Amador, Jose M.

English

Texas A&M University

L-2315
Texas Agricultural Extension Service
Texas Citr'us
Diseases Affecting
Localized Parts of Trees
Jose M. Amador*
Twig Dieback
Twig dieback can be caused by fungi, although
non-pathogenic factors probably play a more
important role. As a result, fungal infection is
often secondary, following freeze damage or
damage resulting from mechanical or chemical
injury. Other factors that can damage twigs are
excessive fertilization, moisture stress and damage
to the root system by cultural practices or heavy
nematode damage. Affected young branches die
back 1 inch or more from the tip, sometimes
showing gum exudation. Damage by twig dieback
usually is not severe.
Severe reaction to twig dieback on Star Ruby
grapefruit.
*Extension plant pathologist, The Texas A&M University System.
Melanose
Melanose, caused by the fungus Diaporthe citri
(Phomopsis citri), is a twig, leaf and fruit disease
of economic importance in the eastern and central
parts of the Valley where humidity is high. The
disease attacks all commercial varieties of citrus,
but grapefruit appears to be more susceptible than
oranges. The fungus attacks young, tender twigs,
leaves and fruit, especially during periods of high
humidity. Mature, hardened tissue usually is
resistant to infection. Melanose symptoms on
leaves first appear as small, circular, dark depres-
sions with a yellow margin. Later, the spots
become raised and turn dark brown.
Leaves turn yellow and may drop prematurely.
Spots begin on twigs as on leaves, but become
more raised than on leaves. In severe cases, twigs
may die. Melanose spots on the fruit are at first
small, light brown and sunken. Later they become
dark and raised. When several spots are close
together, the surface feels rough.
Citrus twig showing severe symptoms of melanose.
This roughness was cause for the name usand-
paper" melanose. Spots sometimes develop into a
tear-streaked pattern, resulting from infection
Texas Agricultural Extension Service. Zerle L. Carpenter, Director. The Texas A&M University System. College Station, Texas
@\---------------------
caused by spores washed down over the fruit
surface by water drops during heavy dews or light
rains. In other cases, large areas of fruit surface
crack in more or less irregular patterns, resulting
in Hmudcake" melanose.
Abundant sporulation takes place in infected or
dead tissue during periods of high humidity.
Melanose usually is worse in years following a
freeze because there are numerous dead twigs
where the fungus sporulates freely. Disease incid-
ence and severity usually are most serious in the
north and northwest quadrants of trees. Prevailing
drying winds from the southeast and the morning
sun result in prolonged wetness in the opposite
side of the trees, allowing more time for the
infection process. Severe outbreaks of melanose
can be traced to rainy periods occurring in the
spring after spores have developed and while
tissues remain susceptible. However, there is
evidence that melanose infection can occur late in
the season, even after fruit is more than 2 inches
in diameter, provided that rain and cooler than
normal weather prevails during summer and fall
months.
Melanose usually is controlled in the Lower Rio
Grande Valley by a single application of a fungicide
after bloom or petal fall and before the fruit
averages 1/2 inch in diameter. Under adverse
conditions, such as those mentioned above, more
than one application may be necessary.
Greasy Spot
Greasy spot is a leaf disease caused by the
fungus Mycosphaerella horii, a weak parasite. The
spots appear yellow initially, then turn dark and
Typical symptoms of greasy spot infection.
A close-up view of greasy spot on citrus leaves.
appear slightly raised and greasy. With severe
infection, leaves may turn yellow and drop pre-
maturely. The fungus responsible for greasy spots
also can cause rind blemishes on some varieties.
These blemishes usually are small. However, on
grapefruit the spots are large and sometimes
coalesce, resulting in a condition commonly known
as greasy spot rind blotch and also pink pitting.
Rust mite injury and humid weather predispose
most varieties to fungal infection, although there
is controversy as to whether rust mite infection
can be correlated to higher incidences of greasy
spot. A combination of long periods of high
humidity (in excess of 90 percent) and high
temperature following heavy infestation of rust
mites usually results in infection by the greasy
spot fungus. The disease, however, will not become
evident until several months after infection.
Control is possible by spraying in the summer
with neutral copper or oil.
Sooty Mold
This is not a true disease because the several
fungi associated with it do not feed on the tissues
of citrus trees. Rather, the fungi feed on rich
honeydew excreted by insects such as aphids,
brown soft scale and whiteflies. The amount of
sooty mold is directly proportional to the number
of honeydew-excreting insects present. A heavy
sooty mold coating on the fruit can result in fruit
downgrading and economic loss to the grower.
Heavy leaf coating can retard growth by interfering
with photosynthesis and other physiological func-
----------------~
Sooty mold on leaf, branch and fruit.
tions of the leaf. Light fruit set and reduced yields
often result.
Control measures are directed at insects which secrete
honeydew, thus, preventing the development of
sooty mold fungi.
Scab
Scab, caused by the fungus Elsinoe /awcetti, is a
disease of minor importance in Texas. The disease
is more severe on lemons, somewhat troublesome
on grapefruit, and seldom a problem on sweet
orange. Sour orange is highly susceptible, thus,
nursery stocks may become infected before young
