium, the other a long, slender, hooked body of unde- 
termined origin and function, referred to in the literature variously 
as scolecospore, stylospore, beta spore, and paraphysis. These slender 
bodies appear to be sterile, as indicated by their failure to germinate 
in common culture media such as corn-meal agar and potato-dextrose 
agar. The ratio of the two types of " spores " varies greatly even 
from one pycnidium to another developing from single spore cultures. 
In some pycnidia these scolecospores appear to be absent, whereas in 
others they occur in numbers about equal to or even many times more 
numerous than the functional spores. In most instances they occur 
in proportions varying from 1 to 20 to 1 to 1 or less,, with an average 
of about 1 to 4 or 1 to 5. 
In nature, pycnidia without scolecospores, presumably of the mel- 
anose fungus, are found frequently, but when present they occur 
in as widely varying proportions as is the case in pure cultures. 
Measurements of spores from pure cultures of P. citri grown on 
corn-meal agar and on sterile grapefruit stems were made for com- 
parison with those produced in nature on dead citrus twigs, as well 
as those grown in the rind of green grapefruits. More than 100 
spores were measured from each source, and little or no difference in 
size was noted regardless of wdiether the spores were grown in pure 
culture or produced in nature. The spores ranged from 6 to 8 
microns in length, averaging about 7 microns, and from 2 to 3 
microns in width, averaging about 2.5 microns. 
The scolecospores vary in length from 22 to 28 microns and in 
width from 0.7 to 1.4 microns. 
Pycnidia of the same fungus produced in the bark of grapefruit 
stems were also measured (pi. 3, B). These bodies are very irregu- 
lar in outline, and therefore the measurements were variable. How- 
ever, the outside dimensions of pycnidia of P. citri are approximately 
250 to 600 microns by 125 to 325 microns. 
Pycnidia of the causal organism are difficult to detect in nature 
without the aid of a hand lens. They occur sometimes in the dead 
bark of the largest limbs or even trunks, but they are found most 
abundantly on the small twigs and fruit stems of the preceding year's 
crop, as well as on the pedicels of young fruit which dropped in late 
spring or early summer. These fruiting bodies (pi. 8, B) are found 
embedded in the outer bark of dead parts with only the ostiole vis- 
ible, which, when the fungus is fruiting, appears as a minute pimple 
with a light or cream-colored head or with a stringy tendril. If the 
5 See footnote 2 (p. 1). 
