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Black Rot (fungus - Alternaria citri)
The infection starts while the fruit is still on the tree. The fruit is infected through cracks or openings on the stylar end of fruit. The fruit colors prematurely and drops.
Brown Rot (fungus - Phytophthora spp.)
The
same species of Phytophthora that causes foot rot can infect fruit, commonly
those close to the ground. Fruit are infected during periods of excessive rain,
resulting in decayed areas that are brown, firm and leathery. Later, a white,
velvety growth is seen on the surface of the fruit [Picture],
accompanied by a strong fermenting odor. The best control for brown rot is sanitation
during transit and in the packing house. Disinfecting the boxes is important.
Treatment with disinfectant solutions and refrigeration are effective for prevention.
Cachexia (Xyloporosis) (viroid)
This
pathogen makes the inner bark surface bumpy [Picture
]. The bark projections are smooth, in contrast to the sharp projections
produced by the citrus tristeza virus. Cachexia affects certain mandarins and
tangelos, particularly Orlando tangelo. Sour orange is tolerant to the virus.
Citrus Tatter Leaf (tatter leaf-citrange stunt virus)
This
virus causes a bud union crease. It can be detected using certain indicator
plants (e.g. swingle citromelo citrus) [Picture].
It is serious on rootstocks of Troyer and Carrizo.
Cotton Root Rot (fungus - Phymatotrichopsis omnivora)
This is a minor disease of citrus and it rarely appears on trees that are more than three years old. The fungus kills trees quickly; most of the dried leaves remain attached to the branches.
See section on Cotton Root Rot
Diplodia Stem-End Rot (fungus - Diplodia natalensis, also known as Physalospora rhodina)
Fruit
are contaminated with the fungus in the field, but the disease commonly occurs
at the packing house or in transit. Decay occurs around the stem end [Picture
] and advances in streaks down the side of the fruit. There is no fungal
growth on the surface of the fruit. Decay is reduced by dipping the fruit in
fungicide solutions before placement in de-greening rooms. Promptness in handling
and shipping, as well as refrigeration during transit, will minimize losses.
Exocortis (citrus exocortis viroid)
This
pathogen causes bark-shelling and stunting of trees on trifoliate orange, trifoliate
hybrids, and Rangpur lime rootstocks [Picture].
Sour orange rootstock is tolerant. In the early stages of the disease, gum exudes
from pustules at the base of the trunk and may extend from below the soil line
to the bud union. New bark forms beneath the pustules, and the outer bark sloughs
off forming the characteristic bark-shelling [Picture].
The tree eventually declines. The pathogen is spread via grafts and also via
pruning tools. The pathogen can be detected following grafting on Etrog citron.
Symptoms on this indicator plant include stunting and leaf deformation [Picture]
and necrosis of the leaf midrib [Picture].
Feeder Root Rot (fungi - Phytophthora spp.)
The fungus infects the cortex of feeder roots, giving the root system a stringy appearance. This can lead to yield loss and a general prolonged tree decline. Sour orange rootstock is relatively resistant. This disease can be controlled with fungicides. Soil solarization may also be an effective treatment.
Flyspeck (fungus - Leptothyrium pomi)
This
is caused by a fungus that keeps citrus fruit from turning yellow in the infected
spots. Small black specks are formed on the rind in areas immediately surrounding
the oil glands [Picture].
The contrast between green and yellow color lowers fruit grade, even though
there is no effect on fruit or juice quality.
Firing (physiological - wind dessication)
The
injury [Picture] is usually
limited to late growth flush (September or October) which has not matured prior
to the arrival of a dry cool front in October or November.
Green Mold (fungus - Penicillium digitatum)
The
fungus causes a rapid breakdown of fruit punctured or bruised during harvesting
and packing operations. The fungus enters the fruit only through wounds. A white
mold that later turns green is seen growing on the skin [Picture].
The decayed fruit becomes soft and shrinks in size. Losses from green mold can
be reduced by preventing injury during harvest and transit. The fruit is treated
in the packing shed with a fungicide solution.
Greasy Spot (fungus - Mycosphaerella citri)
.
This
is a very common disease in Texas. The fungus is primarily a leaf pathogen,
but it can also infect fruit. Yellow spots are seen at first on the upper surface
of the leaf [Picture]. The
corresponding areas on the undersides of the leaves turn dark and appear slightly
raised and greasy [Picture].
With severe infection, leaves may turn yellow and drop prematurely. The infection
is favored by rust mite injury and humid weather. Yellowing caused by scale
insects may be mistaken for early symptoms of greasy spot [Picture].
Either citrus oils or fungicides can be applied during the summer for control. Heavy infections occurring in wet years may require 2 to 3 fungicide treatments. Since infection occurs on the lower leaf surface, thorough spray coverage is essential for effective control. The spores of the fungus originate in the leaf litter on the orchard floor and are released following rainfall or irrigation. Thus, removing litter will reduce the amount of spores present.
