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Melanose is a fungal disease of citrus caused by Diaporthe citri Wolf (also known as Phomopsis citri Fawc). It produces pustules (small bumps) of various sizes on the fruit. A light infection on small to relatively large fruit produces small, discrete pustules. In a heavy infestation, the pustules are larger and may coalesce, often causing the tissue to crack and produce a stage called mudcake melanose.
Melanose disease is directly associated with the presence of dead twigs, especially recently killed ones. Dead twigs often harbor the fungus, which produces several dark, egg-shaped structures called pycnidia that contain many spores.
The spores are released from these structures when they become wet. Rainwater washes the spores onto the young fruit, leaves and twigs. The spores that land on young tissue initiate the infection process.
Grapefruit is most sensitive to melanose infection. Although infected fruit become less marketable as fresh fruit, the internal quality is unaffected.
Some growers are confused by melanose and rust mite symptoms. The following tips may help you identify melanose symptoms:
- Melanose symptoms are found on leaves, stems and fruit.
- The presence of dead wood in the tree canopy may be a sign of possible infection.
- Grapefruit trees show more symptoms than other cultivars
- Raised pimples appear on the fruit and leaves.
- When rubbed with fingers, the pustules feel like sandpaper.
- The size and number of pustules on the fruit may vary with the stage of infection.
- The pustules may be small or large, a few or many coalesced
- The fruit may be cracked and/or coalesced.
- If fruit is washed with spore-laden water, the effect will show as tear streaks.
- Rust mite blemishes are smoother than the rough pustules of melanose.
The melanose fungus can also cause a serious post-harvest disease called Phomopsis stem end rot. Both orange and grapefruit cultivars are susceptible to this disease, in which the infected tissue at the stem end part of the fruit shrinks.
A prominent feature of Phomopsis stem end rot is a clear demarcation line visible between the infected and the noninfected part of the fruit. In contrast, another type of postharvest stem end rot caused by the fungus Diplodiadevelops a characteristic finger-like projection of diseased tissue.
If the citrus tree is exposed to hours of below-freezing temperatures, the fungus could cause leaf drop, twig die back and, in a few cases, tree death.
The most important strategy for both melanose and stem end rot disease control is good cultural practice. Remove dead twigs from the tree by pruning, hedging or topping. Timely application of fungicides in the orchard can also reduce melanose infection.
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Greasy spot (GS) is an important disease of citrus caused by the fungus Mycosphaerella citri Whiteside. It is becoming more severe in the LRGV – within the past 3 years, trees in many orchards have been heavily defoliated. Many growers have had greasy spot-infected fruit.
GS reduces tree vigor and thereby fruit size. GS-infected fruit also tend to regreen; such fruit are culled for fresh-market fruit.
To be able to manage the disease, growers need to understand the three stages of the GS life cycle: the epiphytic, the saprophytic and the parasitic stages.
In the epiphytic stage, the GS fungus grows on the lower surface of leaves for a long period before it enters the stomata (air pores) to begin infection. In this stage, which can last for several weeks, asexual spores or conidia are produced. The hyphal tips (threadlike parts) growing on the undersurface of the leaves can function as a germ tube and begin the infection process.
The most important factor involved in the fungal spread of GS is the ascospores, which are the sexual spores developed in special fruiting bodies called perithecia. Perithecia are developed in decaying infected leaves on the ground. The ideal conditions for ascospore germination and epiphytic (surrounding the leaf surface) growth of the fungus include the presence of water or relative humidity near 100 percent and temperatures between 77 to 86 degrees F. The saprophytic stage of the fungus is the period when the perithecia develop and mature in decaying fallen leaves. Water from rain or irrigation enhances the release of ascospores into the air, and air currents carry the spores to the young growth flushes.
The major factors influencing greasy spot infection and disease severity are the number of infected leaves on the ground, the relative humidity, temperature, insect exudations (discharges), the amount of epiphytic growth and the physiological condition of the trees.
Stem-end rot disease showing tear staining and caused by Colletotrichum sp.
In the parasitic stage, the fungal filaments growing on the leaf surface enter the stomatal openings. The fungal growth causes the underside of the leaf surface to swell. This swelling is the first visible symptom of a new GS infection. The next sign is yellow spots, which develop into dark brown blisters resembling oil spots.
The most effective control material is citrus spray oil. Oil may help the cells resist the fungus. Fungicides will destroy the epiphytic growth of the fungus on the leaf surface. They provide protection for several weeks, which prevents the fungus from recolonizing.
The major source of ascospores for the new cycle of infection is infected leaf litter on the ground. If defoliation is severe, the leaf litter must be destroyed.
Florida citrus trees normally are defoliated more heavily than Texas trees. Florida’s rainy season starts in the summer, whereas our rainy season starts in the fall. Spray chemicals after the rainy season.
