Title: Pruning citrus in relation to disease control
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Title: Pruning citrus in relation to disease control
Series Title: Pruning citrus in relation to disease control
Physical Description: Book
Creator: Childs, James Fielding Lewis,
Publisher: University of Florida Agricultural Extension Service
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AGRICULTURAL EXTENSION SERVICE
GAINESVILLE, FLORIDA



Pruning Citrus in Relation to

Disease Control

JAMES F. L. CHILDS 1
Pruning is perhaps the least understood of all horticultural
practices. It is based largely on results of trial and error ex-
periments. So it is not surprising that methods used for one
kind of fruit tree may be quite different from those used for
another. The objectives in pruning deciduous fruit trees are
largely concerned with fruit production. For example, the
grower seeks to stimulate production of large fruits, to regulate
fruiting habits, or to bring about other desirable changes such
as invigoration or rejuvenation.
The fruiting habits of citrus trees are not greatly influenced
by pruning, probably because citrus is a semi-tropical evergreen
that lacks the strongly defined seasonal changes in growth and
fruiting habits characteristic of deciduous trees. Furthermore,
citrus trees undergo considerable self-pruning by shading out
'Senior Pathologist, Fruit and Nut Crops and Diseases, Bureau of Plant
Industry, Soils, and Agricultural Engineering, Agricultural Research Ad-
ministration, United States Department of Agriculture.

Fig. 1.-Proper and improper pruning. Left, stub left too long will
result in wood decay in spite of wound paint; right, this one is pruned
more nearly flush with adjacent bark.


Circular 116


June 1953







excess branches and twigs. The objectives in pruning citrus
are concerned most with the health and vigor of the tree and
only indirectly with fruit production. Therefore the citrus
grower prunes to remove dead twigs and branches, to shape
very young trees, or to rejuvenate old trees.
It is suggested that certain diseases can be controlled by prun-
ing out infected twigs and branches. On the other hand, poorly
executed pruning often produces wounds through which a num-
ber of disease-producing organisms may enter the tree and do
much damage.
The purpose of this circular is to point out certain relations
between the pruning of citrus trees and the control of diseases
and to emphasize methods that will give improved and eco-
nomical disease control.

RELATION OF PRUNING TO DISEASE ERADICATION
Pruning to Eradicate the Source of Infection.-Although
pruning has been recommended as a method of controlling cer-
tain diseases, relatively little is done in Florida. It is true that
melanose (caused by the fungus Phomopsis citri) can be con-
trolled to a certain extent by pruning out dead or dying twigs
and branches. Phomopsis lives on these dead and dying branches
from season to season, producing the spores that are splashed
by rain to fruits and foliage where they germinate and cause
the blemishes known as melanose. Nevertheless, experience
has shown that on mature trees even careful and close pruning
of the dead wood does not give satisfactory control in the ab-
sence of fungicidal sprays. Much better control can be achieved
by applying suitable fungicidal sprays. However, when citrus
trees have been severely damaged by cold, or grass fires, pro-
tection of subsequent flushes of growth from infection by the
melanose fungus is greatly improved by pruning out the dead
wood.
Pruning to Remove Diseased Tissues.-The surgical treatment
of foot-rot lesions (caused by Phytophthora sp.) is a form of
eradicant pruning that is often necessary under Florida condi-
tions. Diseased bark of foot-rot lesions, as well as healthy
bark extending at least half an inch beyond the diseased or
visibly affected area, should be removed. Infected wood should
be chiseled out also. In Florida this work is usually done during
the spring, when the foot-rot fungus is active and the lesions
are easily recognized. Also the periods of dry weather that






come in the spring are favorable to the drying out of excavated
wounds. Allowing the wounds to dry out for 10 days to two
weeks does much to weaken and kill any fragments of the causal
fungus that still remain, and improves the penetration of dis-
infectants. After a period of drying, the wounds should be
disinfected and dressed as indicated for infected wounds on
page 9.
Pruning the lower branches or skirt (when bare of fruit)
to clear the ground by 18 inches is beneficial,in the treatment
of foot-rot-affected trees and helpful in preventing infection
of healthy ones. It promotes circulation of drying air to the
ground at the base of the tree. This practice is being adopted
by many growers but some are pruning much too high, two to
four feet in some instances. Excessively high pruning sacrifices
too much bearing surface and is unnecessary.
Growers frequently leave prunings piled around the base of
the tree. In time weeds form a canopy over the pile of prun-
ings, which retards air circulation and keeps the soil damp from
one rainy period to the next. This practice favors very active
infections by the foot-rot fungus and may result in girdling
of large trees in one season. In the treatment of foot-rot lesions
it is often helpful to draw the soil away from the root crown,
but this is not part of the pruning operation.

