BULLETIN 109 MAY, 1912
UNIVERSITY OF FLORIDA
Agricultural Experiment Station
H. S. FAWCETT
Fig. 24.-Scab on young oranges.
The Station bulletins will be sent free upon application to the Experiment
E. 0. Painter Printing Co.. DeLand, Fla.
i. Scab attacks lemons and sour oranges badly, and may be severe on Sat-
2- It may also disfigure grapefruit, tangerines, and (rarely) sweet oranges.
3. It is caused by a fungus, and is infectious.
4. It may be prevented, in an orange or grapefruit grove, by destroying all
sour orange or lemon sprouts; by pruning off all scabby young
growth; or, in bad attacks, by spraying with ammoniacal copper
Introduction .............. .. ........ ....... ................. 51
History of Scab ......................... .. ................. 53
The Fungus in Cultures ..................... ................... 54
Germination of Spores ................ ............................ .. 56
Infection Experiments ............................. .... ......... .. 57
Experiments for Control ................................................ 58
Preventive Measures .................................. ............ 59
References ............. ......................... ................. 60
(Cladosporium Citri Massee)
H. S. FAWCETT
The disease called Scab attacks the fruit and leaves, and some-
times the tender shoots. It is caused by the growth of a minute
parasitic fungus which is somewhat similar to the fungus which
we call yeast. The scab fungus arises from a spore or seed-like
Fig. 25.-Scab on sour orange leaves.
body about the shape of a grain of wheat, but less than one-billionth
as large in volume. From the spores there grow out short tubes
or hollow threads filled with living protoplasm, which penetrate
52 FLORIDA AGRICULTURAL EXPERIMENT STATION
the skin, and feed on and injure the growing tissue of the citrus
tree. In consequence of the presence of these minute filaments,
the tissues of the plant become swollen around the point of infection,
breaking the skin and causing projecting warts or scabs (Figs. 24,
25 and 26). During the growth of these little scabs, the fungus
filaments multiply in them, and send short branches outward from
the surface on which are formed thousands of spores like the one
from which the fungus originated. These spores may be blown
or carried by insects to the vigorous young leaves and fruit of heal-
thy trees and infect them in turn. All this goes on unseen by the
unaided eye, and can only be detected by the aid of a compound
This fungus causes sour
orange and lemon fruits (and
sometimes Satsumas) to grow
misshapen and unsightly. In
severe attacks, corky projec-
tions of a dark gray to tan col-
or extend from the surface.
Between these warts the skin
is usually of a normal color.
These irregular corky projec-
tions often grow together into
a large raised corky scab. In
less severe attacks, especially
Fig. 26.-Early stage of pustules of Scab when scab occurs on grape-
on sour orange leaf. Magnified three fruit and on tangerines (or
tims rarely on sweet oranges), in
place of the warty irregular projections there are seen more or less
raised flat patches of variable shape and size. The surface of the
raised portion is finely scabbed or lightly surfed. This milder form
of the disease can usually be distinguished from "thrips marks," or
other forms of scurf, by the patches being raised; but it can be dis-
tinguished with certainty only by the use of the compound micro-
The injury caused by the disease is nearly always confined to
the visible areas of infection. There appears to be little indication
of any poisoning effect on distant tissue. The earliest infections
appear on either surface of leaves just unfolding, as minute light-
brown points.. These spots become depressed on one side and
raised on the other. Older spots become dark brown and some-
times pinkish.-. The spots grow together as they enlarge, forming
BULLETIN 109 53
the irregular corky scabs. The leaves become twisted and warped.
The disease is usually severe on lemons and sour oranges. Satsumas
are often badly attacked, but only occasionally is grapefruit affected.
HISTORY OF SCAB
Scab, or Verrucosis, appears to have been unknown until about
1885 (1), when it appeared (probably near Ocala) in what was
then the heart of the citrus-growing region, and spread rapidly to
all parts of this (3). The first account of the disease was published
by F. Lamson Scribner in October, 1886 (I). A fuller account
(2), with a colored plate, appeared in the annual report of the U. S.