sour orange trees are budded.
Raised, corky lesions on leaves infected with the
scab fungus.
Because citrus tissue is susceptible to scab only
while young, the disease is mainly confined to new
growth. The fruit remains susceptible for longer
periods, but seldom is mature fruit affected.
Small lesions appear as translucent dots that
later become pustules. As the disease progresses,
the pustules turn into warts, consisting of a mass
of corky tissue pale tan in color. The leaves
become twisted and distorted and the entire young
branch may be affected.
Applications of fungicide may be justified in
nurseries if scab becomes a problem. The disease
is not a problem in commercial orchards in Texas.
Mesophyll Collapse
Mesophyll collapse is a non-pathogenic disorder
of citrus of unknown cause. Leaves of both oranges
and grapefruit are affected. The disorder begins as
translucent, irregular areas of different sizes
scattered throughout the leaf blade. One or more
light green spots may appear, later turning brown
as cells within the tissue die. Only the soft tissue
inside the leaf between the veins (mesophyll)
collapses at first, giving these areas the translucent
appearance. Palisade and epidermal tissues are
unaffected initially but may die later.
Symptoms appear readily following periods of
low soil moisture and hot dry winds. For this
reason, water deficiency is often thought to cause
mesophyll collapse, but mite feeding, damaged
root systems and excessive transpiration also are
associated with mesophyll collapse. Nutritional
imbalances also are implicated in mesophyll
collapse of citrus.
Citrus Canker
Citrus canker is considered the most important
bacterial disease of citrus worldwide, but is not
found in the Lower Rio Grande Valley of Texas.
The disease became established in the Valley
earlier but was eradicated after several years. This
disease, caused by the bacterium Xanthomonas
campestris pathovar citri, is highly contagious. It
is transmitted easily by infected nursery stock,
budwood, leaves, twigs and fruit and even by
wind-driven rain, insects, animals, people and
contaminated equipment. The disease can be found
in leaves, twigs and fruit of almost any citrus
cultivar, causing serious damage to the fruit as
well as defoliation and general decline of trees.
@\-------------~
Raised, corky lesions on leaves at a later stage of
citrus canker.
There are several races or strains of the causal
agent. These races or strains have varying effects
on the many cultivars of citrus. Environmental
conditions in an area may prevent some of these
strains or races from becoming established, or
from causing serious damage to trees or fruit.
If any form of citrus canker were to be found
again anywhete in Texas, its immediate and most
damaging effect on the citrus industry would be
the almost certain imposition of quarantines
against the sale of Texas citrus fruit. These
quarantine-related regulations would remain in
effect until the disease was eradicated or until
sufficient evidence was accumulated that its spread
poses no threat to other areas. In either case, the
marketing of Texas citrus would be seriously
impaired for quite some time. Because the disease
presently is found in Florida, Mexico, several
countries in South America, Japan, South Africa
and countries in Asia, the introduction of citrus
canker into Texas remains a possibility. For this
reason, all efforts must be made to prevent the
disease from reappearing in Texas.
Lesions appear on leaves as small, round,
blister-like eruptions, having a whitish color at
first, but turning to tan or brown with age. A
Citrus canker lesions on fruit.
yellow halo commonly surrounds older lesions. As
the lesion progresses, a water-soaked, oily area
develops around the lesion. Lesions on the twigs
are similar to those on leaves, but the areas
affected are bigger. Raised, tan, necrotic tissue
commonly is surrounded by oily, water-soaked
margins. The most conspicuous symptoms are on
the fruit. Although early infection results in
symptoms similar to those found on leaves, blister
or crater-type symptoms are more pronounced.
Infection usually is confined to the surface of the
fruit. Other disease-causing organisms may become
established, causing severe fruit drop.
The most effective control practice for citrus
canker is to prevent its introduction into an area
known to be free of the disease. Quarantines have
been effective in keeping canker out of certain
areas. The outbreak of the disease in previously
disease-free areas can usually be traced to the
introduction of diseased plant material or con-
taminated equipment from other places. If the
disease gains entrance into an area and is detected,
its eradication must be attempted by all available
means. Failing to do so, other expensive and
somewhat ineffective practices must be used, such
as developing resistant varieties, using fungicidal
sprays, taking advantage of certain cultural practices
and using strict phytosanitary practices.
Acknowledgment
Appreciation is expressed to Pete Timmer and Mike Davis, former plant pathologists,
Texas A&I University Citrus Center, Weslaco, Texas, for photographs in this publication.
Educational programs conducted by the Texas Agricultural Extension Service serve people of all ages regardless of socioeconomic
level, race, color, sex, religion, handicap or national origin.
Issued in furtherance of Cooperative Extension Work in Agriculture and Home Economics, Acts of Congress of May 8, 1914, as
amended, and June 3D, 1914, in cooperation with the United States Department of Agriculture. Zerle L. Carpenter, Director, Texas
Agricultural Extension Service, The Texas A&M University System.
1.5M-10-88, New HORT 2-2, PP


