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What Is Sunblotch: Treatment For Sunblotch In Avocado Plants

What Is Sunblotch: Treatment For Sunblotch In Avocado Plants


By: Bonnie L. Grant, Certified Urban Agriculturist

Sunblotch disease occurs on tropical and subtropical plants. Avocados seem particularly susceptible, and there is no treatment for sunblotch since it arrives with the plant. The best recourse is prevention through careful stock selection and resistant plants. So what is sunblotch? Read on to learn more about recognizing and treating avocados with sunblotch.

What is Sunblotch?

Sunblotch on avocados was first reported in California during the late 1920s, and it has subsequently been reported in avocado growing regions around the world. It was several decades until biologists confirmed that the disease, initially believed to be a genetic disorder, is actually caused by a viroid – an infectious entity smaller than a virus. The viroid is known as avocado sunblotch viroid.

Avocado Sunblotch Symptoms

Sunblotch in avocado damages the fruit and is introduced by grafted wood or from seed. Fruit develops cankers, cracks and is generally unattractive.

The biggest issue is reduced fruit yield on trees that are affected. Identifying sunblotch on avocados is tricky because there is such a variation in symptoms, and some host trees are symptomless carriers that may display no symptoms at all. Keep in mind that symptomless carriers have a higher concentration of viroids than trees that display symptoms, thus spreading the disease rapidly.

Typical avocado sunblotch symptoms include:

  • Stunted growth and reduced yields
  • Yellow, red or white discolorations or sunken areas and lesions on fruit
  • Small or misshapen fruit
  • Red, pink, white or yellow streaks on bark or twigs, or in lengthwise indentations
  • Deformed leaves with bleached-looking, yellow or white areas
  • Cracking, alligator-like bark
  • Sprawling limbs on lower part of tree

Sunblotch Disease Transmission

Most sunblotch is introduced to the plant in the grafting process when diseased bud wood is joined to a rootstock. Most cuttings and seeds from diseased plants are infected. Viroids are transmitted in pollen and affect the fruit and seeds produced from the fruit. Seedlings from seed may not be affected. Sunblotch in avocado seedlings occurs eight to 30 percent of the time.

Some infection may also occur with mechanical transmission such as cutting implements.

It is possible for trees with avocado sunblotch viroid disease to recover and show no symptoms. These trees, however, still carry the viroid and tend to have low fruit production. In fact, transmission rates are higher in plants that carry the viroid but do not exhibit symptoms.

Treatment for Sunblotch in Avocados

The first defense is sanitizing. Avocado sunblotch is easily transmitted by pruning tools, but you can prevent transmission by scrubbing tools thoroughly before soaking them with a bleach solution or a registered disinfectant. Be sure to clean tools between each tree. In the orchard setting, the disease progresses quickly from cuts made with infected pruning instruments. Sanitize in a solution of water and bleach or 1.5 percent sodium hydrochloride.

Plant only disease-free seeds, or start with registered disease-free nursery stock. Keep a close eye on young trees and remove any that show signs of avocado sunblotch viroid. Use chemicals to kill the stumps.

Prune avocado trees carefully and keep in mind that stress caused by severe pruning of symptomless carriers may cause the viroid to become more active in new growth and previously uninfected trees.

If you already have trees with the symptoms; unfortunately, you should remove them to avoid spreading the viroid. Watch young plants carefully at installation and as they establish and take steps to nip the problem in the bud at the first sign of sunblotch disease.

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Why Doesn’t Your Avocado Tree Produce Fruit? Reasons for Avocado Trees without Fruit

Although avocado trees produce more than a million flowers when they bloom, most fall from the tree without producing fruit. This extreme flowering is a natural way to encourage pollination visits.

Even with this excessive blooming, there are several reasons why avocados don’t bear fruit. Some of them are:

The tree grew from an ungrafted variety

First of all, grafted trees usually begin to produce fruit in 3-4 years while avocado seedlings (not grafted) take longer to produce (7-10 years), if at all. So one of the reasons why avocados won’t produce fruit is just because it’s not a grafted variety.

