动态第709页 - 绿手指 - 养花技巧、花生病了怎么办、花园打理和设计

2017年09月19日

Oak wilt is a systematic disease caused by a fungus, Ceratocystis fagacearum. The fungus invades the water-conducting tissues of oak trees. The black oak group (red, black, scarlet, and pin oaks) is more susceptible than the white oak group (white, bur, chinkapin, and swamp oaks). Oak wilt ranges from Minnesota east to Pennsylvania, south to South Carolina and Tennessee, west to central Texas, and north through Kansas and Nebraska. Infection through wounds is especially critical between April 1 and July 1 and during later periods of summer rains. There is no cure for oak wilt, so control consists of measures to prevent the disease from spreading.[图片]Symptoms and Diagnosis The first symptoms include a dull-green appearance of wilted leaves. Later, wilted leaves curl and turn tan or bronze, beginning at the outer portions of the leaves. The base of the leaf and the main vein will remain green for some time. Defoliation may be delayed for weeks. Peeled bark or a cut branch from an infected tree may show a brown or black discoloration in the outer annual sapwood ring. Positive diagnosis of oak wilt requires laboratory culturing and identification. [图片]Life Cycle The fungus spreads through the water-conducting vessels of the sapwood. The tree’s response to the presence of the fungus results in the disruption of sap flow, and the affected areas wilt. Oak wilt can spread to healthy trees through natural grafts with roots of adjacent oaks of the same species up to 50 feet apart. Root grafts join together the vascular systems of the trees, forming a network through which the disease can spread. The disease can also spread by sap-feeding beetles that transmit spores of the oak wilt fungus from infected trees to healthy ones. [图片]Integrated Pest Management Strategies 1. There is no cure for the disease. If oak wilt is suspected, a laboratory test is needed to make a positive diagnosis. Contact an arborist or an extension office on sampling procedures and fees associated with the laboratory testing. Samples of freshly wilted stems (not dead) about 1/2 to 1 inch in diameter and 6–10 inches in length are needed for the laboratory test. 2. Sever root grafts. Destroying root grafts with chemicals or by mechanical means can slow the spread of the disease from diseased to healthy oak trees. Since there is a delay between infection and the appearance of symptoms, destroying root grafts is a gamble. Root grafts do not occur between the black oak and white oak groups. [图片]3. Improve plant vigor. Your best guard against getting oak wilt is to keep your oak trees, especially oaks in the black oak group, healthy. If your oak trees do not appear in the best of health, have an experienced arborist evaluate their health and recommend a course of action. Mild cases in white oaks may respond to pruning of diseased wood plus fertilizing and watering to increase plant vigor. 4. Avoid pruning or wounding the tree between mid-March and late June. During this time of year, insects carrying the disease are attracted by the sap which flows freely from wounds. The safest time to prune oaks is during winter before mid-March. [图片]5. Plant white oaks rather than the more susceptible black oaks. If you do plant black oaks, be certain they are more than 50 feet apart to eliminate future disease spread via root grafts. Limit black oak use where oak wilt is prevalent.

