By: Amy Grant
If you have a citrus tree trunk forming blisters that ooze a gummy substance, you might just have a case of citrus Rio Grande gummosis. What’s Rio Grande gummosis and what happens to a citrus tree afflicted with Rio Grande gummosis? The following article contains Rio Grande gummosis of citrus info that includes symptoms and management tips to help.
What is Rio Grande Gummosis?
Citrus Rio Grande gummosis is a fungal disease caused in part by the pathogen Diplodia natalensis along with several other fungi. What are the symptoms of Rio Grande gummosis of citrus?
As mentioned, citrus trees with Rio Grande gummosis form blisters on the bark of trunks and branches. These blisters ooze a sticky gum. As the disease progresses, the wood beneath the bark turns a pinkish/orange color as gum pockets form under the bark. Once the sapwood is exposed, decay sets in. In the latest stages of the disease, heart rot may occur as well.
Rio Grande Gummosis Info
The name citrus Grande Rio gummosis comes from the area where it was first observed, the Rio Grande Valley of Texas, in the late 1940s on mature grapefruit trees. The disease is also sometimes referred to as Florida gummosis or ferment gum disease.
This gumming disease of citrus has been found to be chronic in nature. It is most often observed in mature trees of 20 years or older but has also been found to afflict trees as young as 6 years of age.
Weakened and/or injured trees seem to have a higher incidence for infection. Factors such as freeze damage, lack of drainage, and salt accumulation within the soil also foster the incidence of the disease.
Unfortunately, there is no control for citrus Rio Grande gummosis. Keeping trees healthy and vigorous by practicing excellent cultural controls is the only method for management of this disease. Be sure to prune out any branches damaged by freezing and encourage speedy healing of the injured limbs.
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What Is Rio Grande Gummosis – Treating A Citrus Tree With Rio Grande Gummosis Disease - garden
Fungal Diseases and Nematodes
Jose M. Amador
Extension plant pathologist
The Texas A&M University System
Citrus diseases affecting the entire tree can be classified into two general categories: parasitic and nonparasitic. Parasitic diseases are caused by fungi, bacteria, viruses and nematodes. Some disease-causing agents are restricted to certain parts of the plant, such as the root, trunk or fruit, while others may affect several or all parts of the plant.
Nonparasitic diseases are physiological disorders caused by excesses or deficiencies of certain nutrients or by unfavorable environmental conditions. Among the most common nonparasitic diseases of citrus occurring in Texas are iron chlorosis, mottle leaf, salt and chronic water table injury. In addition to the economic losses that can result if not avoided or corrected, these physiological disorders often predispose the tree to attack by disease-causing organisms, such as the relationship that exists between salt and chronic water table injury and the increased occurrence of several types of gummosis, twig blight, tip-burned yellow leaves and nematode infestations. Physiological disorders are discussed in other sections of this publication.
Cotton Root Rot
This disease, caused by the fungus Phymatotrichum omnivorum , affects many plant species. The fungus attacks the underground parts of the citrus tree and occasionally kills young trees. Once trees reach maturity, they are less susceptible to attack. Cotton root rot commonly causes sudden death of susceptible trees, with most of the dried leaves temporarily retained on the tree. The best protection against cotton root rot is to use resistant rootstocks such as sour orange.
This is one of several well-known gumming diseases of citrus. Gum formation on the trunk or branches is a characteristic symptom. Gum exudes from blisters containing gum pockets, usually located on the trunk. The wood beneath the blister shows a pink-orange color.
Several factors such as freeze damage, high water table and salt accumulation contribute to the disease. Gummosis is believed to be a condition of weak and injured trees and is reported to be infectious. Symptoms of the disease were reproduced when healthy trees were inoculated with the fungus Diplodia natalenris .
No reliable cure exists for gummosis. Keeping trees vigorously growing is the best way to avoid the problem.
| Rio Grande hummosis causes|
amber exudate to be secreted from
pockets beneath the bark.
Foot rot, also known as brown rot gummosis, is caused by one or more species of the fungus Phytophthora . This disease can affect the root system, the trunk below and above ground, branches, leaves, blossoms and fruit. It is especially troublesome during prolonged rainy periods. Trees with the bud union beneath or close to the soil and trees in poorly-drained locations are highly susceptible. Foot rot is commonly found in Lower Rio Grande Valley orchards, but becomes a more serious problem under unusual conditions such as those that occur following hurricanes.
|Foot rot damage from Phytophthora spp.|
Infection of the lower areas of the trunk by Phytophthora spp. results in dark, water-soaked areas in the active areas of infection. Often gum exudes profusely from active lesions. The dead bark frequently breaks away from the wood in vertical strips. Callus tissue begins to form on the margin of the surrounding healthy bark if the fungus becomes inactive because of unfavorable weather conditions. The disease may become active again when conditions become favorable. If the lesion encircles the trunk, girdling occurs and results in death of the tree.