The epiphytic stage of trees on a microjet irrigation system may have different timing. In Texas, researchers are studying the epiphytic growth of GS fungus in young leaves of Rio Red grapefruit trees.
To destroy the fallen leaves, you can use a combination of blowing, raking and burning, but it is expensive.
Anthracnose is a plant disease that causes black blisters or lesions. These lesions are caused by a fungus that produces spores in an acervulus, which is a saucer-shaped fruiting body (plural: acervuli). Anthracnose diseases occur worldwide, but the most severe losses are in the tropics and subtropics.
The major anthracnose diseases are caused by four members of the ascomycetes:
- Diplocarpon, which causes black spot of rose
- Elsinoe, which causes scab of citrus
- Glomerella, which causes bitter rot of apple
- Gnomonia, which causes anthracnose of oak
Anthracnose diseases are also caused by three members of the imperfect fungi:
- Colletotrichum, which causes anthracnose of citrus, fig, olive and avocado
- Coryneum, which causes shot hole of peach
- Melanconium, which causes bitter rot of grapes
Anthracnose in citrus is normally a postharvest disease of grapefruit, navel oranges and tangerines. It requires ethylene degreening.
The causal organism, Colletotrichum gloeosporioides Penz., is common in LRGV citrus orchards. It grows well and produces many spores in acervuli on deadwood. Rainwater may carry spores onto the fruit surface and often produces tear stains.
Normally, the spores germinate, developing a swollen tip, but they remain inactive at this stage. Ethylene treatment for degreening the fruit breaks the dormancy and stimulates the spores to grow.
Anthracnose does not spread from infected fruit to healthy fruit in the packinghouse or in storage. To prevent anthracnose during degreening, keep the degreening process to the minimum amount of time, limit the ethylene concentration to less than 10 ppm, and store packed fruit at 50 degrees F.
Orchard practices that minimize deadwood of the tree canopy are very important in managing anthracnose. A pre-harvest fungicide spray or a post-harvest treatment with a fungicide are effective chemical controls. In recent years, LRGV citrus growers and packers observed unusual blemishes on grapefruit and navel oranges after degreening. The rind of the affected fruit was firm and silvery gray, and some of the fruit showed tear staining. Eventually, the rind turned brown and developed soft rot.
When examined under a microscope, the fruit surface showed many black spores with swollen tips, especially in the tear-stained areas. The problem was more pronounced in grapefruit than in oranges. Also, the blemish problem was apparently connected to the use of a high concentration of ethylene for degreening.
Researchers isolated the cause of the blemishes to be a fungus, Colletotrichum.
A wide variety of diseases and pests can damage our trees and shrubs. Here are some that often appear in our area:
is an insect that congregates on twigs and limbs of trees and sucks sap. They are not readily recognized as insects because they congregate under leathery coverings, which can be of several different colors. Ants sometimes cluster around the scale infestations to live off their secretions, and can function as unwitting protectors for the scale by warding off predators. The fungus sooty mould can also be associated with scale outbreaks. Scale can be treated with horticultural oil or soapy water in combination with thinning of leaves and twigs to increase airflow through the plant.
is a black substance that accumulates on leaves and twigs. Whiteflies secrete a sugary substance on the plant which promotes the growth of the mildew. The mildew is not dangerous to your plant, but it is unattractive. Repeated applications of horticultural oil or soapy water can eventually kill it. Thinning out the limbs a bit to promote airflow around the branches can also be beneficial.
Southern Pine Beetles
periodically infest our area, with fatal effect for many pine species. Outward signs of infestation are whitish-yellow tunnels of resin on the tree and reddish sawdust on the ground, in spider webs on the tree, or in crevices of the bark where the beetles have drilled into the wood. Once they have infested the tree, the beetles’ entire life cycle takes place there. The needles of the tree turn yellow, then red-brown, at which point the tree will die. In a forest setting, the most effective ways of controlling this pest are thinning the stands of pine and controlled burns, neither of which are applicable to city lawns. Insecticides are available for small-scale use, but are considered too expensive to treat entire forests.
results from an iron deficiency in the plant. If there is too little iron available to the plant, the formation of chlorophyll is hampered and the leaves turn yellow. This may be the result of there being too little iron or nitrogen in the soil itself, or in a shortage of the nutrients the plant needs to access it. Fertilization with a high nitrogen fertilizer may help reverse the effects.
is the result of insect or bacterial damage to the plant. Parasitic organisms affect the growth of the plant, causing abnormal growths on various parts of the plant. Sometimes these appear to be fuzzy or hard balls, but they may also be a mass of them on twigs. They are not in themselves harmful to the plant, being essentially an abnormal-looking part of the plant, so it is unlikely that treating them will have any effect, even if you target the insects, fungi, or bacteria themselves.
are tiny insects that feed on leaves, sucking nutrients from their cells. The leaves thus affected appear speckled and eventually drop off. If this goes on long enough, it can result in death. Ladybugs and other beetles are predators that will take care of the mites the way nature intended.