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2017年09月19日

Oak leaf blister (oak leaf curl) is a fungal leaf disease caused by the fungus Taphrina caerulescens. Circular, raised areas ranging up to 2 inches in diameter are scattered over the upper leaf surface. During cool wet springs, almost all species of oak are subject to the leaf blister disease. Members of the red oak family are particularly susceptible to infection. The disease is closely related to Taprina deformans which causes peach leaf curl.[图片]Symptoms and Diagnosis Blister-like, circular, raised areas appear scattered on the upper leaf surface. This raised area causes a corresponding depression on the lower leaf surface at the same site and of the same size. The color of the upper convex area is yellowish white while the bottom concave area is yellowish brown. Leaves with numerous spots may fall prematurely to the ground. If well-established trees defoliate before midsummer, they will sometimes leaf out later in the season. When defoliation occurs in late summer, leaf loss will have little impact on the overall health of the tree.[图片]Life Cycle During mid-spring, microscopic spores are produced in leaf spots. These spores are carried by wind and splashing raindrops onto bud scales and twigs where they remain in a dormant stage until the following early spring. At this time, rain washes the spores onto young leaves where infection takes place. Depending on weather conditions, small circular spots begin to develop in 2 to 4 weeks. Spores produced on these spots will lodge in bud scales and again remain resting until the following spring. Cool wet weather is required for germination on young leaves, and if these conditions continue, severe infection can occur. If weather conditions are not favorable for spore germination shortly after bud break, only minor infection will occur. As the leaves mature, they become more resistant to infection.[图片]Integrated Pest Management Strategies 1. Maintain plant vigor. Keep the tree well watered during drought conditions (approximately 1 inch of water per week) and well-drained during periods of heavy rain. Fertilize according to soil test recommendations. Apply nitrogen every 3 to 5 years for mature shade trees. 2. General health. Oak leaf blister does not seriously affect the overall health of the tree unless the tree is repeatedly defoliated in successive years. Even if this occurs, the second set of leaves should emerge at a time when conditions are not conducive to reinfection of leaves and newly formed bud scales.[图片]3. Fungicides. A single application of a fungicide applied in the spring at the time of bud-swelling is usually adequate. Apply with a power sprayer and coat buds and twigs thoroughly for good control. chlorothalonil (Daconil) is currently registered for use in controlling oak leaf blister. Fungicides will not be effective if applied after bud break. As with the use of all chemicals, carefully read and follow the manufacturer’s directions.

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2017年09月19日

Most plants growing in average garden soil in the St. Louis area receive sufficient nutrients from the soil that deficiencies are not a problem. The most notable exception is iron chlorosis of trees and shrubs, which is more likely to result from a high soil pH than a lack of iron in the soil. See below for more information on this deficiency. Also, because of the higher nitrogen requirement of turf grasses and leafy vegetables in the vegetable garden they may benefit from some additional nitrogen fertilizer. Also, nutrient deficiencies are likely to occur in plants growing in containers that are not fertilized regularly because of the reduced soil volume and the lower nutrient holding capacity of soil-less mixes used for container plants.[图片]Following is some brief information on the symptoms of common nutrient deficiencies: Macronutrients Nitrogen Lack of nitrogen shows up as overall yellow-green leaves instead of a dark green, yellowing and dropping of lower leaves (can be caused by many factors), and overall reduced plant size and slow growth. Although most garden plants receive adequate nitrogen from the soil and dissolved in rainwater, applying nitrogen in a complete, balanced fertilizer such as 20-20-20 once a year around perennials and shrubs is usually sufficient to provide adequate growth. Leafy vegetables can benefit from additional nitrogen. Most trees do not suffer from lack of sufficient nitrogen. Although lawns respond quickly and dramatically to the application of nitrogen, becoming green and lush, its use should be tempered, as this lush growth is also more susceptible to attack by insects and disease. Nitrogen in best applied to cool-season grasses in fall and on warm season grasses as they begin to grow in early summer. Nitrogen should be applied to container plants in a complete fertilizer, such as 20-20-20.[图片]Phosphorus Lack of phosphorus typically results in reduced growth and in some plants purplish foliage, especially older leaves. Although it may be observed on container grown plants most soils in Missouri contain ample phosphorus so more rarely would deficiencies be noticed in a garden situation. If you suspect your plants are showing symptoms of lack of phosphorus, have your soil tested. Then add phosphorus as required by the test results. Container grown plants require regular fertilizing with a complete fertilizer such as 20-20-20.[图片]Potash Again, lack of potash is rare in Missouri soils but deficiency symptoms typically result in stunted growth. Older leaves may yellow and leaf edges may roll up. If a deficiency is suspected have your soil tested and follow the recommendations provided with your soil test results. Container grown plants require regular fertilizing with a complete fertilizer such as 20-20-20. Calcium, Magnesium and Sulfur These three elements complete the macronutrients. They are frequently available in adequate amounts in St. Louis soils.[图片]Micronutrients Several micronutrients are required for good plant growth. These include: iron, manganese, copper, zinc, boron, chlorine, molybdenum, and nickel. Symptoms are almost entirely crop specific, hence, listing general symptoms of little use. Also, their identification can be difficult from symptoms alone. Some are well known and a plant disease reference of well-studied crops may show pictures of the classic symptoms of a particular nutrient deficiency on a crop. Frequently, however, short of sending leaf tissue off to a lab for analysis the gardener is left questioning whether the symptoms they are seeing are caused by a nutrient deficiency or not. If in doubt, treat the plant with a micronutrient fertilizer or a complete fertilizer containing micronutrients. Since most micronutrients are used in very small amounts the fertilizer will provide the plant what it needs if the soil is indeed lacking in a micronutrient. Follow the product’s label directions. In the St. Louis area the most commonly encountered micronutrient problem is with iron.[图片]Iron Lack of iron most notably causes what is referred to as iron chlorosis or yellowing leaves where the veins remain green. New growth is most affected. Lack of iron in the soil, a high soil pH (over 7) that restricts availability of iron in the soil, and environmental conditions can all result in iron chlorosis. For detailed information on iron chlorosis see the IPM page “Iron Chlorosis of Trees and Shrubs” herein. Potted plants showing iron chlorosis can be watered with a fertilizer containing iron or a specific iron fertilizer such as chelated iron.

