Bulletin 161 May, 1922
UNIVERSITY OF FLORIDA
Agricultural Experiment Station
H. E. STEVENS
Bulletins will be sent free upon application to the Experiment Station,
INTRODUCTION ...................................--.....-- --- ---- ...----........... 3
AVOCADO SCAB................. --............ -----................. ....................... 3
BLACK SPOT ............................................--------------........................ ...... 10
AVOCADO BLOTCH....- -------.............---------- -.......-- .....--....-- 17
RUSTY BLIGHT....................................................................................................... 21
POWDERY MILDEW................................ .. ... ............................ 22
RUSSET FRUIT........................ ...... ....... ......... .......... ......... 23
In his field work on these diseases the writer received valuable
help from various members of the staff of the State Plant Board,
and from Messrs. W. J. Krome, G. W. Kosel, E. R. Goldberg and
J. W. Ross of Redland, Dade County. Full appreciation is due
Mr. Hamilton Michelsen of Miami for his interest and help in
the investigation of black spot.
By H. E. STEVENS
The production of avocados on a commercial scale is rapidly
gaining prominence in those localities in Florida where the trees
can be safely grown. Increasing interest is being manifested in
the planting and production of this fruit. More information is
desired in regard to the following: Those varieties best suited
for commercial plantings, best cultural practices, use of ferti-
lizers, and insects and diseases affecting the crop. Much infor-
mation on these subjects which would be of value to the avocado
grower, if it were brought together, now exists in various pub-
The object of this bulletin is to bring together the available
information on avocado diseases which may be of practical use
to the avocado grower of Florida. The greater part of the in-
formation contained herein is the result of personal investiga-
tions and observations. Other sources have been drawn on and
credit given them.
Cladosporium citri, Massee
This is a disease of the foliage and fruit of the avocado that
has developed in Florida within the last few years. It is closely
related to citrus scab and, since the two diseases have many
points in common, it has been designated Avocado Scab.
The disease was first called to the writer's attention during
the spring of 1916 by W. J. Krome of Redland, Dade County. A
few specimens were sent in to the laboratory but the brief, pre-
liminary study made of the disease at that time resulted in
nothing of interest.
In the summer of 1917 further reports and specimens of the
disease were received from Dade County, and the disease ap-
peared to be confined chiefly to young seedling avocados. A field
study of scab was made during the fall and winter of 1917, in
cooperation with the State Plant Board, in an effort to deter-
mine the extent of the disease and something of its nature. The
new disease was found more or less widely scattered thru the
seedling plantings in the county, especially in the nurseries. It
was observed on the foliage and fruit of bearing seedling trees
and on some of the budded varieties in the nurseries where scab
4 Florida Agricultural Experiment Station
was present as well as on young seedling stock. The cause of the
disease was later determined and a report' was published the
Avocado scab probably is more prevalent on young plants
in the nursery, where it has proven itself difficult to control. It
is common as a foliage trouble on seedlings of the West Indian
Fig. 1-Avocado scab on young foliage
group. However, many of the budded varieties of this plant
are susceptible to scab. During the period the disease has been
under observation it has been noted as severely affecting the
fruit of both seedling and budded varieties. Fruit of the Trapp,
Taylor and Fuerta seem very susceptible to scab. Therefore,
the disease is apt to be a serious factor in the production of these
'H. E. Stevens: Press Bulletin 289, Fla. Agr. Expt. Sta., 1918.
Bulletin 161, Avocado Diseases 5
varieties in localities where the scab is established, unless pre-
cautionary measures are taken to avoid it.
The injury from scab on the fruit is superficial and does not
affect the quality of mature fruit. However, the outward ap-
pearance of the fruit is marred, and as the markets become more
discriminating the effect of scab on the price of avocados un-
doubtedly will be more evident. It is also probable that the
disease is responsible for a considerable shedding of young fruit,
especially where severe infections occur about the time the fruit
is setting. Later attacks may cause dwarfed and misshaped
fruits where the infection is severe.
Fig. 2-Avocado scab on fruit
Scab is a fungous disease and its further development on
young foliage is influenced by the presence of the fungus on
older foliage, providing moisture and temperature conditions
6 Florida Agricultural Experiment Station
are favorable for its development. A cool wet period is favor-
able to the development of scab in those localities where it is
established. The disease attacks only young and tender growth,
so the period of susceptibility of foliage and fruit probably is
short. The fruit may become infected from the time the bloom
drops until six or eight weeks later, but the principal injury is
done during the first week or two after the bloom drops.