Temperature

Avocados planted in the area with a hot temperature zone can bear fruit, but if you are in a cooler region, the tree can survive but never produce fruit. In the Fuerte variety, for example, there is no good fruiting below 13 ºC and above 40-45 ºC.

The high temperature causes the fruit to drop and the low one leads to the formation of parthenocarpic fruits, without commercial value. Also, avocados often produce fruit that is heavy one year and the following year produces a much lighter fruit set. This is called a biennial fruit.

Lack of water

As the avocado is considered very demanding in water, the low availability of water causes a reduction in the size of the fruit and the excess is also not tolerated by the plant.

Excessive watering

It is often thought that if the water is life, the more we give plants it will be better, but the reality is very different. If we water too much, the roots will rot. For this reason, it should be watered only when necessary.

The frequency will vary depending on the weather and the season we are in, but it will generally be about 4 times a week in summer and every 3-4 days the rest of the year.

If we have a fruit tree that has suffered from excessive irrigation, it is important to treat it with fungicides since the fungi may be attacking it.

Lack of nutrients

As for nutrients, nitrogen and potassium are the most important for the production of avocado. If there is a lack of these nutrients in the soil where the avocado is planted, it may not bear the fruit.

Excess nutrients

The fact that the fruits do not develop is due to excess nutrients. Therefore, it is best to acquire nutrient meters so that it can be controlled in the best way. Also, the mulch helps to avoid this problem.

Lack of light

If the tree or plant does not receive enough sunlight, it may not produce the fruit.

Excess use of fertilizer

An excess of a fertilizer, especially if it is synthetic chemical can burn the roots and greatly weaken the trees. For this reason, it is necessary to follow the instructions specified on the packaging to avoid the risk of overdose.

Not yet fruitful period

It is possible that your plant does not want to bear fruit because their age has not yet reached the age limit for fruiting. The age limit for fruiting depends on the type of plant and the type of seed. Is it from seed, cuttings, or grafting?

Infertility flowers

One of the other causes can also be due to infertile flowers. These changes usually occur due to gene mutations. The nature of plants that were originally flowers should be fruit, changed from their original nature due to environmental influences.

Climate and environment do not support

All types of fruit plants will live and bear fruit well if their living conditions such as climate (rainfall, wind, temperature, humidity, sunlight), soil conditions, and the environment are met. If not, plants will be hampered by growth and productivity.

Poor care and attention

Your plants may be reluctant to bear fruit because of their need for light (for photosynthesis), air temperature and humidity, irrigation, air circulation, and nutrients are insufficient.

Plants will flower and bear fruit just in time with productivity when selecting seeds, environmental conditions, and how to care well.

Root rot

If the soil has been over-watered or the water is not drained properly, there may be an accumulation of the liquid rotting the roots and, therefore, it will not be able to bear fruit.

Lack of pollination in the garden

The lack of insects can cause the absence of fruits. So brightly colored plants would need to be secured to attract pollinating insects. As these visits are generated in the garden, you can have trees full of fruits.

Flowering pattern

Avocados have an extraordinary flowering behavior called protogynous dichogamy. All the meaning of this complicated word is that the tree has both male and female organs that function in each flower.

During the two days, the first bloom opens as a woman, and the next day as a man. Each flower opening lasts about half a day.

To further complicate matters, the pattern of the flowering of avocados is divided into two groups: type A and type B. Type A flowers open as females in the morning and then as males, while Type B blooms open as males followed by females.

For these reasons, if you have a type A avocado plantation only, pollination will not occur because the flowers have a different fertilization time than pollen release. With that in mind, planting should be done by intercalating type A avocados with others of type B, so that the release of pollen from one coincides with the reception time of the other.

Temperature plays a role in how well-synchronized bloom patterns are achieved. The optimal temperature for flowering is 68-77 F. (20-25 C.). Higher or lower temperatures can change how well the tree pollinates.