DISEASES ASSOCIATED WITH PRUNING WOUNDS
An important factor in pruning is the employment of meth-
ods that guard against infection. Unfortunately, much of the
pruning of citrus trees is done in such a slovenly manner that
it is actually an invitation to infection by disease-producing
organisms. Growers seem to forget that many disease-pro-
ducing organisms are unable to penetrate healthy, undamaged
bark but enter readily through wounds. To emphasize the
nature and importance of the diseases associated with pruning
wounds, this phase of the subject is discussed from the stand-
point of several diseases important in Florida.
Young-tree Die-back or Gangrene.-A disease of unknown
cause that attacks young trees soon after they are planted has
been called to our attention many times in the last three years.
We have called this disease gangrene. In Florida and elsewhere
young nursery trees are often pruned back or topped when they
are transferred to the permanent planting. Trees so topped
appear to be subject to gangrene until they are completely es-







tablished, but the disaese ap-
Shpears to be most active in
late spring and summer.
Summer-planted trees us-
ually have made one or more
flushes of growth in the
nursery, and therefore are
somewhat deficient in re-
serve starch and slower to
become established a f t e r
ia transplanting. This condi-
L tion of depleted starch may
contribute to their suscepti-
bility to disease.
Some young trees die back
but a few inches; other die
I to the ground in a period of
S. i two to three weeks (Fig. 2).
In most cases that have
come to our attention;
young-tree die-back or gan-
grene has started at the
wound made by topping, but
Fig. 2.-Young-tree dieback. Ar- in a few cases the cut ends
row marks the advancing margin of
dead tissue. of roots and wounds made in
budding have been t he
points of infection. In one instance, 1,198 of 1,200 recently
planted Temple orange (Citrus reticulata hybrid) trees on Rough
lemon rootstock died to the ground in this manner. When the
area was replanted with the same variety, approximately 50 per-
cent of the new trees died in the same way. Losses of 25 to 50
percent of the trees in other young groves have been brought to
our attention and doubtless others are unreported. This rapid
type of die-back has occurred on newly-planted trees of several
varieties of sweet orange (C. sinensis) and of grapefruit (C.
paradisi) in widely separated areas and also on topped trans-
planted seedlings of Poncirus trifoliata. Temple orange and P.
trifoliata trees seem to be particularly susceptible.
A common fungus, Diplodia natalensis, frequently invades the
dead tissues of trees affected with this type of die-back, but ex-
amination of diseased tissues indicates that it is a secondary in-
vader rather than the primary cause of the disease. Evidence







has been found that another
microorganism is present in
wood of affected trees ap-
proximately 11/2 inches in
advance of the dead tissue.
It has been our suggestion
that topping wounds be pro-
tected from infection by
painting with asphalt emul-
sion on the day the trees are
set out. Growers have re-
ported good control from
this method. For example,
in 1951 a grower set out 750
trees and arranged to have
the topping wounds painted
at the time they were set in
the ground. Two
days after plant-
ing it was found
that only half the
trees had been
painted. The un-
painted trees
were immediate-
ly cut back one
inch (three in-
ches was recom-
mended) and
painted. The
owner reported:
"F i fty percent
infection in the
late painted
trees and practi-
cally no infection
in the trees that
were painted as
they were set."
Of course, paint-
ing the topping
wounds in t h e tree,


Fig. 3.-Bark scaling typical of
Florida gummosis.


-Gum pocket on a mature grapefruit
a symptom of Florida gummosis.






nursery prior to digging would be desirable if it could be
managed.
Florida Gummosis (also called Rio Grande gummosis and
lemon gummosis).-Primarily a trunk and limb disease of older
trees, Florida gummosis is common on lemon and grapefruit
and to a minor extent on tangerine and sweet orange.


Fig. 5.-Wood decay caused by the fungus Fomes applanatus.


Trees frequently become infected with Florida gummosis
through improperly cared for pruning wounds. However, evi-
dence has been found that in the absence of pruning wounds
infection may take place through the bark. This disease causes
the bark to scale off in more or less circular patches that are
sometimes confused with the bark scaling caused by psorosis
(Fig. 3). Large quantities of gum frequently exude from gum-
mosis lesions and the gum pockets that are found in the wood
underneath the bark (Fig. 4). Neither of these symptoms (ac-
tive gumming and gum pockets) is found in psorosis lesions.
Control of Florida gummosis is largely a matter of cleanly (asep-
tic) methods in the handling of pruning wounds discussed on
page 9.