The disease forms definite spots or patches on the young,
tender leaves and shoots, and severe attacks may cause the af-
fected leaves to curl and become distorted. Infection takes place
only on young tender growth and as the leaf tissue hardens it
becomes immune to the disease. However, the older leaves fre-
quently will be found bearing scab spots that were formed when
the tissue was young. The spots are generally small, circular to
irregular in outline and vary from one-sixteenth to one-eighth
of an inch or more in diameter. They are purplish brown to
dark in color and may appear scattered over the surface or
several may grow together to form irregular areas. The spots
penetrate the leaf tissue and become visible on both sides of the
leaf. They are usually more prominent on the upper surface of
the leaf, in which case the under surface of the spot may be
slightly bulged and marked by a purplish discoloration. The
centers of the older spots are composed of dry, dead cells, more
or less spongy and brownish in color. Sometimes the dead cen-
tral tissue falls away, leaving a small hole in the leaf. In the
early stages of development the surfaces of the spots may show
a whitish growth. As the spots grow older their surfaces be-
come brown or black, due to the invasion of other fungi.
On the young shoots, twigs and leaf petioles the spots appear
darker and more elevated. They are more or less oval in shape
with comparatively smooth surfaces and in general outline may
resemble one of the soft scale insects.
On the fruit occurs the same oval-shaped, raised type of spot
that is found on the twigs. The spots may be scattered or
clustered together to form an irregular, scabby mass. Severe
infections on fruit frequently cause a roughened or russet ap-
pearance similar to the disease on grapefruit known as scab.
In this case the markings are light brown in color. The appear-
ance is marred and fruit badly attacked is undersized and of
Bulletin 161, Avocado Diseases 7
The fungus which causes scab has been isolated from the
diseased spots on leaves, studied and identified as Cladosporium
citri, Massee, which also causes the scab of citrus. The fungus
producing avocado scab appears to be identical with the fungus
producing citrus scab and will cause typical scab infections on
the tender growth of either citrus or avocado. However, the
writer has not succeeded in producing typical avocado scab with
Fig.. 3-An avocado fruit badly affected by scab
the strains of the fungus isolated from citrus scab. Only a
limited number of attempts have been made; but all of these
have given negative results. It is probable that all strains of
C. citri are not pathogenic to the avocado but that certain strains
recently have adapted themselves to this host. Such strains are
parasitic to both avocado and citrus. C. citri has the habit of
8 Florida Agricultural Experiment Station
passing from host to host in the citrus group and may follow
the same trend with the avocado. Many varieties of the avocado,
and even individual plants, show a marked resistance to scab
but it may be only a question of time until all of our common
varieties are susceptible to this disease.
In certain sections the disease is so well established that
growers will be compelled to rely on control measures. Where
the disease does not occur precautions should be taken against
its introduction and establishment.
The control of avocado scab may be considered from two view
points; namely, controlling the disease in the nursery and con-
trolling it on the fruit. Controlling the disease in the nursery
is the more tedious and difficult and will require considerable
effort where it has become established. We have not been in a
position to carry out a satisfactory spraying experiment for the
control of the disease on nursery stock, and the suggestions of-
fered are largely derived from the results obtained by several
nurserymen who have sprayed with varying results. Apparently
the ammoniacal solution of copper carbonate gives better re-
sults than bordeaux mixture for the control of scab on nursery
stock. At least one prominent avocado nurseryman in the state
is known to rely on this fungicide to keep his stock free of
As a means of controlling the disease in the nursery the fol-
lowing method is suggested: If the old leaves are badly infected
with scab, spray the plants thoroly with 4-4-50 bordeaux mixture
before any new growth puts out. The purpose of this spraying
is to thoroly cover the scab spots with a coating of the fungicide
and to kill any spores that may have lodged on the surface of
the foliage. This is merely a clean up spray. When the new
foliage begins to put out, spray the plants thoroly with am-
moniacal solution of copper carbonate. Repeat this spraying
every week or ten days until the new growth becomes well hard-
ened. This system of spraying may have to be repeated thru-
out the season whenever new growth appears.