Heart Rot/ Wood Rot (various fungi)
The
visible external symptom is the fruiting structures ("conks") of the fungus
projecting from the trunk [Picture
]. Eventually, the tree declines. Species documented on citrus in Texas
include Ganoderma lucidum [Picture],
which has a shiny, brown appearance on the upper surface of the conk, and Phellinus
robustus [Picture], which appears
flat on the bark surface.
These fungi, and other species, cause decay of internal wood. The fungi enter the tree through injuries to the roots or scaffold limbs or improper pruning cuts. Growth of the pathogen is favored when the tree is under environmental stress. Once infected, the tree can not be treated. Proper pruning practices, which promote rapid wound healing, will minimize the risk of infection.
The
larvae tunnel within the leaf, leaving snaking trails of dead tissue [Picture].
Leaf Yellowing (physiological - various causes)
Injury
by the herbicides Krovar (a mixture of diuron and bromacil) or Hyvar (bromacil)
can cause yellowing [Picture].
Magnesium deficiency causes yellowing of the leaf, except for a lanceolate area
along the petiole [Picture].
However, this type of yellowing can also be caused by iron or zinc deficiency,
or it may be a consequence of foot or root rot.
Melanose (fungus - Diaporthe citri)
This
is primarily a disease of grapefruit. The first symptoms on leaves are small,
circular, dark depressions with a yellow margin. Later, the spots become raised
and turn dark brown [Picture].
Leaves turn yellow and may drop prematurely. Raised spots are also found on
twigs, which may be killed. Fruit can also become infected. The disease can
be severe following rainy periods in the spring, particularly when such periods
follow a freeze that has left an abundance of dead twigs.
Melanose is usually controlled by a single fungicide application after bloom or petal fall and before the fruit averages one-half inch in diameter. With a particularly wet spring, a second application may be needed. Citrus should be sprayed for melanose control in the spring following a freeze. Chemical control efforts in heavily infected orchards may need to be combined with supplemental pruning to reduce the quantity of the pathogen.
This
fungus also infects fruits. 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 has a rough feeling to the touch, hence the name "sandpaper"
melanose. Spots sometimes develop in a tear-streaked pattern [Picture
], resulting from infection caused by spores which wash down over the fruit
surface during heavy dews or light rains. In other cases, large areas of the
fruit surface crack in irregular patterns, resulting in the "mud cake" type
of melanose.
The
damage is caused to rind cells by peel oil release as a result of abrasion,
rough handling or other injuries. Foggy and wet weather predispose rind to oleocellosis
by making the fruit more turgid. The first symptoms are spots [Picture],
which later become sunken [Picture
]. The damage can be minimized by harvesting later in the day on days with
cool, foggy mornings.
Phytophthora Foot Rot/Gummosis (fungus - Phytophthora spp.)
This
is one of the more important pathogens of citrus in Texas. It primarily infects
the roots and the trunk [Picture].
Infection of the trunk results in a dark, water-soaked areas, often with profuse
exudation of a dark resin from the lesion [Picture].
The dead bark frequently sloughs off the wood in vertical strips. If the lesion
encircles the trunk, girdling occurs, leading to the death of the tree. Trees
killed by this fungus are defoliated and show an abundance of fruit [Picture].Trees
with the bud union beneath or close to the soil and trees in poorly drained
locations are especially susceptible to infection.
Phytophthora foot rot can be prevented through the use of resistant sour orange rootstock, budded 4 to 6 inches high, and planting at the proper depth in well-drained soil. Flood irrigation water must be kept below the scion and micro-irrigation emitters should be placed so that the trunk is not left wet for prolonged periods. Soil banks should be removed as soon as the danger of frost has passed. When tree wraps are used on young trees, trunks should be treated with a recommended fungicide and they should be inspected for bark damage or gumming if there has been fire ant activity.
Pruning and sprout removal are best done only in dry weather. Cultural activities that result in occasional trunk injury, such as disking or boom spray applications should be done at least 2 days in advance of irrigation.
Trunk lesions can be treated with fungicides. Severely damaged or unproductive trees should be removed from the orchard.
Sweet
orange, grapefruit, and tangerine are severely affected. The pathogen is spread
by budding nursery stock with budwood from infected trees. Variable symptoms
can be produced on young leaves and can include pale flecks or mottling. The
earliest bark symptom is scaling [Picture].
Bark scaling of the trunk and larger branches is typical of psorosis symptoms
in citrus trees eight to 12 years or older. As the disease progresses, the tree
declines rapidly and eventually becomes unproductive [Picture
].