Texas Citrus Diseases Affecting Localized Parts of Trees.

Texas A&amp;M Repository

L-2315Texas Agricultural Extension ServiceTexas Citr'usDiseases AffectingLocalized Parts of TreesJose M. Amador*Twig DiebackTwig dieback can be caused by fungi, althoughnon-pathogenic factors probably play a moreimportant role. As a result, fungal infection isoften secondary, following freeze damage ordamage resulting from mechanical or chemicalinjury. Other factors that can damage twigs areexcessive fertilization, moisture stress and damageto the root system by cultural practices or heavynematode damage. Affected young branches dieback 1 inch or more from the tip, sometimesshowing gum exudation. Damage by twig diebackusually is not severe.Severe reaction to twig dieback on Star Rubygrapefruit.*Extension plant pathologist, The Texas A&M University System.MelanoseMelanose, caused by the fungus Diaporthe citri(Phomopsis citri), is a twig, leaf and fruit diseaseof economic importance in the eastern and centralparts of the Valley where humidity is high. Thedisease attacks all commercial varieties of citrus,but grapefruit appears to be more susceptible thanoranges. The fungus attacks young, tender twigs,leaves and fruit, especially during periods of highhumidity. Mature, hardened tissue usually isresistant to infection. Melanose symptoms onleaves first appear as small, circular, dark depres-sions with a yellow margin. Later, the spotsbecome raised and turn dark brown.Leaves turn yellow and may drop prematurely.Spots begin on twigs as on leaves, but becomemore raised than on leaves. In severe cases, twigsmay die. Melanose spots on the fruit are at firstsmall, light brown and sunken. Later they becomedark and raised. When several spots are closetogether, the surface feels rough.Citrus twig showing severe symptoms of melanose.This roughness was cause for the name usand-paper" melanose. Spots sometimes develop into atear-streaked pattern, resulting from infectionTexas Agricultural Extension Service. Zerle L. Carpenter, Director. The Texas A&M University System. College Station, Texas@\---------------------caused by spores washed down over the fruitsurface by water drops during heavy dews or lightrains. In other cases, large areas of fruit surfacecrack in more or less irregular patterns, resultingin Hmudcake" melanose.Abundant sporulation takes place in infected ordead tissue during periods of high humidity.Melanose usually is worse in years following afreeze because there are numerous dead twigswhere the fungus sporulates freely. Disease incid-ence and severity usually are most serious in thenorth and northwest quadrants of trees. Prevailingdrying winds from the southeast and the morningsun result in prolonged wetness in the oppositeside of the trees, allowing more time for theinfection process. Severe outbreaks of melanosecan be traced to rainy periods occurring in thespring after spores have developed and whiletissues remain susceptible. However, there isevidence that melanose infection can occur late inthe season, even after fruit is more than 2 inchesin diameter, provided that rain and cooler thannormal weather prevails during summer and fallmonths.Melanose usually is controlled in the Lower RioGrande Valley by a single application of a fungicideafter bloom or petal fall and before the fruitaverages 1/2 inch in diameter. Under adverseconditions, such as those mentioned above, morethan one application may be necessary.Greasy SpotGreasy spot is a leaf disease caused by thefungus Mycosphaerella horii, a weak parasite. Thespots appear yellow initially, then turn dark andTypical symptoms of greasy spot infection.A close-up view of greasy spot on citrus leaves.appear slightly raised and greasy. With severeinfection, leaves may turn yellow and drop pre-maturely. The fungus responsible for greasy spotsalso can cause rind blemishes on some varieties.These blemishes usually