Scab injury can be prevented easily on avocado fruit by
timely applications of bordeaux mixture. This is indicated by
a spraying experiment for this purpose which was made in
1920. The object of the experiment was to determine the number
Bulletin 161, Avocado Diseases 9
of applications necessary to control the disease and the periods
at which they should be applied to give effective results.
This experiment was made in a grove of the Trapp variety
and the trees were in their second or third year of bearing. The
fungicide was applied at monthly intervals thruout the season
and beyond the period in which the fruit was considered liable
to infection. Eight blocks of trees were included in the ex-
periment, and the application of bordeaux varied in seven of
these blocks. The 3-3-50 formula was used in the first appli-
cation and the 4-4-50 in the later ones. The change in formula
was not due to necessity, but rather to convenience in regard to
some other spraying experiments that were being conducted
at the same time. The first application was made into the open
bloom near the latter part of the blooming period and was fol-
lowed by subsequent sprayings at monthly intervals. The fol-
lowing table is explanatory of the sprayings made and of the
0 0 dI
3 Mar. 23 Apr. 21 May 21 ............. 9
1 Mar. 23--..-..------ ..------------ --.......... 25 infected
2 Mar. 23 Apr. 21-..-.-....-..... ------..---6
4 Mar. 23 Apr. 21 May 21 June 22 4 infected
52 .................. Apr. 21 May 21 June 22 9
6 ................. -Apr. 21 May 21 -................. 20
7 ................. Apr. 21 ---............ .................. 63 infected
Not Not Not Not Very badly
8 sprayed sprayed sprayed sprayed 92 infected
On August 1, 100 fruits were examined from each block and
carefully inspected for scab. The results as tabulated above
indicate the effectiveness of bordeaux mixture for keeping the
disease in check. One application made in the bloom materially
reduced the amount of scab over that found on the unsprayed
trees, as indicated by Block 1. However, this block was located
at one corner of the field, bordering on the road, and the trees
"By mistake a majority of the trees in this block were sprayed (March
23) into the bloom, which accounts for the low percentage of scab on the
trees in this block.
10 Florida Agricultural Experiment Station
probably were exposed less to scab infection than those in the
In summarizing the results the bloom spray seems to be
necessary and advisable for the control of scab. There is some
objection, however, to spraying into the open bloom, fearing
a loss due to the application of the fungicide. In this ex-
periment no difference was noted in the shedding of bloom from
the sprayed blocks over that of the unsprayed block, as both
sprayed and unsprayed trees seemed to set fruit equally as
Probably three applications are necessary in order to keep
the scab under control. The critical period for infection seems
to be from the time the bloom drops until the fruit is six or
eight weeks old. For a period of two months the fruit should be
protected, and this can be done easily with three applications of
bordeaux mixture. The 3-3-50 solution of bordeaux mixture is
of sufficient strength to control scab on fruit. In outlining a
schedule for scab control it is suggested that the first applica-
tion be made into the bloom during the latter part of the bloom-
ing period. A second application should be made three weeks
later and a third six weeks later, or three weeks after the
These conclusions are based upon the results obtained from
one season's experiments. However, it seems clearly evident
that bordeaux mixture is effective in controlling scab.
Black Spot has caused considerable loss to seedling avocado
fruit on the lower East Coast during the last few years. The
disease was first brought to the attention of the author in the
summer of 1917. At that time it seemed to be new or unde-
scribed as no report of a similar disease could be found in
available literature. Preliminary studies were begun in both
field and laboratory. Investigation soon indicated that the dis-
ease was of a fungous origin and was widespread among seedling
avocados in Dade County. In previous years certain growers
had observed this type of injury on seedling avocado fruit. As
the years passed it appeared more prevalent and serious.
Bulletin 161, Avocado Diseases 11
The disease belongs to the Anthracnose group,' but is des-
ignated as Black Spot, the common name by which it is gen-
erally referred to on the lower East Coast.
At present black spot is confined chiefly to the lower East
Coast but recently a few specimens of the disease have been
observed in the vicinity of Ft. Myers. It severely attacks the
Fig. 4-Avocado fruit spot
fruit of seedling trees and causes serious injury to the crop
where it is well established. In some cases 90 percent or more
of the fruit on a tree will show the disease, and most of the
'H. E. Stevens: Avocado Diseases (Fruit Spotting), Fla. State Hort. Soc.