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Citrus tatter leaf virus (CTLV)
Citrus tatter leaf virus (CTLV) was first discovered in California in 1962 on Meyer lemon trees introduced from China. This disease was also reported from other places in the 1960s and ‘70s. In Texas, CTLV-like symptoms were discovered in indicator plants inoculated with tissue from Meyer lemon.
Most citrus species and commercial cultivars are symptomless carriers of this virus. A bud-union crease (with or without fluting of the stem) may develop when infected scions are grafted to trifoliate orange or its hybrids. When the bud-union crease is severe, the tops may shear off at the union in high winds.
Citrus tatter leaf virus is an important disease that growers should take into consideration when replacing sour orange with rootstocks of trifoliate orange or its hybrids. Always use virus-free plants for propagation. CTLV-free Meyer lemon is already available in Texas, and nurseries are encouraged to use only virus-free budwood.
The presence of CTLV is generally detected based on reactions of indicator plants such as Citrus excelsa and citranges. The common symptoms of CTLV infection are tattering of young leaves, chlorosis and asymmetric leaf distortion.
Citrus exocortis viroid (CEVd)
Field symptoms are rare among commercial citrus trees in Texas. This is because the common rootstock, sour orange, is tolerant to CEVd. However, commercial citrus on tolerant rootstock can be stunted to some degree. Susceptible rootstocks that show field symptoms include trifoliate orange and its hybrids. These rootstocks show bark scaling and tree dwarfing. In the 1990s, several samples suspected to have CEVd were collected from commercial plantings. The presence of CEVd was tested by indexing on to ‘Etrog’ citron indicator plants. Of 45 suspected samples, 16 were found to carry CEVd based on the reaction of Etrog citron indicator plants for CEVd.
Cachexia (also called xyloporosis) is another viroid disease of citrus. Unlike exocortis, it affects the scion part of mandarins, tangelos and Palestine sweet lime trees. Cachexia produces wood pitting and gumming in the bark and distorted leaves in mature trees. Although this viroid is present in many citrus-producing areas, the disease symptoms are more prevalent in some of the Mediterranean countries.
Citrus nematode, Tylenchulus semipenetrans Cobb, is an important pest in the LRGV. It feeds on young root tissue, using a spear or stylet (a slender probe) protruding from the “head.” Feeding by an inconceivably large number of nematodes often results in the general decline of tree health and the production of fewer and smaller fruit. Affected trees do not die from citrus nematode infection alone. The effect of citrus nematode is often referred to as “slow decline.” Sour orange, the predominant rootstock used in the Valley, is susceptible to attack by citrus nematodes. The greatest concentration of nematodes is in the upper 1 foot of soil.
A typical life cycle of citrus nematode can take 1 to 2 months. Eggs that are destined to be males do not develop a stylet and therefore cannot feed on root tissue. Female juvenile larvae feed on root surface cells, each larva often embedding 1⁄4 of its anterior body into root tissue (this is about 4 to 5 cells deep). Adult females penetrate deep into roots. Thus, a typical feeding nematode becomes a sedentary pest that feeds and develops a comparatively larger body (the posterior part) outside the root tissue. Females excrete gelatin and deposit many eggs into it. Nematode populations are influenced by such factors as climate, soil type and root mass. Peaks in citrus nematode population in soil and roots are often associated with new growth of roots. Therefore, spring is an ideal time to have soil samples tested for nematodes.
The best strategy to control citrus nematode is prevention. Start with buying and planting trees grown in nematode-free soil. Unfortunately, most nurseries in the Valley have no programs to eliminate nematodes or to plant trees in nematode-free nursery plots.
In soils heavily infested with nematodes, the preferred control method is to fumigate the orchard site before planting. In dooryard situations, nematode populations can be managed by solarizing the soil with plastic mulch before planting.
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When managing nematode problems, remember:
- Plant trees that are free of nematodes, other pests and diseases.
- When sampling, collect as many soil and root samples as possible.
- Before planting, have the soil tested to assess the nematode levels. The test can help you decide whether to fumigate the soil.
- To collect a soil sample, scrape off the top 1 inch of soil and collect the upper 8 to 10 inches along with many feeder roots.
- Collect soil samples in the spring or fall.
- Keep the samples at room temperature or in a cooler; avoid direct sunlight before shipping them to a laboratory.