are small. However, ongrapefruit the spots are large and sometimescoalesce, resulting in a condition commonly knownas greasy spot rind blotch and also pink pitting.Rust mite injury and humid weather predisposemost varieties to fungal infection, although thereis controversy as to whether rust mite infectioncan be correlated to higher incidences of greasyspot. A combination of long periods of highhumidity (in excess of 90 percent) and hightemperature following heavy infestation of rustmites usually results in infection by the greasyspot fungus. The disease, however, will not becomeevident until several months after infection.Control is possible by spraying in the summerwith neutral copper or oil.Sooty MoldThis is not a true disease because the severalfungi associated with it do not feed on the tissuesof citrus trees. Rather, the fungi feed on richhoneydew excreted by insects such as aphids,brown soft scale and whiteflies. The amount ofsooty mold is directly proportional to the numberof honeydew-excreting insects present. A heavysooty mold coating on the fruit can result in fruitdowngrading and economic loss to the grower.Heavy leaf coating can retard growth by interferingwith photosynthesis and other physiological func-----------------~Sooty mold on leaf, branch and fruit.tions of the leaf. Light fruit set and reduced yieldsoften result.Control measures are directed at insects which secretehoneydew, thus, preventing the development ofsooty mold fungi.ScabScab, caused by the fungus Elsinoe /awcetti, is adisease of minor importance in Texas. The diseaseis more severe on lemons, somewhat troublesomeon grapefruit, and seldom a problem on sweetorange. Sour orange is highly susceptible, thus,nursery stocks may become infected before youngsour orange trees are budded.Raised, corky lesions on leaves infected with thescab fungus.Because citrus tissue is susceptible to scab onlywhile young, the disease is mainly confined to newgrowth. The fruit remains susceptible for longerperiods, but seldom is mature fruit affected.Small lesions appear as translucent dots thatlater become pustules. As the disease progresses,the pustules turn into warts, consisting of a massof corky tissue pale tan in color. The leavesbecome twisted and distorted and the entire youngbranch may be affected.Applications of fungicide may be justified innurseries if scab becomes a problem. The diseaseis not a problem in commercial orchards in Texas.Mesophyll CollapseMesophyll collapse is a non-pathogenic disorderof citrus of unknown cause. Leaves of both orangesand grapefruit are affected. The disorder begins astranslucent, irregular areas of different sizesscattered throughout the leaf blade. One or morelight green spots may appear, later turning brownas cells within the tissue die. Only the soft tissueinside the leaf between the veins (mesophyll)collapses at first, giving these areas the translucentappearance. Palisade and epidermal tissues areunaffected initially but may die later.Symptoms appear readily following periods oflow soil moisture and hot dry winds. For thisreason, water deficiency is often thought to causemesophyll collapse, but mite feeding, damagedroot systems and excessive transpiration also areassociated with mesophyll collapse. Nutritionalimbalances also are implicated in mesophyllcollapse of citrus.Citrus CankerCitrus canker is considered the most importantbacterial disease of citrus worldwide, but is notfound in the Lower Rio Grande Valley of Texas.The disease became established in the Valleyearlier but was eradicated after several years. Thisdisease, caused by the bacterium Xanthomonascampestris pathovar citri, is highly contagious. Itis transmitted easily by infected nursery stock,budwood, leaves, twigs and fruit and even bywind-driven rain, insects, animals, people andcontaminated equipment. The disease can be foundin leaves, twigs and fruit of almost any citruscultivar, causing serious damage to the fruit aswell as defoliation and general decline of trees.