H. E. Stevens: Some Diseases of the Avocado and other Subtropical
Fruits, Fla. State Hort. Soc. Proc., 1920.
H. E. Stevens: Avocado Diseases, An. Rpts. Fla. Agr. Expt. Sta., 1917,
1918, 1919, 1920.
12 Florida Agricultural Experiment Station
infected fruit is worthless for shipping purposes. Some seedling
fruit is highly resistant to the disease while others are very
susceptible to it. Occasionally, specimens of black spot have
been observed on the Trapp variety.
The disease appears on fruit as it approaches maturity and
forms a characteristic spot that is easily recognized. A similar
injury is formed in the green bark of young twigs and branches
and on the fruit stems. It is probably carried over from season
to season in the infected bark of the branches.
From the results obtained in several series of inoculations
the fungus appears to be a secondary invader and unable to
penetrate the surface of the fruit except thru some injury.
Typical spots of the disease were obtained only when the fungus
was introduced into punctures made in the rind of the fruit.
Black spot can be controlled readily by timely applications
of bordeaux mixture.
The injury appears as definite spots scattered over the
surface of the affected fruit. These spots are round, brown to
dark brown or black in color and vary from one-eighth to one-
half of an inch in diameter. They are composed of hard, dry,
corky tissue which penetrates the skin of the fruit down to the
The surface of a spot is slightly sunken, often cracked or fis-
sured, and in some cases a zonated effect is observed. When
once formed, the spots do not appear to increase in size on the
surface of the skin, but a decay of the meat below may follow,
especially in matured fruit. Affected fruit may show a few or
many spots of various sizes. Frequently spots merge to form
irregular patches, the surfaces of which are deeply cracked or
broken. Severe attacks on less matured fruit may misshape or
Spots also appear in the bark of young shoots and on fruit
stems somewhat similar to the spots on fruit. Infections on the
fruit stems generally appear sometime in advance of those on
Black spot is caused by a species of Colletotrichum which has
been isolated repeatedly from the tissue of the spots on fruit,
fruit stems and twigs. The fungus has been inoculated into the
rind of healthy fruit and in several instances infection resulted,
forming spots typical of those from which the fungus was ob-
Bulletin 161, Avocado Diseases 13
tainted. In all cases of infection the fungus was introduced into
the rind of the fruit thru punctures in the epidermis.
No exhaustive study has been made to identify the fungus;
however, it appears identical to Colletotrichum gloeosporioides
Penz. This fungus is of common occurrence in citrus groves
thruout the state, and it causes anthracnose and withertip of
citrus in addition to other minor injuries. C. gloeosporioides is
found on other plants beside the citrus group. Generally it is
considered a weak parasite and the fungus rarely becomes de-
structive to citrus trees except those of low vitality. The same
Fig. 5-Fruit badly affected by avocado fruit spot
appears true regarding the species of Colletotrichum, which
causes black spot. It only gains entrance thru a break or
weakened place in the tissue. When an avocado tree once be-
comes infected with the fungus, it may be carried over from
year to year, in the bark of dead twigs, spots in the bark of
living twigs, dead fruit stems, the tissue of fallen leaves, and
dried fruit rinds carrying the spots that collect on the ground
beneath the trees. The fungus is also saprophytic in habit and
can exist and propagate readily on dead avocado tissue. The
wide prevalence of the fungus excludes any possibility of com-
14 Florida Agricultural Experiment Station
plete eradication of the disease from a community in which it
has become troublesome, but it can be controlled or held in
check by a proper system of spraying.
The results from one season of spraying experiments indi-
cate that black spot can be controlled with bordeaux mixture.
Probably two or three applications of the fungicide will give
satisfactory results in controlling the disease on seedling fruit.
The period during which the fruit may become infected is
perhaps comparatively short and lies somewhere between the
time the fruit is one-third to two-thirds mature. Young fruit
shows no indication of the disease and spotting is rarely observed
until after the fruit is half mature. As the fruit nears maturity
it shows less tendency to spot. Most of the spots that appear on
infected fruit are evident by'the time the fruit is half grown.
In 1920 several groups of trees were sprayed to control the
spotting of the fruit of seedling avocados. For this experiment
36 trees were selected in a grove of seedling trees where black
spot and other fruit diseases had been severe for several seasons.