Rio Grande Gummosis (fungus - Physalospora rhodina, also known as Diplodia natalensis, possibly other fungi involved)
This is not a common disease. Narrow cracks appear in the bark which exude a pale yellow gum. Symptoms are usually found in association with injured or dead branches. The wood beneath infected tissue is pink to orange in color. Later, gum pockets develop beneath the bark. Stressful growing conditions, such as freeze damage, high water table, salt accumulation, and poor cultural practices predispose trees to the disease. Removal of dead wood and pruning which promotes quick healing is recommended as a preventative measure.
The
fruit can be scarred as a result of mite feeding [Picture].
Early infestation results in a sharkskinned fruit texture of tan color, and
fruit size is reduced. Later infestation causes russetting or bronzing of the
rind.
Slow Decline (nematode - Tylenchulus semipenetrans)
The nematode causing this disease attacks the roots. Symptoms associated with the presence of nematodes are wilting, lack of vigor, poor fruit production, small fruit size, and poor response to watering and fertilization. Poor drainage, excess salt, improper watering and damage caused by other soilborne organisms will affect trees in much the same way as the citrus nematode.
Positive diagnosis requires laboratory analysis of root and soil samples, which are obtained following specific sampling guidelines. Losses caused by the citrus nematode in heavily infected orchards can be reduced by applying nematicides to the soil. The table of citrus nematode action thresholds can be used as a guide to determine whether nematode populations warrant chemical controls.
When establishing a new orchard, avoid old sites known to be infested. Soil should be sampled before establishing a new orchard to determine if parasitic nematodes are present in damaging numbers. Ideally, nematode-free plants should be used when establishing new orchards, i.e. grown in containers or in fumigated nursery soil.
Sooty Mold (fungi - Capnodium spp.)
Leaves, fruit and sometimes branches have a black, moldy appearance. The fungi causing sooty mold do not actually infect the plant, instead they grow on the sugary exudates of insects such as aphids, brown soft scale, blackflies and whiteflies. The amount of sooty mold found is directly proportional to the number of honeydew-secreting insects present.
A heavy sooty mold coating on the fruit can result in a lower grade of fruit, while a heavy coating of fungi on the leaves could retard growth, causing lighter flowering and reduced yields. Insect control will prevent sooty mold development. Heavy sooty mold can be treated by adding one-half percent oil to the insecticide spray.
The fruit has a dry, brown sunken spot [Picture
33]. 
Sunscald (physiological - strong sunlight)
Irregular,
brown pustules are found on the underside of leaves [Picture].
This occurs when the undersides of leaves are suddenly exposed to direct sunlight,
for example, after harvest because of repositioning of limbs and leaves after
removal of the fruit that had bent them downward.
Tristeza (citrus tristeza virus)
The virus infects most types of citrus and it is transmitted by certain aphids or by the use infected budwood. The symptoms produced depend upon the variety, environmental conditions, and the virus strain. The virus can cause a decline [Picture ] or death of the tree [Picture]. Mild strains were identified in Texas in 1991, although tristeza was also detected in the 1950's. There are several serious strains that are not known to occur in Texas. Three strains are of particular concern: one causes the decline of sweet orange on sour orange by killing phloem, resulting in girdling at the bud union; and the stem pitting strains (one on grapefruit, the other on sweet orange). The stem pitting strains causes limbs of trees to become brittle, forms deep pits on the underside of bark, and a roping appearance of the trunk that results in a decline of the tree. Although the tree doesn't die, there is reduced set, size and quality of fruit.
The virus is inefficiently vectored by three species of aphids present in Texas: Aphis gossypii, A. spiraeola, and Toxoptera aurantii. It is very efficiently vectored by the brown citrus aphid, Toxoptera citricida, which is present in Florida, but not in Texas. If the brown citrus aphid arrives in Texas, it could move severe strains of tristeza suspected to be present on symptomless hosts in dooryard (i.e. non-commercial) trees to commercial orchards, potentially destroying the Texas industry. This is because sour orange rootstock, which is predominantly used in Texas, is susceptible to severe strains.
Control efforts in Texas are mainly directed toward keeping infected budwood out of the Lower Rio Grande Valley by quarantine measures and a shoot tip grafting and budwood certification program to clean up infected material already in Texas and place it in commercial orchards. Should these measures fail, a proposed alternative control is cross-protection of trees with mild strains of the virus and/or alternative rootstocks which may tolerate the disease.
Twig
dieback can be caused by several fungi, as well as by other factors. Fungal
infection often occurs following a freeze or mechanical or chemical injury.
Affected young branches die back from the tip, sometimes producing gum exudation.
Wood is discolored underneath the bark [Picture
]. Damage by twig dieback usually is not severe.
This
damage occurs as the result of twigs or leaves rubbing against young fruit [Picture]The
injury develops early, i.e. within the first couple of months post-bloom.
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