Commissioner of Agriculture for 1886 (pp. 120-121). Scribner
described the fungus as Cladosporium sp. L. M. Underwood (4),
writing in 1891, speaks of it as a widespread disease. A detailed
account of the disease and of the fungus was published by W. T.
Swingle and H. J. Webber in 1896 (7). They said:
The introduction of this disease into the United States is comparatively
recent. It first appeared in Florida about the year 1884, and spread rapidly
over the State and to Louisiana. Although many thousands of trees affected
with scab have been sent to California, it seems that the disease is unable to
exist there permanently * The malady also occurs in Australia and
Japan, from which latter country it was undoubtedly introduced into America.
Their investigations with lemon showed that the disease could
be controlled by the use of Bordeaux mixture or ammoniacal solu-
tion of copper carbonate. They also found that the removal of all
infected fruits before the blooming season was highly important.
In Tubeuf and Smith's Diseases of Plants, published in 1897,
reference is made (p. 509) to Scribner's publication under Clado-
sporium elegans Penz. This is evidently an error( (23), as may be
seen by examination of Penzig's description and figures in Studi
Botanici sugli Agrumi, published in 1887. The dimensions of the
spores of C. elegans are too large and the pathological effects on
the leaf as figured by Penzig are not the same as those described
by Scribner. The spores of C. elegans are given as 18 to 20 by 5
to 6, while those of the Scab fungus are 8 to 9 by 2.5 to 4 microns.
G. Massee, in his Text-book of Plant Diseases, published in 1899
(Io), described both fungi. On page 310, he described the scab
under the name of Cladosporium citri "pro ter." and says: "This
Cladosporium species is evidently quite distinct from Cladosporium
elegans Penz. which forms arid brown spots on living leaves of
oranges in Italy."
W. T. Swingle in 1897 (8) (then recently back from a trip to
Mediterranean countries) said before the Florida State Horticul-
54 FLORIDA AGRICULTURAL EXPERIMENT STATION
tural Society: "They (the Italians) do not have the dreaded Scab
which seems to exist in Florida alone of all lemon-producing coun-
tries, although originally introduced from Japan."
Scab was reported in Hawaii (14) in 1905 and in Cuba (15)
about the same year.
T. Petch, writing from Ceylon under date of August 20, 190o,
says, "Our lime trees are all affected with 'Scab' (Cladosporium?),
on the leaves especially." Specimens of badly scabbed leaves of
citrus were received through H. G. Keppel from Japan in the same
year. G. Gandara reports the scab in Mexico in 1910 (25).
THE FUNGUS IN CULTURES
The fungus was isolated from sour orange leaves in petri-dish
cultures of 2 per cent. glucose agar in March, 1906 (18), and since
then it has been isolated a number of times both from fruit and
leaves. There was first a tortuous growth of minute colorless by-
Fig. 27.-Growth of Scab fungus on 3 per cent. glucose agar. (A), 15 days
old. (B), maximum growth when not crowded. Natural size.
*BULLETIN 109 55
phae, which soon combined into a kind of stroma. In three or four
days the mycelia became visible to the unaided eye. In twelve days
the mycelia were 2 to 4 millimeters in diameter, hemispherical and
flesh-colored, with a yellowish to reddish-brown tinge (Fig. 27).
The central mass became raised, conical, firm and gummy. The
margin was fringed by small out-growing hyphae. Stromas that
were not crowded grew to a maximum diameter of 6 to 8 millimeters
(Fig. 27, B).
