Posts Tagged ‘disease’

Last month’s article covered basic information about Oak Wilt and its symptoms, along with an introduction to its transmission. The article noted that there is much confusion and misinformation regarding Oak Wilt, and research goes only so far in providing the answers to important questions. Future research will likely change current recommendations — which is a good thing. Early detection and an accurate diagnosis, paired with a solid management plan, are critical to managing the disease successfully. In some cases, there may be a need to develop a management plan for an entire city block or particular part of a community.

Oak Wilt Management/Diagnosis

An Oak Wilt diagnosis can be confirmed in a laboratory by isolating the fungus from diseased tissues. Taking tissue samples is not a simple process and should be left to someone with specific training and experience. Each step of the process must be done properly. Although all the symptoms indicate Oak Wilt is present, lab reports may return as a “negative.” A negative from the lab does not mean the sampled tree is not infected, only that the fungus could not be found in the particular sample provided. Note that “someone with specific training and experience” is typically a consulting arborist, but may also be a forester or Oak Wilt researcher.

Oak Wilt Management/Suppression

Fungal Spore Mat

As previously noted, fungal spore mats form only on species in the Red Oak group and produce a fruity odor that attracts the Nitidulid beetle — believed to be responsible for the overland spread of the disease. As a result, infected and dying red oaks are a greater concern, due to the potential for aiding in the overland spread. There is no evidence that live oaks contribute to the overland spread. The firewood and stumps from removed live oak trees are not a concern.

Since the spore mats have been known to form on stumps and firewood of species in the Red Oak group, infected trees should be promptly removed from the site and chipped, burned or buried. If this is not practical, remove the infected tree’s bark from the soil line up to 2 or 3 feet to help prevent the formation of spore mats. In urban areas, grinding the stump below the soil surface is also recommended.

Avoid storing infected red oak firewood near healthy trees unless it is “seasoned” or completely dried for at least one year. If infected red oak wood must be left on a site, it should be covered with clear plastic and the edges buried in the soil. Avoid purchasing red oak firewood that appears green and not ready to burn. However, burning infected oak firewood of any species will not spread the disease since the heat of the fire will destroy fungal spores.

The disease is most often spread by root-to-root contact. In a dense group of live oaks, Oak Wilt can expand outward to 75 feet or more each year. Trenching between infected and non-infected trees is a method of suppression in a rural setting, but it is difficult and less successful in an urban area due to the number of underground utilities and obstructions.

By comparison, Oak Wilt centers in the DFW area do not appear to be expanding outward as rapidly as centers found in Austin. One theory is that the disease expands more rapidly in communities with indigenous populations of red oak and live oak. These populations have root systems that are more interconnected than an urban forest established primarily by planting new trees. One of the primary indigenous oaks in North Central Texas is post oak (Quercus stellata), which is extremely tolerant of the disease. The same is true for chinquapin oak (Quercus muehlenbergii) and bur oak(Quercus macrocarpa), which should be planted more often for this reason. Are you starting to lose some of your fears yet?

Next month`s article will cover Oak Wilt Management/Pruning. The final article will address Prevention and Treatment. Afterward, we hope to have fewer nervous tree mothers!!

About the author: Steve Houser is a Dallas native with more than 30 years of experience as a consulting arborist. He is the president of Arborilogical Services Inc., “The Experts Your Trees Deserve.”  www.arborilogical.com.

Although the deciduous trees have lost their leaves for the winter and we humans consider the trees to be dormant, physiologically the tree is very active.  New roots are being developed and the tree is preparing itself for the upcoming spring.  It is important to note that photosynthesis may have stopped when the leaves drop, however, respiration does not.  Respiration is the process where food manufactured in the leaves is burned with oxygen obtained through the roots to provide the energy necessary for growth.

What’s the point?  The past several winters have been extremely wet with the soil saturated for long periods of time resulting in low oxygen levels in the soil and consequently a low rate of respiration.  Many of our tree species which prefer a dry site (Texas Red Oak, Chinquapin Oak, etc.) have experienced some problems related to weakened root systems that I believe to be caused by the excess water and low oxygen levels in the soil.

What can I do?  Although no person can control the weather, each of us can control the use of our irrigation system.  Our clay soil should be allowed to cycle through wet and dry periods.  Prior to irrigation check the soil at a depth of 6 to 8 inches.  If it crumbles, it is time to irrigate.  If it packs into a wet clay ball suitable for making pottery, no irrigation is necessary.  Allowing the soil to dry increases the oxygen content and then the respiration rate.  You will have healthier, stronger plants able to withstand our hot Texas summers, if you irrigate less frequently, but thoroughly saturate the root zone when you do irrigate.  The benefits are stronger roots, healthier plants and a lower water bill.