Phytophthora spp. also may attack nursery stock and young orchard trees during rainy weather. Examination of the crowns of infected trees shows symptoms similar to those described for older trees.
Phytophthora foot rot can best be controlled by preventative practices, including use of resistant rootstock and planting in well-drained land. Sour orange is the most resistant rootstock for the Lower Rio Grande Valley. Other resistant rootstocks include Swingle citrumelo and Troyer and Carrizo citrange.
Budding seedlings high, avoiding wounds, and keeping soil off the lower trunk are among recommended control practices. Soil fumigation of seedbeds should be practiced in field nurseries.
Plant parasitic nematodes are very small worms that cannot be seen with the naked eye. Several plant parasitic nematodes are known to attack the root system of citrus plants, especially the citrus nematode, Tylenchulus semipenetrans. The burrowing nematode, Radopholus similis , causes a disease known as spreading decline and is a serious pest of citrus in other production areas. This nematode has been found in the Valley on ornamentals, but not in citrus orchards. Serious losses to the Texas industry could result should it ever become established in citrus groves.
The citrus nematode occurs commonly in Valley citrus orchard soils. Sour orange is rated as highly susceptible to nematode infection. Of the common rootstocks with potential use in the Valley, only Swingle citrumelo is rated as highly tolerant to the citrus nematode.
|A microscopic view of a|
swollen female citrus nematode.
Populations of the citrus nematode usually are higher in older orchards (6 years and older). Young trees also may be injured if planted in orchard sites where nematodes previously prevailed. Other plant parasitic nematodes, mainly the lesion nematode ( Pratylenchus spp. ) often have been found associated with citrus roots.
Because the presence of the citrus nematode cannot be detected visually, positive diagnosis is based on external symptoms and laboratory examination of root and soil samples. Above-ground symptoms associated with nematode-infected trees are wiIting, lack of vigor, poor fruit production and poor response to watering and fertilization. Texas Cooperative Extension operates a Plant Nematode Detection Laboratory at College Station where samples can be sent for analysis and recommendations. County Extension agricultural agents can assist growers in collecting and sending samples.
Losses caused by the citrus nematode in heavily infected orchards can be reduced effectively by applying nematicides to the soil. Before treating an orchard, however, consider the overall condition of the orchard. Application of nematicides to freeze-damaged orchards grown under poor cultural conditions may not be profitable. Soil should be sampled before establishing a new orchard to determine if nematodes are present in damaging numbers. If the analysis reveals large numbers of parasitic nematodes, treating the soil before establishing trees may be profitable. Ideally, nematodefree plants should be used when establishing new orchards.
At present, products cleared for use on established citrus trees are also known to have an effect on mites and other insects. Careful attention must be paid to instructions on the label, not only for directions on how to apply the material, but also for current rates and regulations that may affect the legal use of these products.
Appreciation is expressed to Pete Timmer and Mike Davis, former plant pathologists, Texas A&M University Citrus Center, Weslaco, Texas, for photographs in this publication.
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How to Trim Orange & Grapefruit Trees
If you already have apple or stone-fruit trees in your yard, you may expect your new citrus tree to require the same severe early pruning. This is not the case. Unlike deciduous fruit trees, evergreen citrus trees produce strong wood, shape themselves nicely and ripen fruit even in shaded branches. Orange and grapefruit trees (Citrus spp.) generally require no major pruning for the first 20 years of their lives other than cutting out dead wood. If you wish, you can trim orange trees to create a more pleasing shape, but limit grapefruit pruning to the absolutely essential because of the risk they will develop Rio Grande gummosis disease.
Prune out dead branches from your citrus trees whenever you notice them. Remove dead wood at the point of origin. If you are unsure whether a branch is dead, scratch it with your fingernail or a sharp knife, exposing the bark layer beneath. Green wood beneath means the branch is still alive.
Cut back broken or damaged limbs at least 6 inches into healthy wood. Remove crossing branches to prevent them rubbing together. This pruning can be done at any time of the year.
Trim out suckers -- vigorous shoots that appear below the graft point of the tree -- whenever you see them. These shoots are the same wood as the root stock, not the cultivar, and the sucker leaves may appear different than others on the tree. Suckers will not produce fruit and you should remove them.