The trees were large bearing ones and were from fifteen to
eighteen years old. None were selected except those known or
suspected to have suffered from "spotted fruit" in seasons past.
The first spraying was made into the open bloom with 3-3-50
bordeaux mixture about the middle of the blooming period.
Other sprayings were made at monthly intervals until four ap-
plications had been made on some of the trees. Bordeaux mix-
ture, 3-3-50 formula, or its equivalent, was used in the first
applications, and the 4-4-50 formula was used in the last appli-
cations. The spraying was done with a power sprayer and spray
guns, using a pressure of 200 pounds. While the spraying was
fairly thoro in some cases it was not possible to cover completely
the foliage and fruit in the tops of some of the tallest trees.
The object of this experiment was to determine the number
of applications of bordeaux mixture necessary to prevent black
spot or blotch and the time at which these applications should
be made. The results obtained were encouraging and indicate
that avocado fruit can be kept comparatively free from these
diseases, if sprayed at the proper time. The experiment is
summarized in table 2.
Bulletin 161, Avocado Diseases 15
1 4 Mar 23 Apr. 21 May 21 June 22 1.6
2 4 Mar. 23 Apr. 21 May 21 ................. 5.
3 4 Mar. 23 Apr. 21 .................... ........... 1.2*
4 4 ........... Apr. 21 May 21 June 22 2.6
5 4 .................. Apr. 21 M ay 21 ................ 2.6
6 4 .................. Apr. 21 ............ -----................ 18.
7 4 ................................... May 21 June 22 60.
8 4 .................. ................. M ay 21 ................. -97.
9 4 .................. -..................-................. ................. 84.
*There were practically no fruit on these trees this season.
The trees in Group 1 were sprayed four times during the
season. The first application was made March 23, about the
middle of the blooming period, and was sprayed directly into
the open bloom. A sceond spraying was made April 21. The
3-3-50 bordeaux mixture was used in the first two applications.
A third spraying was made May 21 and a fourth June 22, both
of these being with the 4-4-50 solution.
About the first of August the fruit on these trees was ex-
amined and noted for black spot and blotch. Only 1.6 percent
of the fruit from the trees in this group showed any spotting
and this to only a slight extent.
The results from a single tree in this group demonstrated
clearly the effectiveness of bordeaux mixture in preventing the
spotting of the fruit. This tree had been observed for two or
three seasons and the mature fruit always was affected badly
with black spot and blotch. Little of the fruit was of a market-
able character, altho the tree bore a good crop each season. At
the end of the experiment this season (1920), the fruits on the
tree were carefully examined, but not a single one showed any
indications of black spot or blotch. Fully 90 percent of the
fruit on this tree the year before was more or less affected by
Trees in Group 2 received three applications of bordeaux
mixture during the season, as indicated in table 2. On August
1 the fruit was examined and 5 percent was found slightly
spotted. This increase in percentage of infected fruit over that
of Groups 1, 4 and 5 is due probably to the failure to spray
thoroly the fruit in the top of the taller trees.
16 Florida Agricultural Experiment Station
Trees in Group 3 were sprayed only twice and with the weaker
formula. No fruit was produced on two trees this season, and
little was found on the other two trees, hardly enough to form
a basis for estimating the percentage of infection. Of the fruit
observed, 1.2 percent showed slight infection, a small percentage
Fig. 6-Decay of inner tissue of fruit following avocado fruit spot
when taking into consideration the number of applications and
the times at which they were made.
Trees in Group 4 were sprayed three times, beginning after
the fruit had set. The fruit was checked over early in August
and 2.6 percent was found to be spotted.
Trees in Group 5 were sprayed twice, in April and in May.
Only 2.6 percent of the fruit in this group was found to be
spotted at the close of the experiment. Trees in this group gave
a fair test for the effectiveness of bordeaux mixture in control-
Bulletin 161, Avocado Diseases 17
ling spotting. The results indicate the number of applications
and the time they should be given to be most effective. Fruit
from trees in this group the previous year (1919) showed a
high percentage of black spot infections.
Trees in Group 6 were sprayed in April and, of the fruit
observed, 18.8 percent was infected at the end of the experi-
ment, August 1.