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2017年09月19日

Worldwide, Nectria fungi cause several common canker and dieback diseases, especially in hardwood trees. Nectria canker, which is caused by the fungus, Nectria galligena, may occur on over 60 species of trees and shrubs including apple, ash, birch, dogwood, elm, sweet gum, holly, maple, pear and walnut. A similar disease infects members of the magnolia family. Nectria canker is usually not a fatal disease, but it can cause considerable damage as the cankered area is weakened and susceptible to breakage. It may also adversely affect the appearance of the affected plant. This disease is important commercially as it reduces the quality and quantity of forest products.[图片]Another member of the Nectria genus, Nectria cinnabarina, causes the disease Nectria dieback. Also known as coral spot Nectria canker or Nectria canker, this disease occurs on many plant species, including apple, ash, barberry, birch, boxwood, crabapple, elm, hickory, honey locust, linden, maple, pear, rose and Japanese zelkova. Nectria cinnabarina usually grows as a saprophyte on dead wood, but if a plant is wounded or otherwise stressed, the fungus becomes an opportunistic weak parasite, producing cankers and causing dieback of twigs and branches. Maples are especially affected by this disease as are recently wounded or severely pruned trees and shrubs, urban ornamentals and new transplants of other species.[图片]Symptoms and Diagnosis Although it is most common in spring and fall, the Nectria fungus can infect plants throughout the year as long as there is sufficient moisture and the temperature is above freezing. Plants that are stressed by cold, drought, mechanical injuries or other disease are especially susceptible. Infections may be worse in autumn and winter when the host plant is dormant and wound recovery is weaker than in the growing season. The first symptom of a Nectria infection is a depressed discolored area of bark near wounds or at the base of dead twigs or branches. These cankers are usually not noticed until other symptoms appear.[图片]The first easily visible signs of Nectria canker are small creamy white or red to reddish orange fruiting structures and the development of callus tissue. This callus tissue is produced as the host plant attempts to isolate the fungus. If the callus does not isolate the infection, the fungus will continue to grow into healthy wood and the plant will respond by growing another ridge of callus tissue. This alternation of fungal growth and callus ridge, which may occur for many years, results in a rounded or elongated target-like shape. The bark of older ridges may decay and weather away exposing the ridges of wood underneath. This disease grows slowly and larger stems are rarely girdled, although multiple lesions may grow together and kill a branch or the entire plant. Plants that are stressed are most severely affected by the disease. This fungus may also affect apple fruit causing it to rot during storage.[图片]The first obvious sign of Nectria dieback may not occur until spring when the plant begins to grow. Affected twigs, branches, or even entire plants will not produce leaves or may wilt suddenly. Larger branches or small trees may be girdled and killed. The fungus produces reproductive structures that vary in color from creamy, coral pink, pink-orange, light purplish red or orange-red and that darken as they mature. Life Cycle Nectria galligena overwinters in the callus tissue growing slowly while its host is dormant. During moist periods, creamy white cushion-like fruiting structures will develop. These are followed by a second type of reproductive structure, which is red to reddish orange, pin-head sized and lemon-shaped, in autumn through spring. During rain or other moist weather, spores are released and dispersed by wind or water infecting susceptible plants through natural openings such as leaf scars or through wounds from improper pruning, sunscald, storm damage, frost cracks or other mechanical damage. As the fungus grows, it kills bark, cambium, and the outermost sapwood.[图片]The life cycle of the Nectria dieback fungus is similar to that of Nectria canker. Creamy to coral pink to pink-orange or light purplish red spore-producing structures develop in spring or early summer. These will age to tan, brown, or nearly black. Orange-red fruiting structures, which mature to dark reddish brown and may persist through winter, are produced in summer and autumn. Both structures release spores that are dispersed by water and can invade susceptible tissue producing cankers and dieback.[图片]Integrated Pest Management Strategies 1. Proper selection. Choose trees and shrubs that are well adapted to the climate of the area to minimize infection due to freeze damage and other environmental stresses. 2. Maintain plant vigor. Keep plants healthy and growing vigorously by using good cultural techniques. These include choosing the appropriate planting site, watering during dry periods, using mulch around the base of the tree or shrub and fertilizing and pruning properly. Pruning is best done in late winter. Avoid pruning in spring when higher moisture can increase risk of infection or in late summer and autumn, which can delay the plant’s natural cold hardiness response. Minimize any wounding due to root pruning, transplanting or lawnmowers to reduce infection sites.[图片]3. Prune. Prune out branch cankers during dry periods when conditions are unfavorable for infection. Disinfect pruning tools in a 1-part bleach to 9-parts water solution between each cut.