The dormant season is an excellent time for tree pruning.  Live Oaks and Red Oaks should be pruned at this time or in the heat of summer to cut down on the risk of Oak Wilt disease.  Live Oaks and Red Oaks should not be pruned in the spring.

If you have any questions concerning the health of your trees, please call us.  One of our Certified Arborists will be happy to answer your questions.

Oak Anthracnose – introduction.

Oak Anthracnose

Perhaps you have noticed your Live Oaks dropping new leaves this spring. The moist conditions of the past winter and early spring, in conjunction with seasonably cool temperatures, has allowed a number of fungal diseases to develop on the leaves of many of our shade trees. One of these more pre

valent this year is Oak Anthracnose. This disease infects the foliage of nearly all Oak species. It is most prevalent in our Live Oaks this year. Although it can cause problems, generally this disease is not a serious threat to the long-term health of the tree.

Oak Anthracnose – symptoms, characteristics and effects.

The spores of the fungus infect young supple leaves just emerging in the spring. After a period of rapid infection, the leaves begin to fall. Symptoms are evident on a portion of the leaf and are expressed by the margin of the new leaf turning brown or becoming necrotic. This area of browning is usually along one of the leaf margins, causing the leaf to become curled and fall to the ground. Not all infected leaves fall. In fact, many will remain on the tree throughout the season causing the overall appearance of the tree to look a bit odd. Trees in areas of little air movement and those with thick, dense foliage are most readily infected. The lower, inside canopy usually exhibits the most intense symptoms, as these leaves dry more slowly.

Temperatures consistently over 80-85 degrees Fahrenheit cause the infection cycle to cease. Fortunately, the portions of the canopy that drop leaves, will produce new leaves. Single year infections rarely cause much damage to the tree. If the disease repeats next season and infects a larger portion of the tree, preventative fungicide applications may be needed. Foliar sprays, which reduce the incidence of the disease, should begin when the new leaves appear in March. It usually requires 2-3 applications at 21-30 day intervals to adequately manage this disease. If evidence of the disease is observed in late spring or early summer, foliar sprays are normally not recommended. Your arborist may however, recommend deep root fertilization in order to encourage the production of new leaves and improve the overall health and vigor of the tree.

Of course, if you have any further questions regarding Oak Anthracnose, Live Oaks, or trees in general, we are here for you.

THE DISEASE AND ITS BIOLOGY:

Oak Wilt is a vascular wilt disease of Oak trees caused by the fungus Ceratocystis fagacearum. All species of Oaks are susceptible with some species being somewhat tolerant. Red Oaks, both Shumard and Spanish, are highly susceptible to the pathogen with Live Oak following a close second.

The disease in Red Oak is expressed in two primary ways. The first is an oily green appearance to sections of the canopy, which rapidly turn a tan color as the foliage dries on the tree. The other and less common expression is a heavy and rapid shedding of foliage with a variety of the above symptoms on fallen leaves. The shedding is fairly uniform throughout the canopy. There is no effective control for Red Oaks that express active symptoms of infection. However, injection with the fungicide Alamo has proven to be effective as a preventative. Red Oaks properly diagnosed with Oak Wilt should be removed as soon as possible as these species are responsible for the overland spread of the pathogen. There is more and more evidence that Red Oaks infected with the Oak Wilt fungus can be effectively treated if the infection has not progressed too far.

There are a variety of possible foliar symptoms observed in infected Live Oaks. The most common is termed a veinal necrosis or yellowing and browning of the main leaf vein and side veins (see photograph below). The foliage will usually fall once this symptom has appeared. It is possible to see leaves still on the tree with veinal necrosis.The defoliation in Live Oaks can occur at anytime of the year, but should not be confused with the normal process of leaf drop and flowering which generally occurs late February to late March. A small percentage of diseased Live Oaks can potentially be treated with the fungicide injection. However, it should be noted that the curative qualities of the fungicide are limited.

The disease spreads from tree to tree in two methods that we are aware of to date. The first and most common is root to root contact. It is well known that Live Oaks commonly form functional root grafts with other Live Oaks. Red Oaks appear to form these root grafts as well. Once the pathogen has entered a tree, it can potentially move from tree to tree quite readily and efficiently. Less common, but reported to occur, is root grafting between Live Oak and Red Oak. This phenomenon is not nearly as common, but the potential is there. This is why we will commonly recommend to inject all Oaks on the property for protective purposes.