The plant doesn’t flower

The tree has not flowered, either because it has grown too much in previous years and it needs to replenish its reserves, or because the climatic conditions were not favorable for flowering. So obviously no flower no fruit. The other reasons for the tree not blooming flowers can be:

  • It is perfectly natural for young avocado trees to drop flowers in their first or even second year.
  • Avocados need a cold period to promote flowering and fruiting. They need to experience temperatures between 0 and 7 C during the period of inactivity. Temperatures should be fairly constant for several months. A sudden cold snap could affect flower production. As the buds form, a late frost could cause them to die and fall.
  • A common mistake is to prune at the wrong time and remove too much wood from the tree. Avocados do not need much pruning, removing more than a third of the wood, especially the terminal ends, can remove the yolk. However, light pruning can improve light circulation and penetration, promoting budding.
  • Fertilizing a tree, especially with nitrogen, can also help prevent the avocado fruits from blooming.

    A flower doesn’t get fertilized

    The tree has flowered but the flowers have not been fertilized: lack of pollinating insects, dry wind at the time of flowering, etc.

    1. Another possibility: The flowers are fertilized but the young fruits fall before reaching maturity.

      External factors

      It can take about 10-15 years from the time the seed is struck to become productive. An avocado tree ripens 150-200 fruits a year. It prefers well-drained soil, so make sure the soil you plant it in is always moist, but it suffers from over-irrigation. It is not cold and frost tolerant, it likes diffused light the most.

      Diseases of the avocado tree

      External factors such as animals, parasites, or diseases caused by unsuitable soil can affect the development of the avocado tree. Let’s look at some of these:

      • Phytophthora root rot: The Phytophthora root rot, also known as ring rot, a type of fungal disease, a parasitic fungus that is located below ground level. Phytophthora cinnamomi is the cause of this disease. As a result of the infection, the roots will become black and sensitive. The parasite can also attack the trunk of the tree, which manifests itself in the form of bark death. It spreads quickly if not treated in time. The disease detected at an early stage can be stopped by dissecting the infected tissues.
      • Anthracnose fruit rot: This disease is caused by a fungus called Colletotrichum gloeosporoides. It infects the young stem, leaf, flower, and fruit and destroys it. Dark, patchy depressions appear on the infected fruit and spread rapidly. Regular use of fungicides can effectively treat anthracnose, thus preventing rot of the avocado fruit.
      • Powdery mildew: Disease caused by powdery mildew in the avocado tree can become severe if treatment is neglected. The leaves have dark green or purple-brown spots on the backs and yellowish-green spots on the upper sides, followed by a white or gray powdery protrusion. Spray the leaves with an officially approved fungicide against powdery mildew to stop the spread of infection.
      • Black stripes of avocado: The disease is characterized by the appearance of black streaks on the trunk and young shoots of the avocado tree, the yellowing of the leaves, and the sparse fruit. Symptoms include the death or alteration of the bark of the tree, which is a serious problem for the grower. Effective intervention can be soil disinfection or proper irrigation to prevent the disease from destroying the tree.
      • ASBV (Avocado Sunblotch Viroid): The virus, known as sunblotch, infects the bark and fruit of the avocado tree, which shows colored streaks. White or yellow spots may also form on the leaves. This disease usually spreads through infected seeds, so ask for the virus-free nature of the seeds at the source of their purchase.

      The diseases and pathogens of the avocado tree must be known to all growers to be able to defend themselves effectively against them. The tree can fall victim to strong sunlight, frost, infections, insects, mites, naked snails, and other pests.

      Requirements for an avocado plant to Bear Good Fruit

      1. Selection of good fruit plant seeds. Choosing a good and superior fruit seeds are very necessary. We should know and understand whether the seeds of the fruit plants that we will be planting are good or not. Plant seeds that are not good or seeds that are descendants of infertile plants will certainly not be good results.
      1. Fulfillment of plant needs including the environment, favorable weather climate, rainfall, nutrients, and also sufficient sunlight. The nutrient element in the soil that is needed and most important for plants is Phosphorus, so you have to make sure that this element is in the field to be used.
      1. The fulfillment of micro-climate is the other micro things that affect the growth of fruit plants including land conditions, soil water quality, air humidity, and temperature. Plants or fruit trees that we plant must be free and protected from diseases that affect plants.

      How to Take Care of Avocado Trees for Fast Fruit in Different Seasons

      Planting and then waiting for years in the hope that it will bear fruit one day means tiring out your patience. On average, the avocado tree of any species can take 5 years to bear the fruit.