Where the sprayings were delayed until May or June, little
was accomplished in the way of preventing either black spot or
blotch, as indicated from the results in Groups 7 and 8.
Just why Group 9, the check group of unsprayed trees, should
show a smaller percentage of spotted fruit than the trees in
Group 8 is not easily explained. However, fruit on two of the
trees in this group showed considerable resistance to black spot
and blotch. It has been noted on former occasions that some
seedling fruit is quite resistant, if not immune, to attacks of
both of these diseases.
In summarizing the results of this experiment it will be noted
that the two applications of bordeaux mixture made in Group
5 during April and May, were about as effective as the four
sprayings in Group 1. In Group 7 where the April spraying
was omitted and two applications made later in the season the
diseases showed decided increases.
For the control of black spot the bloom spray seems hardly
necessary, and it appears that two timely applications of bor-
deaux mixture, or three at most, will be sufficient to hold the
disease in check.
The first application needs not be made until the fruit has
set well, probably three or four weeks after the bloom has dis-
appeared. A second application should follow from three weeks
to a month later, and possibly a third application should follow
three weeks after the second. Bordeaux mixture of the 4-4-50
formula will probably give better results than the weaker one
(3-3-50). This solution should be prepared from fresh stone
lime, rather than hydrated lime.
When black spot is troublesome in the grove, pruning out the
dead and sickly branches and removing all dropped fruit from
beneath the trees will help materially to reduce the fungus.
Avocado Blotch is another type of fruit spotting that was
discovered during the investigation of black spot. It is dif-
18 Florida Agricultural Experiment Station
ferent from black spot in many respects altho the two diseases
generally are found together on the same fruit, occurring under
similar conditions. Blotch easily may be mistaken by one un-
familiar with the two diseases for the initial stage of black
Blotch usually appears on the fruit sometime in advance of
black spot and on individual seedling trees it has been observed
to be more abundant and more serious. As yet it has not been
observed on the fruit of budded varieties. This disease was
referred to previously as Avocado Blotch,' a name suggestive of
its general appearance. It is a fungous disease and has been
found widely scattered on seedling avocado plantings along the
lower East Coast. Blotch is responsible for a large percentage
of inferior or low grade fruit and severe attacks may result in
the greater part of the crop's becoming unfit to ship. The disease
is confined chiefly to the rind of the fruit which it makes un-
sightly in appearance and indirectly opens the way for other
organisms that may enter and cause a decay of the meat. Blotch
in itself does not penetrate far beneath the rind and causes no
decay of the meat.
While the disease has caused considerable loss of seedling
fruit in seasons past, it can be controlled readily, if steps are
taken in time. It is useless to spray after the disease appears
on the fruit.
Blotch is a surface spotting of seedling avocado fruit which
is most noticeable as the fruit approaches maturity. The first
spots may occur when the fruit is less than half grown, after
which a succession of spots will follow until the surface is nearly
covered. Mature blotch spots appear as small, slightly sunken,
irregular blotches on the surface of the fruit, usually black in
color, but often showing a short, white fungous growth at the
Fully developed blotch spots may vary from one-eighth to
one-fourth of an inch in diameter. The beginning of such a spot
is indicated by a pale, green area, showing one or more brown
or black dots which are smaller than a pin's head. Gradually the
pale green area becomes brownish to black in color, and even-
tually develops into an irregular sunken pit or spot which is
'H. E. Stevens: Some Diseases of the Avocado and other Subtropical
Fruits, Fla. Sta. Hort. Soc. Proc., 1920.
Bulletin 161, Avocado Diseases 19
typical of the disease. These spots may be scattered freely over
the surface of the infected fruit or several may merge to form
irregular black patches. The spots are confined chiefly to the
rind of the fruit, but more advanced stages may penetrate into
the edge of the meat. The interior of a spot is composed of
brown, spongy tissue made up of dead, collapsed cells of the
fruit rind, intermingled with the dark colored mycelium of the
fungus. The disease is confined apparently to the fruit. How-
ever, in a few cases the fungus has been found in spots on fruit
stems similar to those on the fruit.
Avocado blotch is caused by a fungus belonging to the genus
Cercospora. This is the type of spore produced in the white
fuzzy growth on the surfaces of spots and the only spore type
that has been observed as yet by the writer. However, it is be-
lieved that the fungus has another spore form which carries it
over from one season to another.