The second method of disease transmission is overland spread. This occurs when fungal mats are produced on dying Red Oaks and visited by the suspected insect vector, the Nitidulid Sap Feeding Beetle.

It should be noted that fungal mats are produced primarily in the Spring or cooler times of the year and only form on Red Oaks, which are at a particular phase of the mortality process. Mats will be viable, that is producing viable spores of the fungus, for a short period of time. This time frame is usually 2 – 3 weeks at the most. Although new infection centers do form from time to time, it is not fully understood whether it is from the insect spreading the disease or possibly other methods of transmission such as infected pruning tools, squirrel damage during the Spring, or methods not yet discovered.

THE ROOT FLARE INJECTION PROCESS:

Oak Wilt

The root flare injection process is three fold. The first is to expose the buttress roots or root flare of the tree or trees being injected. This is accomplished with careful excavation of the base of the tree where plant material and soil are removed from around the base of the tree. The distance away from the trunk in which plant material and soil will need to be removed will vary according to the size or diameter of the tree being treated.

The second step is to physically perform the high volume root flare injection with the registered fungicide “Alamo”. This is the only material registered and labeled for this purpose. It is now available in a water-soluble formulation, which will have a distinct blue color. Anything else being used is ineffective or being illegally applied.

A series of holes will be drilled up and down the exposed root flares at about 4-6 inch increments. Proper location of these injection sites is critical for proper distribution of the fungicide. Inadequate distribution leaves trees poorly protected. A series of injection tees are inserted into the injection sites and connected with tubing to create a closed system. The system is usually attached to a pressure tank containing the fungicide solution and the injection process begins. Once the tree has taken up the allotted amount of fungicide, the system is broken down and removed from the tree.

The third step is to return and fill in the exposed area. There will be many instances where trees have been planted too deep to begin with, or fill soil has been brought in. In these cases we will commonly recommend to fill in with a large stone or leave the root flare exposed entirely. Each situation will differ and your Arborist will recommend what is best for the long-term health of the tree.

In general, Live Oaks and Red Oaks not expressing active disease symptoms are good candidates for preventive root flare injection with the fungicide. It appears that Oaks can be infected with the fungus up to a year before active symptoms are expressed. What this means is that an apparently healthy Oak adjacent to a diseased tree may already be infected and in this instance, uptake of the fungicide may be poor.

COMMONLY ASKED QUESTIONS REGARDING ROOT FLARE INJECTION WITH ALAMO:

Q: Why does the process seem expensive?

A: The cost of the process is primarily driven from the cost of the fungicide. This is dictated by the manufacturer. There is also quite extensive labor involved in properly exposing the root flare of a tree or trees in the injection process itself.

Q: Does it matter how much fungicide the tree receives?

A: It is critical that the tree receives a minimum of 10 milliliters of Alamo for every inch in diameter. This diameter measurement is usually taken about 4.5 feet from soil grade. We will commonly recommend 12 milliliters per inch if infected trees are being injected or high disease pressure exists.

Q: Which trees should I treat?

A: Your Arborist will generally recommend to treat all Oaks on the property, and certainly all that potentially could form root grafts. This will vary from site to site and will usually not include trees already showing active symptoms.

Q: How fast does the disease spread in a population of Oak trees?

A: The disease has been reported to move up to 100 feet per year in a solid stand of Oaks. This can vary greatly due to variables unique to each site.

Q: Should my neighbor treat their trees?

A: The best candidates for preventive injection are those Oaks out ahead of the active disease center. Many times we are called to the property with trees showing active symptoms of Oak Wilt. We ask the fungicide to do much more than it is designed to do when we are always treating trees adjacent to infected trees.

Q: Is this injection process guaranteed?

A: The process has no guarantee. We are however, much more successful when we are treating trees that, number one, are not showing active symptoms. And, number two, are some distance from infected trees. We have had very good results with high volume root flare injection to date. We still see failures from time to time, which is usually due to the tree already being infected which leads to poor uptake and distribution.

Q: Are there alternative methods for the prevention of Oak Wilt?