The mycelia grew differently from and had no resemblance to
mycelia of other Cladosporiums (such as those of C. herbarum var.
citricolum shown in Bul. io6, Fig. 23). The central part was made
up of close-septate hyphae in a fairly firm gelatinous mass, from
the edges of which extended chains of oval cells, many of them ap-
parently formed by budding. When these gelatinous stromas were
transferred to sweet potato plugs, a white downy growth was
formed over the surface and on the edges. It was formed by small
hyaline hyphae. This would often disappear and a tough compact
stroma be left (Fig. 28). The fungus did not usually spread out
much over the surface of the medium, but piled up in time into
wrinkled tough masses. When transferred to
sterilized leaves or stems, more of the looser
less-septate hyphae were developed. If these
stromas were kept growing for some months
and then gradually allowed to dry out, there
were formed compact masses made up of
thick-walled colored packet-like clusters or
chains of cells. In one case it was found that
growth took place from a culture four months
old which had thoroughly dried out. Another
culture, which had been made on November
16 and had dried out, was dropped on to an
agar tube medium on June 8 of the following
year, and developed a pure culture by contact.
On January 2 of the second year, this culture,
which had also dried out, was dropped on to a
sweet potato plug and produced a pure growth;
thus making two periods, one of about seven
months, and the other of about six months,
between transfers. Some attempts were made
to get growth from cultures from 12 to 14
months old, but no growth of the fungus was Fig. 28.-Growth o f
evident. Scab fungus on sweet
56 FLORIDA AGRICULTURAL EXPERIMENT STATION
The same thick-walled packet forms of cells that were seen in
old cultures were found in old warts and scabs that had wintered
over on the trees. It is unnecessary to assume that the fungus need
have any other form of reproduction to enable it to live over from
one season to another. Search for other forms of reproductive
bodies, and several attempts to develop a perfect stage by placing
scabbed fruit and leaves for a long time in sand under different
conditions failed to show any new forms of spores.
GERMINATION OF SPORES
The spores of this fungus germinated readily in hanging drops
of tap-water, 3 per cent. glucose and orange-leaf agar. They be-
gan to germinate in from 5
to 25 hours. In water, new
short rounded cells were
formed immediately from
the original spore of about
A the same size. Later an ir-
regular body of constricted
Fig. 29.-Spores germinated in water, cells, with oval or spherical
(A) and (B) after 24 hours. (C) cells wth oval or ca
after three days Magnified 500 times, conidial cells at their pro-
jecting ends, was formed
(Fig. 29). In glucose, the
germination began about as
in water. As growth pro-
ceeded a mass of coalescing
thick-walled short septate
cells formed, with looser
vegetative hyphae radiating
^ B from the central part. With
Fig. 3o.-Growth of two spores in age. the protoplasm of some
orange-leaf agar after 3 days. (A) Bud- of the cells seemed to be
ding form. (B) Elongated hyphae. concentrated in a body or
Magnified 375 times.
Magnified 375 tes. bodies resembling spores.
Fig. 30 shows two forms of
growth in orange-leaf agar.
o B one of which (a) appeared
to be formed by budding as
in Saccharomyces. Spore-
Fig. 31.-(A) Conidia from edge of like bodies inside some
stroma on glucose agar. (B) Thick- of the cells were also com-
walled spore-like cells from culture one
year old. mon.
BULLETIN 109 57
Several successful infections were made from inoculations with
pure cultures of this fungus. A number of complete failures in
causing infection were at first experienced. It was found that under
ordinary conditions the disease would not spread naturally in the
greenhouse. Small sour orange trees badly infected with scab
were brought into the greenhouse. As new growth came out, it
was invariably free from scab, even when similar plants remaining
outside continued to show scabs on the leaves and young twigs.
Some infection was produced in the greenhouse by inoculation with
cultures, but the scabs thus formed did not communicate the disease
to subsequent new growth.
On August 25, 1906, 9 small sour orange trees, which had been
cut back and were coming into new leaf, were treated by spraying
with cultures shaken up in distilled water; 9 similar plants were
treated by putting on the culture with a camel's-hair brush; and 9
others were treated by putting on spores from infected leaves shaken
up in water; while eleven others were kept untreated as checks.