The fungus has been isolated repeatedly from the interior
tissue of spots on the surface of fruit and from spots represent-
ing all stages of development. It is frequently found associated
with Colletotrichum in black spot lesions, especially where the
two diseases occur on the same fruit. On a few occasions this
Cercospora has been isolated from spots on the fruit stems.
The fungus grows readily on the ordinary laboratory media
and forms a typical growth which is at first grayish in color
and later turns to brown or blackish brown. In cornmeal agar
it produces a round, raised, tufted, gray colony which is hemis-
pherical in outline. The surface growth is composed of short,
thickly tufted hyphae, and the colony has a tough or leathery
consistency. Cultures made from Cercospora spores, taken from
the surface of a blotch spot on an infected avocado, produced
identically the same growth characteristics on artificial media as
the fungus isolated from the interior tissue of blotch spots.
Various attempts to make the fungus fruit under laboratory
conditions have failed and no spore forms have been observed
in the pure cultures isolated and kept under study. The identity
of the species is undetermined yet.
The parasitic nature of the fungus has been demonstrated
by artificial inoculations into healthy avocado fruit. The fungus
was applied on the surface and introduced thru the rind into
healthy Mexican avocado fruit which was about half grown at
20 Florida Agricultural Experiment Station
the time. These avocados were growing at the Florida Experi-
ment Station, Gainesville, far removed from any region where
blotch was known to occur naturally. Some of the fruits were
punctured and the mycelium of the fungus from a pure culture
inserted into these punctures. With others the mycelium
was laid on the surface of the fruit within a marked area. The
surface of the fruit was sterilized before the inoculations were
made and after the fungus was applied each fruit was wrapped
in moist, sterilized cotton and protected by a covering of waxed
paper. Checks were treated in the same way except that no
fungus was introduced. The inoculations were made on June
30, 1920. Twenty-three days later the fruit was examined and
no indications of infection were noted. It was examined again
about two weeks later, or on August 14, and eight out of ten
individual fruits that were inoculated showed typical infections
of blotch. A larger percentage of infections resulted where the
fungus was placed on the surface than where it was introduced
thru the rind. In some of the inoculations well-developed spots
were found in which the fungus was fruiting and the typical
Cercospora type of spore was present.
Isolations were made from spores taken from the surface of
typical spots and also from the interior tissue of typical spots
on the inoculated fruit. It was found that the fungus from the
surface spots was identical in growth characteristics and mor-
phology to the fungus that had been applied or introduced into
the fruit. There is apparently a period of four or five weeks
from the time the fungus enters the fruit until symptoms of
the disease are visible.
The writer is of the opinion that another spore form exists
and that this probably is produced in the bark of dead twigs
as a pycnidial spore. In some of the laboratory cultures on
sterilized twigs of the avocado, which had been kept for a num-
ber of months, small, black bodies that appeared to be imma-
ture pycnidia were observed. These were found never to contain
spores during the time the fungus was under study and no
method was found to make the fungus fruit in artificial cul-
tures during the period of investigation.
The fungus is highly parasitic to the rind of the avocado
fruit. However, young fruit and those nearly mature show a
great degree of resistance. The critical period of infection prob-
ably lies between the time the fruit is one-fourth to three-
Bulletin 161, Avocado Diseases 21
fourths mature. By protecting the fruit thru this period with
some fungicide the disease should give little trouble.
The control of avocado blotch is similar to that of black spot
as indicated by the results of the spraying experiment reported
on page 14. The two diseases occur in close association and
spraying for one controls the other. Two or three thoro appli-
cations of bordeaux mixture should prevent loss from blotch.
The first application should be made after the fruit has set,
about four or five weeks after the bloom has disappeared. A
second application should follow three or four weeks after the
first and possibly a third three weeks after the second. The
4-4-50 bordeaux formula should be used and care should be
taken to thoroly cover the fruit.
The control of blotch is probably more important than the
control of black spot, since it is much more injurious than the
latter disease. If blotch were eliminated, it is probable that black
spot would be less evident for there is little doubt but that blotch
opens the way for a large percentage of the black spot infections.
This is a fungous disease that has become quite troublesome
on the avocado in Hawaii," and it may be present to some extent
in Florida. It is found chiefly on the foliage and young branches
of the trees; however, it also may attack the bloom and imma-
ture fruit. In habits it corresponds very closely to withertip on
citrus. A species of Gloeosporium has been identified as the
cause of the disease.