A: NO, there are currently other types of injection, such as “microinjection” which has no data to date that suggest any type of control or prevention. This process is strictly experimental and will take a number of years to provide adequate data one way or the other. DO NOT LET anyone tell you differently. This can be a confusing point because the manufacturer has packaged Alamo in a microinjection system. This was done primarily to increase revenue for the chemical company and offer what appears to be a quicker and easier method of injection. If an easier method becomes available that is equally successful and cost effective with repeatable results, we will be the first to implement such a method.

Q: If I keep my trees healthy will they be less susceptible to infection by the Oak Wilt fungus?

A: No, it is unfortunate that this is not the case. We are dealing with an aggressive vascular wilt disease and a tree, which is genetically predisposed to infection by the fungus. To draw an analogy, the Chicken Pox Virus does not infect children, which are unhealthy. The healthiest children will still get the infection at some time or another because humans are genetically predisposed to the Chicken Pox Virus.

Q: Will the root flare injection ever need to be repeated?

A: Once trees are injected, it appears they are protected for about 2 years. If disease pressure is high, that is to say the Oak Wilt center is still active next door or across the street, we may recommend to retreat the following year. If we are treating trees with active symptoms and the symptoms reappear the following season, we will commonly recommend to retreat. Retreating high value or unique or historic trees annually as a preventative is not unusual.

Dormant oil applications to trees and shrubs can be a quite effective tool when managing many of our insect and disease problems.

The application is done once a year during the dormant season. In our area this is roughly January through Mid March. Many times Dormant Oil applications are used in conjunction with other foliar sprays, such as fungicide sprays for the management of fungal leaf diseases. The Oil helps reduce the levels of pathogenic fungi that can “over winter” (survive) in bark fissures and leaf scales or unopened buds. The Oil alone does not provide control, additional spray applications with an approved fungicide will need to be done in the spring after the tree begins to leaf out.

Dormant Oil applications are quite effective against “over wintering” insect pests. Many of our early season pest problems can be reduced with Oil applications. We need to mention again that the Oil alone does not always provide complete management. Additional foliar applications with an approved miticide or insecticide may need to be scheduled for a complete management program.

A particular insect pest that Dormant Oil is especially effective against is the various scale infestations we get in North Texas. If you own Red Oak, Live Oak, Pecan, or Silver Maple, you can find one of the various scale species. Sometimes the level of infestation may not warrant an application. However, this pest can increase in severity quite quickly. Many times it’s best to include Dormant Oil applications as part of an overall health management program. Applications on an annual basis will help to keep scale populations below damaging levels.

The material is a very finely refined oil product, with practically no odor, which acts to suffocate the “over wintering” target pest. It is by far one of the safest products used in pest management.

Many times you may find us recommending the Dormant Oil application to include your shrubs. For example, there are a variety of scale problems which Oil applications can reduce, especially on Holly sp.

NOTE: If you own a swimming pool and we are applying Dormant Oil to trees and or shrubs near the pool, you may experience a cloudy look to the pool when we are done. This phenomenon is temporary and does not damage the pool or filter. It will dissipate in 24 – 48 hours. It is also completely safe to swim under these conditions as well.

by Walter J.Walla and Everett Janne Extension plant pathologist and Extension landscape horticulturist, The Texas A&M University System. Published originally as TAEX L- 056, November, 1982.

Cotton root rot, caused by the fungus Phymatotrichumomnivorum, also is known by several other names such as Phymatotrichum root rot, Texas root rot and Ozonium root rot.

It is one of the most destructive plant diseases and attacks more than 2,000 species. However, either the fungus infects but does not kill monocotyledonous plants (grasses, etc.), or these plants are all highly resistant. In Texas, the disease is economically important in cotton, alfalfa, ornamental plants, and fruit, nut and shade trees. The fungus is prevalent in calcareous clay loam soils with a pH range of 7.0 to 8.5 and in areas with high summer temperatures. Therefore, the disease is limited to the southwestern United States.

Phymatotrichum root rot has been reported in Texas counties from the Red River to the Rio Grande and from Tom Green County to the Neches River.

Disease Symptoms

Disease symptoms are most likely to occur from June through September when soil temperatures reach 28°C (82°F). The first symptoms are slight yellowing or bronzing of leaves followed by wilting. Plants die suddenly after the first symptoms of wilting. Leaves remain firmly attached to the plant. Affected plants die suddenly, often after excellent growth. Large trees and shrubs may die more slowly.

Usually roots are invaded extensively with the fungus by the time plants have wilted. When roots are pulled from the soil, root bark is decayed and brownish, and wooly strands of the fungus frequently are apparent on the root surface. Affected plants pull from the soil with little effort.