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2017年09月19日

Cornus species are riddled with leaf diseases. After the discovery of dogwood anthracnose that devastated the dogwood population in the northeastern U.S., it has been increasingly more significant to be on alert for disease outbreaks. Three leaf spot diseases that show themselves in varying degrees are dogwood anthracnose, spot anthracnose, and powdery mildew. Dogwood anthracnose and powdery mildew are of particular concern. Dogwood anthracnose was confirmed in St. Louis in 2012 and again in 2015. Powdery mildew can occur yearly.[图片]Symptoms and Diagnosis Symptoms of Dogwood Anthracnose: Dogwood anthracnose is a disease caused by the fungus, Discula destructiva. Since its discovery in the 1970s, the spread has been swift and the results disastrous to the native Cornus florida. Symptoms include leaf spots that are soft and very wet with purple borders, twig blight, dead leaves that cling to the tree, epicormic shoots (water sprouts with many shoots occurring near the base of the tree), lower branch dieback, and eventually death. Conditions that favor disease include low light levels, high humidity, and temperatures of 60 F. In the St. Louis area it was found in the Kirkwood/Ladue area in 2012 and the Brentwood area in 2015. It is considered established in St. Louis.[图片]Symptoms of Spot Anthracnose: Spot anthracnose is caused by the fungus, Elsinoe corni. Symptoms include small, dark, pinprick lesions with purple borders on leaf surfaces. The lesions are numerous and usually occur on dry leaves. The disease is unsightly, but typically not very harmful. Symptoms of Powdery Mildew: Powdery mildew is caused by the fungus, Microsphaera pulchra. Early symptoms include round, white or gray powder-like spots that appear on upper leaf surfaces. These spots form a dry white layer on leaves. As young leaves become infected, they appear twisted, dry, and leathery. Older leaves look bronzed. Moderate temperatures, low light, and high humidity accelerate this disease. High nitrogen levels cause the disease to flourish.[图片]Integrated Pest Management Strategies 1. Choose disease resistant varieties. A soon-to-be released cultivar for dogwood anthracnose resistance is called ‘Appalachian Spring’. Cultivars less susceptible to spot anthracnose include ‘Cherokee Princess’, ‘Cherokee Sunset’, and ‘Springtime’. ‘Cherokee Brave’, C. kousa, and C. kousa x C. florida crosses are resistant to powdery mildew. 2. Water the roots of the tree. Avoid using overhead irrigation in order to reduce the incidence of leaf spot.[图片]3. Practice good sanitation. Promptly remove and dispose of any dead or dying twigs or branches found in trees as well as leaves that show the symptoms of dogwood anthracnose. Leaves with spot anthrancnose and powdery mildew can remain but rake and remove them in the fall when they drop. 4. Remove plants confirmed with dogwood anthracnose. Removing an infected tree confirmed with dogwood anthracnose is strongly recommended. The infected trees should NOT be converted to mulch to avoid spreading the disease.[图片]5. Apply fungicides. If you suspect dogwood anthracnose, get it diagnosed by a professional prior to a fungicide application. Powdery mildew should also be controlled, especially if the disease has occurred previously.