@\-------------~Raised, corky lesions on leaves at a later stage ofcitrus canker.There are several races or strains of the causalagent. These races or strains have varying effectson the many cultivars of citrus. Environmentalconditions in an area may prevent some of thesestrains or races from becoming established, orfrom causing serious damage to trees or fruit.If any form of citrus canker were to be foundagain anywhete in Texas, its immediate and mostdamaging effect on the citrus industry would bethe almost certain imposition of quarantinesagainst the sale of Texas citrus fruit. Thesequarantine-related regulations would remain ineffect until the disease was eradicated or untilsufficient evidence was accumulated that its spreadposes no threat to other areas. In either case, themarketing of Texas citrus would be seriouslyimpaired for quite some time. Because the diseasepresently is found in Florida, Mexico, severalcountries in South America, Japan, South Africaand countries in Asia, the introduction of citruscanker into Texas remains a possibility. For thisreason, all efforts must be made to prevent thedisease from reappearing in Texas.Lesions appear on leaves as small, round,blister-like eruptions, having a whitish color atfirst, but turning to tan or brown with age. ACitrus canker lesions on fruit.yellow halo commonly surrounds older lesions. Asthe lesion progresses, a water-soaked, oily areadevelops around the lesion. Lesions on the twigsare similar to those on leaves, but the areasaffected are bigger. Raised, tan, necrotic tissuecommonly is surrounded by oily, water-soakedmargins. The most conspicuous symptoms are onthe fruit. Although early infection results insymptoms similar to those found on leaves, blisteror crater-type symptoms are more pronounced.Infection usually is confined to the surface of thefruit. Other disease-causing organisms may becomeestablished, causing severe fruit drop.The most effective control practice for citruscanker is to prevent its introduction into an areaknown to be free of the disease. Quarantines havebeen effective in keeping canker out of certainareas. The outbreak of the disease in previouslydisease-free areas can usually be traced to theintroduction of diseased plant material or con-taminated equipment from other places. If thedisease gains entrance into an area and is detected,its eradication must be attempted by all availablemeans. Failing to do so, other expensive andsomewhat ineffective practices must be used, suchas developing resistant varieties, using fungicidalsprays, taking advantage of certain cultural practicesand using strict phytosanitary practices.AcknowledgmentAppreciation is expressed to Pete Timmer and Mike Davis, former plant pathologists,Texas A&I University Citrus Center, Weslaco, Texas, for photographs in this publication.Educational programs conducted by the Texas Agricultural Extension Service serve people of all ages regardless of socioeconomiclevel, race, color, sex, religion, handicap or national origin.Issued in furtherance of Cooperative Extension Work in Agriculture and Home Economics, Acts of Congress of May 8, 1914, asamended, and June 3D, 1914, in cooperation with the United States Department of Agriculture. Zerle L. Carpenter, Director, TexasAgricultural Extension Service, The Texas A&M University System.1.5M-10-88, New HORT 2-2, PP

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Texas Citrus Diseases Affecting Localized Parts of Trees.

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Texas Citrus Diseases Affecting Localized Parts of Trees.

Abstract

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Full text

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Texas A&M University

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Texas A&M Repository