Common Tree Pests & Diseases Of Central Texas
Stress and Trees
City life can be stressful, and trees feel stress too. Common urban tree issues include:
- Limited growing space
- Poor soil conditions
- Urban Heat Island
- Lots of people
1. May be limited in their ability to acquire essential resources from their environment
2. May not have the resources to respond to an attack
3. Become increasingly vulnerable with each additional stress factor
So improving a tree’s vigor through good tree care practices can help it be less likely to attract pests and disease, and can help it to fight them off and recover if it gets attacked
Common Central Texas Insects
- Gall Makers
There are many different diseases that affect landscape trees and shrubs. Control of tree and shrub diseases cannot be properly accomplished until the disease pest is identified. Identification of tree and shrub diseases is crucial because, although most diseases can be controlled, there are some diseases that cannot be controlled.
Disease control on landscape trees and shrubs can sometimes be accomplished by more than one method, depending on the particular disease that if infecting your landscape plants. Fungicides are often used to control diseases on landscape trees and shrubs and fungicides may be sprayed, injected into the tree trunk, or even injected into the soil surrounding the roots of the tree or shrub. In other cases cultural changes, such as tree pruning, tree fertilization, or altering watering habits, may reduce disease infection on landscape trees and shrubs.
Even if you are going to attempt to control a disease that is infecting your landscape trees or shrubs you should consider consulting a local arborist. The arborist can identify the disease and advise you if there are treatments available to control the disease and the proper time to apply the tree disease control treatments.
SYMPTOMS OF ANTHRACNOSE DISEASE
Evidence of Anthracnose varies depending on the tree species being infected. Tree leaves may have small dead spots or large irregular dead blotches. The spots, or blotches, on the leaves may be black, brown, or purple and infections are often found along the veins of tree leaves. Infected leaves often become distorted due to the unequal growth occurring from healthy and infected portions of the leaf.
Ash Anthracnose (Apiognomonia errabunda)
Ash leaves with brown blotches littering the ground in late spring is an indication the tree may be infected with Anthracnose. The infection often occurs at the tip of the leaflet or in the middle of the leaflet, causing the leaflet to have a bend in it from uneven growth. Infection is generally confined to the lower half of the tree canopy.
Dogwood Anthracnose (Discula destructiva)
Extensive loss of both native woodland dogwoods and ornamental dogwood trees has resulted from Dogwood Anthracnose infection. This disease infects both Dogwood leaves and twigs.
Leaf infections appear on the upper surface of the leaf as brown blotches with purple borders. The infected areas appear tan colored on the underside of the leaf. The infected areas on one side of a leaf usually stop when they reach the midvein of the leaf.
As the disease moves into the wood of the tree twig dieback will begin to occur. Elliptical cankers may form at the base of dead branches and fungal fruiting bodies may be seen on dead twigs. Epicormic sprouts may grow out from the main trunk below dead branches.
Maple Anthracnose (Gloeosporium spp., Discula spp., Kabatiella apocrypta)
Anthracnose on Maple trees is limited to leaf infections. Irregular tan, brown, or reddish-brown blotches may occur along the leaf veins or on the leaf margins. Anthracnose on Maple trees is rarely serious, but can cause leaf drop and unsightly leaves.
Oak Anthracnose (Apiognomonia quercina)
When prolonged cool, moist weather occurs during leaf development Anthracnose infection can be severe on oak trees, especially White Oaks. Large brown dead areas will develop between the leaf veins, usually on the lower half of the tree. Leaves may appear shriveled, cupped, and distorted. Twig infection can occur and twig or branch dieback may be evident next spring.
Sycamore Anthracnose (Apiognomonia veneta)
Sycamore trees can be seriously affected by Anthracnose infection. Anthracnose on Sycamore trees infects the leaves, buds, twigs, and shoots. When optimal conditions are present almost complete defoliation of the tree may occur.
Tan to reddish-brown blotches develop along the veins of infected leaves. Eventually, the disease moves into the petiole of the leaf and then into the twigs of the tree. Cankers develop on the twigs from which new spores are produced. Anthracnose is worse in coastal regions due to elevated moisture levels.
Trunk injection of fungicides may be an option to control Anthracnose in large Sycamore trees.
Walnut Anthracnose (Gnomonia leptostyla)
Anthracnose on walnut trees starts as a brown lesion surrounded by a yellow margin on the underside of the leaflet. Eventually, the lesion can be seen on the upper leaf surface as well. Leaves begin to turn yellow, curl, and fall prematurely.
Anthracnose on Black Walnuts can affect nut production. Lesions on Black Walnut fruit appears as sunken spots on the husk of the nut. The infected nuts will have reduced quality and may drop prematurely.
Anthracnose on walnuts may increase as the summer progresses if frequent rainfall occurs. Often, secondary spores, called summer spores, are produced. These spores are spread throughout the tree by wind or rain.