      However, there is an effective way that can make avocado plants bear fruit within 3 years. Then here is how to take care of an avocado tree to quickly bear fruit.

      1. Cut avocado trees as high as 50 – 60 cm from the soil surface and select a diameter of plants with a size of 25-30 cm.
      2. The first method is to connect the skin, where a gap between the bark and the bond is as deep as 5-7 cm. Then take the stem (entries) with a diameter of 0.5-1 cm with a length of 10-15 cm or consist of 3-5 buds and sliced on both sides below. Insert the entries in the gap that has been made.
      3. The second method is a slit connection, where the bark is cut 5 – 7 cm long with the width adjusted to the size of the entries. Entries will be sliced slant and affixed to the incised bark. After that, tie around the patch with raffia or plastic rope. Apply liquid wax to the surface of the rootstock and exposed bark to prevent excessive evaporation. In one tree, 3 entries can be placed with a balanced distance around the trunk.
      4. So that the connection is not exposed to direct sunlight, cover the entries with a cement bag, and plastic coated. To help air circulation, make 2 holes in the front and back.
      5. After 1 month, the plastic cover is opened and the entries will bring up new green shoots. If it is brownish, the process has failed.
      6. The figure of the plant will appear after 5 months. With proper care, the plant will bear fruit after 3 years.

      This is how to take care of an avocado tree to bear fruit quickly. This process is not difficult to do and is suitable for beginners who want to start cultivating avocado trees.


      Prevention and control

      Removal of sunblotch-expressing or symptomless carrier trees is the primary mode for the control of disease spread. Mechanical transmission by razor slashing ( Desjardins et al., 1980 ) or graft inoculation with filter paper containing extracts from infected trees ( Allen et al., 1981 ) is possible but of very low efficiency. ASBVd can be effectively inactivated by dipping pruning and propagating tools in solutions of 20% sodium hypochlorite, 2% formaldehyde + 2% sodium hydroxide, or 6% hydrogen peroxide ( Desjardins et al., 1987 ).

      A phytosanitation programme for the indexing and dissemination of registered rootstock and scion materials offers the best approach for the control of sunblotch. ASBVd was reported to be inactivated in infected tissue following exposure to 56°C for 15 minutes (da Graca and van Vuuren, 1980a ).


      Texas Plant Disease Handbook

      Anthracnose (fungus – Colletotrichum gloeosporioides): Black circular spots up to one-half inch appear on fruit. The center of the spots may be slightly sunken and spots may develop cracks. During moist periods, the spots produce pinkish, moist masses of fungal spores. As the fruit ripens, the infection spreads rapidly into the flesh causing a greenish-black, fairly firm decay. Fungicides can control the disease and should be applied at bud swell.

      Cotton Root Rot (fungus – Phymatotrichopsis omnivora) : Although avocado is susceptible to cotton rot root, it is not a common disease. It is characterized by a sudden wilt of the tree. The leaves turn brown and remain on the tree. Very young trees tend to become diseased, rather than old ones. The only control is to avoid planting trees in soil known to be infested with the fungus. See Section on Cotton Root Rot.

      Leaf Burn (physiological): Leaves brown at the tips and the edges. Affected leaves may drop prematurely. These symptoms can be caused by accumulation of salts in the soil, inadequate soil moisture, wind dessication, and frost. Light irrigation should be avoided since this fails to leach accumulated salts out of the root zone. The soil should have good drainage.

      Phytophthora Root Rot (fungus – Phytophthora cinnamomi): Infected trees have small leaves which are lighter green in color than healthy leaves. The leaves wilt and drop, eventually leading to complete defoliation of the tree. Twigs and branches die back. There is a light fruit set with small fruit. The fungus infects roots up to one-quarter inch in size, although it is the feeder roots that are primarily affected. These roots become blackened and brittle before they die. The disease occurs if the pathogen is present in soil and if there is excessive soil moisture. Poor drainage can be a contributing factor to disease development. The fungus can be introduced to new areas by movement of infected nursery stock or infested soil. Soil on implements can also serve as a means for introduction of the fungus to non-infested areas. Fungicides should be used in combination with sanitation in the nursery and maintaining adequate drainage in the field.