None of these 39 plants showed any infection, and they continued to
remain free from scab as long as they were in the greenhouse.
In November, 1908, 5 different trees were inoculated in the
greenhouse by puncturing the young rapidly growing leaves and
twigs and placing in them portions of cultures 14 days old. These
twigs were then wrapped in paraffined paper, which was taken off
after 24 hours. In about fifteen days characteristic diseased areas
had developed. Noticeable irritation of the punctured points had
started before that time. The scabs were produced both on the
twigs and on the leaves. (Checks not inoculated were not at all in-
fected.) The leaves became wrinkled and contorted just as in case
of natural infection. In about 60 days the diseased warts were hard
and corky and fully matured, but while the plants remained in the
greenhouse no new points of infection developed.
On January 6, 1909, 6 sour orange trees were sprayed with
cultures two months old which had developed on sterilized orange
twigs. The cultures were mashed up in distilled water and sprayed
on with a large atomizer.' Two of the six trees were covered with
bell-jars. The others were left uncovered. All the trees were
growing rapidly and bore many tender twigs and leaves. In six
days there were small infection points to be seen on the leaves of
the two trees under bell-jars. In one month all the trees showed
more or less infection. Those which had been under bell-jars when
inoculated were badly infected. Those uncovered were only slightly
58 FLORIDA AGRICULTURAL EXPERIMENT STATION
infected with the scab. Check trees not inoculated showed no
signs of the disease. Later observation showed that there was no
spread from these first scabs to the new leaves that came out later.
EXPERIMENTS FOR CONTROL
In 1896, Swingle and Webber reported that their experiments
had shown that either Bordeaux mixture or ammoniacal solution
of copper carbonate was effective in controlling scab.
In the fall of 19o9 and spring of 1910, the writer conducted
some spraying experiments on the Experiment Station grounds with
Bordeaux mixture and various strengths of Avenarius' carbolineum.
It was found that weak Bordeaux mixture was more effective than
any mixture of carbolineum up to 25 per cent.
In September, 1909, small sour orange trees which were badly
affected with scab were sprayed with Bordeaux. Others next to
them were left as checks. All the trees put on some new growth.
The new growth on the sprayed trees was healthy and vigorous,
while the growth on the unsprayed trees continued to show infection.
These lots were contiguous to one another, there being a space of
only six inches between them. As would be expected, a little in-
fection came over from the unsprayed lot on growth which came
out after the spraying was done; but the effect of the spraying was
In March, 1910, half of the trees sprayed in September, 1909,
were sprayed with Bordeaux for the second time, and- half of the
check trees were sprayed with Bordeaux for the first time. The
effect of the Bordeaux was again quite apparent. Those trees
sprayed in November and March had their leaves entirely free from
scab. On those sprayed only in November the lower leaves were
free from scab, while the upper and new growth was affected. On
those sprayed only in April, the lower old leaves of the summer be-
fore were badly scabbed, while the upper leaves that were rapidly
growing at the time of the spraying were free from scab. Those
not sprayed at all showed scabs on leaves and on twigs all the way
out from the oldest to the newest growth.
At the same time a number of the larger trees on the Experi-
ment Station grounds were sprayed with Bordeaux, and others
near, them left as checks. While the growth subsequent to the
spraying showed no infection for at least two months, the unsprayed
trees showed considerable infection on the growth of the same age.
Mr. H. B. Stevens, an orange grower at DeLand, reported that
he had been successful in preventing the spread of scab on grape-
BULLETIN 109 59
fruit by pruning off and destroying all the infected growth as soon
as it appeared. This was made possible by the fact that the scab
began on certain branches and not all over the tree at once. When
this was pruned off, the subsequent new growth came out entirely
free from the disease. To test this method further, the writer, in
March, 1910, pruned off all the growth showing scabs on a row
of sour orange trees on the Experiment Station grounds. Later
observations of the fresh new growth following this pruning showed
that it was nearly free from scab for at least two months. On new
growth later in the year some infection occurred, probably from
other trees which were left as checks in another part of the grounds
not far distant from the pruned ones. Those unpruned continued
to show scabs on the leaves as fast as the leaves unfolded.