Under moist conditions, sporemats sometimes appear on the soil surface. These mats, 2 to 16 inches in diameter, are first snow-white and cottony and later tan and powdery. On large roots and tubers, there are numerous small, cushion-like sclerotia or resting bodies about the size of a pinhead. At first they are light tan but later appear dark and warty.

The fungus generally invades new areas by continual slow growth through the soil from plant to plant. Occasionally, it spreads more rapidly on the roots of infected transplanted plants. The fungus can survive in the soil for many years, and often it is found as deep in the soil as roots penetrate. Affected areas often appear as circular areas of dead plants in fields of infected crops. These areas gradually enlarge in subsequent years as the fungus grows through the soil from plant to plant. Infested areas may increase 5 to 30 feet per year.

Causal Organisms

Phymatotrichum omnivorum exists in the soil in three distinct forms: (1) hyphae and strands (rhizomorphs), (2) sclerotia and (3) sporemats and conidia.

Hyphae and strands. The fungus produces root-like strands (rhizomorphs) that grow through the soil until they contact the descending plant roots. Strands surround a root and grow toward the soil surface. Immediately below the surface, the fungus proliferates around the hypocotyl, producing a cottony, mycelial growth. Below this mycelium, the bark is destroyed, and the fungus fills the vascular tissue of the plant. Following death of the plant, sclerotia form in the strands.

Sclerotia. Strand cells divide, grow and enlarge to form sclerotia. These sclerotia are small (1 to 2 millimeters in diameter), densely compacted masses of thick-walled cells. Sclerotia are first white, changing to buff, brown and black with age. They are irregular shaped, generally taking the shape of the soil space where they are formed. Sclerotia enable the fungus to persist in fallow soil or soil planted to resistant crops for several years. Sclerotia have been found as deep as 12 feet in some soils.

Sporemats and conidia. The fungus often forms sporemats on the soil surface during warm, rainy weather. These mats vary from 2 to 16 inches in diameter and are white to tan colored. They are composed of large-celled, branched fungal strands that later produce conidia. The conidia appear sterile, and their role in the spread of the pathogen has not been documented.

Phymatotrichum root rot is one of the most difficult plant diseases to control. Fungal behavior in different crops and soils and its activity from year to year in the same field are so erratic that it is ineffective to rely on one approach. Use a control program consisting of a systematic course of treatment involving several recognized control methods.

Control Methods

Organic amendments. Significant control of Phymatotrichum root rot has been achieved by using various crops as organic matter amendments. A delay in infection is readily apparent and has resulted in 90 percent reduction in root rot. Wheat, oats and other cereal crops are effective in delaying infection and reducing losses when incorporated in soil in the spring.

Plant barriers. This technique consists of planting resistant species around an infected area. These barriers either exclude or limit the spread of the pathogen. This technique assumes that the barrier plant does not harbor the pathogen in its root system. Make ornamental plantings of cotton root rot-susceptible species with isolated plants or groups of plants rather than in continuous rows as hedges. When the disease occurs in an ornamental planting, replace diseased plants with resistant species.

Fertilizer applications. To reduce root rot, apply fertilizers high in certain nitrogen forms. When nitrogen is applied as ammonia in a manner to fumigate as much soil as possible, research shows a reduced incidence of root rot.

In some cases, valuable ornamental plants and orchard trees have been treated successfully even after root rot infection has taken place. First prune the tree (or shrub) back and build a circular ridge (equal in diameter to the top of the plant) of soil some distance from the trunk. Work 1 pound of ammonium sulfate into the soil for each 100 square feet of surface within this ridge. Fill the area within the ridge with water to a depth of about 4 inches. Repeat the treatment and watering after 5 to 10 days. Do not apply more than two treatments in the same season.

Following this treatment, water frequently to prevent drought injury. Acidifying the soil with sulfur around susceptible trees or shrubs may help delay or prevent root rot infection in areas where the disease is prevalent.

Resistant varieties. Development of resistant plants using conventional breeding concepts, has been difficult due to the pathogen’s wide host range. However, the following list of woody and herbaceous plants has shown resistance or tolerance to cotton root rot and should be considered by the homeowner where the disease is prevalent. The hardiness zone is given for each woody plant listed. Check the map to determine the zone in which you wish to use the plant. Use any plant with that zone number or a lower number. Plants with a higher zone number usually will not be hardy in that area. Check the list for size and foliage type to aid you in selecting the plants best suited for your particular purpose.