Remove the lowest layer of branches and trim the next highest layer to raise the "skirt" of the tree, if you wish to do so for aesthetic purposes. Note that this is not essential pruning as it does not affect the health of the tree or increase fruiting. Many citrus growers allows orange branches to sweep the ground. This makes fruit easier to reach and prevents weed growth.
Green Mold Post Harvest
It is not uncommon to find green mold on a citrus tree after harvest. The cause is the mold spore penicillin digitatum, which primarily affects fruit. The spores are dislodged when fruits are harvested and fly on a breeze to cling to the trunk or any other part of the tree. They can even remain lodged in the soil until conditions are right for them to ripen. They produce the classic mold look as seen on old bread. They will not kill the tree and usually the colder temperatures of winter will remove them.
Citrus greening disease takes root in Texas
1 of 3 These oranges show symptoms of citrus greening disease. USDA photo User Show More Show Less
2 of 3 Horticulture professor Fred Gmitter displays root stock of a citrus tree at the University of Florida Citrus Research and Education Center, in Lake Alfred, Fla. Gmitter is studying the citrus greening disease, which is also in Texas. Lynne Sladky/STF Show More Show Less
3 of 3 In this Friday, July 25, 2014 photo, Nick Howell, 13, a member of the McLean family who owns Uncle Matt's organic orange juice company, places a vial containing the tamarixia wasp to release in their orange groves in hopes of combating the citrus greening disease, in Clermont, Fla. Florida's $9 billion citrus industry is facing its biggest threat yet by a tiny invasive bug called the Asian Citrus Psyllid, which carries bacteria that are left behind when the psyllid feeds on a citrus tree's leaves. (AP Photo/Lynne Sladky) Lynne Sladky/STF Show More Show Less
A bacterial disease that has infected 75 percent of Florida's citrus trees - ruining fruits' shapes and flavors and ultimately killing the trees - has a stronghold in the Rio Grande Valley.
Known as citrus greening, it first appeared two years ago with a handful of infected trees in the border city of San Juan but is now showing in alarming numbers, mostly in a "hot zone" in the mid-Valley.
Texas Citrus Mutual, a growers' trade group, said the disease had been detected in 430 trees in commercial groves and 207 in residential backyards as of last week. More than 100 of the infected trees were found in La Blanca, an unincorporated community north of Donna. Hundreds more were found in orchards and a former nursery to the south.
"The question weighing heavily on the minds of growers and many others in South Texas is whether Texas can avoid a catastrophic situation for our citrus industry, which wasn't the case for our eastern neighbors in Florida," Texas Citrus Mutual President Ray Prewett said.
The disease's latency period makes it difficultto know how longa tree had been infected,he said.
"Trees will have the disease in a lot of cases for at least two years before they show symptoms," he said. "If they don't show symptoms, you can't fully even run tests."
Also known as huanglongbing, or yellow dragon disease, citrus greening causes misshapen, off-tasting, green-tinted fruit and is kills trees. It is carried by the Asian citrus psyllid, a type of jumping lice that originated in Asia and made its way to the Western Hemisphere.
While Texas' citrus industry is tiny when compared with Florida's, the Valley's sweet oranges and trademark red-fleshed grapefruit have an annual local economic effect of $150 million.
Growers and scientists say Texas has, unlike Florida, been able to take pre-emptive steps against the disease, and Congress and the U.S. Department of Agriculture have allocated millions of dollars for research into methods to stop its spread. The industry has funded a Spanish- and English-language awareness campaign.
"They've been detected at a reasonably early stage. It might be two years after infection, but it still hasn't spread through the whole tree," said John da Graca, director of the Citrus Center at Texas A&M University-Kingsville.
At present, all of Hidalgo and Cameron counties are under quarantine, and the USDA is continuing "door to door" testing.
Trees with the disease also were recently found in nurseries in Webster, in the south Houston suburbs. That prompted a third quarantine, in Harris County.
The industry is offering tree removal services for homeowners and is encouraging commercial growers to likewise destroy infected trees.
Texas growers are hoping to avoid the kind of devastation Florida has experienced.
"They wish they were where Texas is, where removing trees would make a difference," Prewett said. "They got to the point where 70 to 80 percent of their trees are infected. Well, you can't take out 70 to 80 percent of your trees and have any of your industry left."
Periodic testing of trees in cities including San Antonio and Corpus Christi so far hasn't found signs of citrus greening.
"It is simply too early to know how the situation will unfold," Prewett said. "However, we know that all Texans, from commercial growers to nursery owners to homeowners, must continue to be aggressive to slow the spread of the disease."
He said the industry was planning a strategic "spray surge" in September, a typically wet month that is prime spawning time for the disease-carrying psyllid.