      Scab (fungus – Sphaceloma perseae): Circular, brown, scabby areas are found on mature fruit (See Photo). Leaves have brown spots and become crinkled. The disease can become a problem if there is cool, moist weather when fruit and leaf tissue is young. Fungicides can control the disease and should be applied when flower buds appear, near the end of the main bloom period, and 3-4 weeks after that.

      Seedling Blight (fungi – Phytophthora spp.): This can be an important disease in nursery production. Leaves show irregular reddish-brown areas that enlarge along the larger veins. The terminal bud may be killed. The disease is favored by periods of heavy rainfall and high humidity, when seedlings are young and succulent. The following cultural practices can prevent or minimize damage. Plants should be grown on benches. The use of soil should be avoided to prevent introduction of the pathogen. Irrigation water from ponds may also serve as a pathogen source. Providing good air circulation between plants will provide a less-favorable microclimate for disease development. Fungicides should be used only in combination with cultural control practices. See section on Seedling Blight.

      Sunblotch (avocado sunblotch viroid): Twigs have a light yellow, sunken streak that follow the length of the twig. Fruits have white or yellow areas. Trees are stunted and have a sprawling growth habit. The pathogen is transmitted in budwood, graftwood, or seed from infected trees. Disease-free nursery stock should be used and trees with symptoms should be removed from the orchard.


      Common Diseases

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      It is prone to diseases of various kinds which can hamper its growth. There are a number of causes, some of them being―improper irrigation, injury of some kind, environmental and soil conditions, and infections.

      Powdery Mildew

      Powdery mildew is basically a fungal disease which affects the leaves. It is characterized by the appearance of purplish and brownish spots on the lower side of the leaves, and greenish or yellowish spots on the upper side. In mild cases, this infection can be removed by rubbing the leaves. In severe cases, a powdery substance appears on the leaves. A fungicide is generally used to treat this disease.

      Avocado Root Rot

      This is a fungal disease which basically affects the roots of the avocado tree. The roots start getting brittle once they are affected. Eventually the twigs and stems begin to die, and the leaves fall off. This disease is characterized by the appearance of pale yellow leaves and low fruit production. It can be cured using a fungicide.

      Pseudocercospora Spot

      This is another disease that affects the avocado tree, especially in warm and humid weather. The fruits, stem, and leaves are commonly infected, and brown-black lesions appear on the infected leaves. It is treated by using azoxystrobin spray.

      Anthracnose Fruit Rot

      This is another fungal disease which affects the fruits, stems, and flowers of the tree. It is caused by a fungus called Colletotrichum gloeosporioides. Regular use of fungicides help in prevention of this disease which is characterized by the appearance of dark sunken spots on the avocado fruit, which rapidly spread to the surrounding tree structures.

      Avocado Black Streak

      This disease is characterized by the appearance of lesions and cankers, and affects the fruit production of the tree. It also causes yellowing of the leaves. It can be best prevented by using proper irrigation techniques and soil fumigation.

      Avocado Sunblotch

      This viral disease lowers the yield of the tree and also results in poor quality fruit. It is caused by a virus called ASBV (Avocado sunblotch viroid). This disease is serious and the virus can be passed to other trees as well.


      RNA silencing as related to viroid induced symptom expression

      Evidence of post-transcriptional gene silencing (PTGS) in avocado infected by Avocado sunblotch viroid (ASBVd), the type species of family Avsunviroidae, was suggested by detection of ASBVd-specific 22-nucleotide RNAs. PTGS was observed in infected bleached and variegated symptomatic tissues as well as symptomless carrier foliar sources and fruit with typical sunblotch disease lesions. Tissues with the different symptom expressions, characterized by the presence of different predominant ASBVd variants, were found to induce PTGS at differential levels. Detection of the PTGS-associated small interfering RNAs (siRNAs) as well as relative concentration was also related to viroid titer. PTGS induced in Gynura aurantiaca infected with two closely-related variants of Citrus exocortis viroid, a member of family Pospiviroidae, was not directly related to viroid titer with initiation of symptoms.

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      Watch the video: UC Ag Experts Talk: Gibberellic Acid Use on Avocado to Improve Fruit Set