I. Destroying All Sour Orange or Other Worthless Growth.-
All sour orange or lemon sprouts in an orange or grapefruit grove
should be cut out and destroyed, as they are almost always badly
infected year after year, and are a constant source of danger to the
grapefruit and other varieties which are usually less susceptible. All
scabbed sour oranges and worthless lemons or other scabby fruits
should be picked up and destroyed.
2. Spraying When Necessary.-In case of a severe outbreak
of scab that must be checked at once, spray with ammoniacal solu-
tion of copper carbonate (3 pints ammonia and 5 ounces of copper
carbonate to 50 gallons of water) or Bordeaux mixture (3-3-50).
The copper carbonate solution is to be preferred, since it usually
does not cause so great an increase of scale insects as does Bor-
deaux. In bad attacks, the first spraying should be done soon after
the petals fall, and a second may be necessary two weeks or a month
later. A third spraying will probably not often be necessary. In
many cases of light attack, the loss by the scab is less than the cost
of spraying with the fungicide and following it up with an insecti-
3. Cutting out Scabby Growth.-New growth coming out at
unseasonable times, and the late bloom (known as "June bloom")
are most apt to be infected. Such infected growth can be pruned
out and destroyed. This will help to prevent a recurrence of the
1. SCRIBNER, F. LAMSON, Bul. Torrey Bot. Club. XIII: 181-183. 1886.
2. Annual report of Commissioner of Agri. U. S. Dep. Agri. pp. 120-121, 1886.
.3. CURTISS, A. H., Bul. Fla. Agr. Exp. Sta. II: 35-38, 1888.
4. UNDERWOOD, L. M., Jour. of Mycology, U. S. Dep. Agr. p. 34, 1891.
5. WEBBER, H. J., Proc. Fla. State Hort. Soc. P. 52, 1895.
6. P. 75, 1896.
7. SWINGLE, W. T., and WEBBER, H. J., Bul. Div. of Veg. Phys. and Path.
VIII: 20-24. 1896.
8. SWINGLE, W. T., Proc. Fla. State Hort. Soc. P. 36, 1897.
9. WEBBER, H. J., Proc. Fla. State Hort. Soc. P. 28, 1898.
o1. MASSEE, G., Textbook of Plant Diseases. Pp. 310, 436. 1907.
In. HUME, H. H., Bul. Fla. Exp. Sta. LIII: 155-157. 1900.
12. HUME, H. H., Proc. Fla. State Hort. Soc. P. 65, 1901.
13. WAITE, F. D., Proc. Fla. Hort. Soc. P. 54, 1904,
14. HIGGINs J. E., Bul. Hawaii Agr. Exp. Sta. IX: 24, 25. 1905.
15. EARLE, F. S., First Annual Report Cuba Exp. Sta. 19o6.
16. ROLFS, P. H., Proc. Fla. State Hort. Soc. Pp. 39-40, 19o6.
17. CooK, M. T., and HORNE, W. T., Bul. Cuban Exp. Stat. IX: 34, 35. 1908.
18. FAWCETT, H. S., Fla. Agr. Exp. Sta. Rep. for 1907. Pp. 45, 46.
19. 9o08. P. 67.
20. 1909. Pp. 57-59.
21. 191o. Pp. 54,55.
22. Proc. Fla. State Hort. Soc. P. 85, 1909.
23. Mycologia II: 245-246. 191o.
"24. Bul. io8, Fla. Agr. Exp. Sta. P. 41, 1911.
25. GANDARA, G., Bol. 31, Est. Agr. Cent. (Mexico), Pp. II, 12. 191o.
26. BANCROFT, C. K. West Indian Bul. X: 256. 191o.