Infected leaves turn rusty brown and the affected parts often
are marked by concentric circles of lighter color, showing the
progress of the fungus. The leaf may be attacked in any part of
the blade, the disease spreading rapidly until a large part of
the tissue is invaded. Leaves thus affected later fall, and in
severe attacks the tree may be nearly defoliated. The fungus
may pass from an infected leaf into the young twig and kill it.
Affected branches become dark in color and may continue dying
back, carrying the disease into the larger parts of the tree.
"J. E. Higgins: Hawaii Expt. Sta., Bul; 25, pp. 23-26, 1911.
22 Florida Agricultural Experiment Station
The bloom also may be attacked and it is often thru the
infected flowers that the disease gains entrance to the young
twigs. It is probable that in severe attacks the fungus causes
an excessive dropping of the newly set fruit.
Attacks on leaves, bloom and fruit may be kept down by
spraying with bordeaux mixture, 4-4-50 formula. As soon as
the trouble appears an application of the spray should be made.
Other applications should follow at weekly intervals until three
or four sprayings are made. Any dead or sickly branches should
be removed from affected trees to prevent further progress of
the disease in the branches.
This is a foliage trouble that has been observed on several
occasions during the last few years on both young and old trees.
It probably is not a serious pest, especially on bearing trees;
however, it might become troublesome in the nurseries or on
young trees located in damp, shaded places. It is a fungous
disease and should yield readily to proper treatment.
Attacks on young trees may result in the killing back of the
tips of the tender shoots. The affected terminal leaves may
show a dark, watery discoloration on the upper surfaces, along
the midribs, and may be curled or dwarfed. From one-third
to one-half of the leaf tissue may be affected with the disease.
On the under surface of an infected leaf the same dark, watery
area is visible and this generally is covered with a white, pow-
dery fungous growth, the spore-bearing parts of the fungus. On
the under surfaces of more mature leaves, irregular spots or
blotches occur. These vary from one-half to an inch or more in
extent. They have a purplish cast, or show the white, powdery
growth. They are rather characteristic on account of the net-
work or vein-like appearance of the affected areas. The spots
are not conspicuous on the upper surfaces of the more mature
leaves but may be outlined faintly by pale green areas.
While no control experiments have been tried out for this
particular disease, as have been with most of the powdery mil-
Bulletin 161, Avocado Diseases 23
dews, it seems that it should yield readily to the sulphur treat-
ment. Spraying with lime-sulphur solution, 1 to 30, should keep
the disease in check, or dusting with powdered sulphur probably
would be equally as effective. Where only a few small plants
are involved the dust method may be employed to advantage.
Where the disease occurs to any extent in the nursery two or
three applications of lime-sulphur solution, at intervals of two
weeks, should keep the disease under control.
Russeting of avocado fruit is not uncommon, and varying de-
grees of injury of this nature may be observed any season. Such
injuries are probably the results of several causes, some of which,
no doubt, are mechanical injuries, such as the young fruit's
rubbing against twigs and branches of the trees. Thrips are
probably responsible for a certain amount of the russeting and
perhaps certain fungi are concerned with some of it.
There is one type of russet which is rather striking and which
is thought to be the effect of fungous injury. This closely re-
sembles in outward appearance melanose injury of citrus fruit
and may be caused in a somewhat similar manner. The surface
of badly affected fruit is thickly studded with small, hard, brown,
angular spots that are raised above the surrounding tissue. This
gives the fruit a roughened surface, similar to that of coarse
sandpaper. The streaks, bands and circles that are characteristic
in melanose of citrus fruit are absent. However, there are found
on the fruit large caked masses with firm glazed surfaces which
finally crack to form small angular spots. Slight attacks may
show a scattering of spots, more or less circular in shape, dis-
tributed over the surface of the skin, or the spots may occur
in broken or irregular lines. The injury is confined to the sur-
face of the fruit and does not penetrate deeply into the skin.
Apparently the meat is unaffected and no decay has been ob-
served following this type of injury. Fruit thus affected often
are misshaped and undersized. The outward appearance of
such fruit is completely marred. This condition has been noted
more often in the Trapp variety.