Diplodia Disease.-The fungus Diplodia natalensis is ordi-
narily an invader of weak and injured tissues and attacks many
plants in addition to citrus. Uncared for pruning wounds and
branches damaged by cold, insect attack or other causes are
readily invaded. After gaining a foothold in weakened tissues
Diplodia often invades healthy tissues and then may cause con-
siderable damage. The
presence of Diplodia usu-
ally may be recognized
by the dark gray color
it imparts to the in-
fected wood. In addi-
tio n, considerable a-
mounts of gum are of-
ten produced. Infections
on small branches may
be removed by pruning,
but on larger branches
and trunks it may be
necessary to cut out dis-
eased wood as in the
treatment of foot-rot le-
sions.
Concentric Can k er
and Heart Rot.-It is
doubtful whether wood
rotting fungi enter cit-
trus trees except through
dead wood resulting
from uncared for prun-
i n g wounds, foot-rot
legions, and other injur-
ies. Several saprophytic Fig. 6.-Concentric bark canker on a
fungi, such as species of grapefruit tree infected with Fomes ap-
planatus.
Polyporus a n d Schizo-
phyllum, are found on dead stubs and branches but do not
appear to attack living tissues. The most serious offender un-
der Florida conditions and the only one apparently capable of
invading healthy tissues is the fungus identified as Fomes ap-
planatus. This fungus is probably the principal cause of heart
rot (Fig. 5) in Florida citrus trees and is much more common
than is generally recognized. Comparatively young trees have
7






been severely damaged by Fomes applanatus, but it is more com-
mon on older trees, possibly because these have been "pruned"
more often. Typical cankers marked by more or less concentric
cracks results when the bark adjacent to the point of infection
is killed (Fig. 6). Although the conks, or brackets, may not
appear until several years after infection, they are perennial
and have been estimated to liberate up to 30 million spores daily
for a period of several months. The spores are carried by air
currents to suitable places where they germinate. The fungus
cannot penetrate living bark but the spores can start infection
when they alight on dead wood. Therefore, wounds and injuries
should be properly disinfected and dressed to prevent infection.

CARE AND HANDLING OF PRUNING WOUNDS
The previous discussion has emphasized the fact that a con-
siderable portion of the deterioration and even death of citrus
trees, young and old, can be attributed to careless methods of
pruning. One of
the most frequent
and serious mis-
takes in prun-
ing is the failure
to do a neat,
smooth job in the
first place. Care-
less operators of-
ten leave project-
ing stubs of
branches (Fi g.
1) that seldom
ever callus over.
In time such
wounds may be-
come overgrown
by bark but that
usually occurs
Fig. 7.-Damage resulting from poorly long after gum-
executed pruning. mosis, heart rot,
Diplodia, or a combination of these diseases has become estab-
lished and done considerable damage (Fig. 7). Painting a
poorly pruned stub only delays infection; in time the bark will
die back, exposing unpainted dead wood to infection.
8






Principles of Good Pruning.-The underlying principle of
good pruning is to take advantage of the natural growth tenden-
cies of the tree so that wounds may heal over in the shortest possi-
ble time. It should be remembered that the larger the wound the
longer the time required for it to callus over and the longer it
must be protected. Furthermore, a wound that is most nearly
flush with the adjacent bark surfaces heals most rapidly (Fig.
1). On large branches it may be necessary to make a preliminary
cut to remove the branch and then a second cut to bring the
wound surface to the desired flush condition. It is usually de-
sirable to make a first cut underneath the branch and a finishing
cut from above to prevent splitting of the wood or tearing of
the bark. At times it may be necessary to smooth off the wound
with a chisel.
Wound Dressings and Coatings.-It seems apparent from the
discussion of die-back of young trees and of gummosis and heart
rot in older trees that making a properly executed pruning cut
is only half the job in pruning. It is equally essential that the
cut be covered with a suitable wound dressing.
For small wounds such as occur in topping or shaping young
trees it is sufficient to paint the cut surface with an asphalt emul-
sion dressing such as "Tree-Seal" or "De Ka Go" or others. This
type of material seems ideal for the purpose because when fresh
it can be thinned with water to any desired consistency but after
setting it is completely impervious to moitsure. Also, it remains
slightly plastic and thus allows the expansion and growth of
callus tissue.
When diseased wood has been removed in the recutting of
diseased pruning wounds or in the surgical treatment of foot-
rot lesions it is important to apply to the new surface a good
penetrating disinfectant such as Avenarius carbolineum (Fig.
1). If the wound is allowed to dry through exposure to the air
for several days, penetration of the disinfectant will be im-
proved. A carbolineum-treated wound will remain water-repel-
lent for several months but in time moisture will penetrate it.
Therefore, a final coating of asphalt emulsion should be ap-
plied within 30 days.
If the job is worth doing at all, it is worth doing right, and the
cheapest way is to do it right the first time.





































































COOPERATIVE EXTENSION WORK IN
AGRICULTURE AND HOME ECONOMICS
(Acts of May 8 and June 30, 1914)
AGRICULTURAL EXTENSION SERVICE, UNIVERSITY OF FLORIDA
FLORIDA STATE UNIVERSITY
AND UNITED STATES DEPARTMENT OF AGRICULTURE
COOPERATING
H. G. CLAYTON, DIRECTOR




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