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2017年09月19日

Leaf spot is a common descriptive term applied to a number of diseases affecting the foliage of ornamentals and shade trees. The majority of leaf spots are caused by fungi, but some are caused by bacteria. Some insects also cause damage that appears like a leaf spot disease. Leaf spots on trees are very common and generally do not require spraying. Leaf spot may result in some defoliation of a plant. An established plant can tolerate almost complete defoliation if it happens late in the season or not every year. Small or newly planted trees that become defoliated are more at risk of suffering damage until they become established.[图片]Symptoms and Diagnosis The chief symptom of a leaf spot disease is spots on foliage. The spots will vary in size and color depending on the plant affected, the specific organism involved, and the stage of development. Spots are most often brownish, but may be tan or black. Concentric rings or dark margins are often present. Fungal bodies may appear as black dots in the spots, either in rings or in a central cluster. Over time, the spots may combine or enlarge to form blotches. Spots or blotches that are angular are generally referred to as anthracnose (see entry on “Anthracnose of Trees”) Leaves may yellow and drop prematurely.[图片]Life Cycle The organisms that cause leaf spots survive in fallen infected leaves and twigs. Some may remain in dead twigs on the tree. Most damage occurs in spring. During wet weather, spores may splash or be windblown onto newly emerging tender leaves where they germinate in the moisture and infect the leaf. Overhead watering can also provide prolonged wet periods that are ideal for spreading leaf spot diseases.[图片]Integrated Pest Management Strategies 1. Live with the disease. Most trees tolerate leaf spots with little or no apparent damage. A tree affected early in the year will re-leaf and the new leaves may not be affected. Only if defoliation occurs three or more years in a row will most established plants be adversely affected. 2. Remove infected leaves and dead twigs. Raking up and disposing of infected leaves as they drop and pruning out dead twigs can help control the disease by removing spores that can reinfect the new leaves. This is not a cure but may help limit infection by reducing the total amount of inoculum.[图片]3. Keep foliage dry. Avoid overhead watering. Use soaker hoses or water early in the day so the foliage can dry off before night. Watering can also spread the disease by splashing. Pruning plants to allow for good air circulation and reducing crowding will also help keep the foliage dry. 4. Keep plants healthy. Since most plants can tolerate some defoliation, keep them in good health so they can rebound quickly. Avoid over fertilizing by testing the soil first. Abundant, young, tender growth is very susceptible to attack by disease and insects. Overuse of nitrogen can cause an abundance of succulent growth.[图片]5. Use fungicides if needed. In rare cases of severe infection and where the size and value of plants make it practicable, applications of fungicides may be helpful. Sprays will not cure infected leaves. Therefore, once the damage is noticed, spraying may have limited value. Spraying generally needs to be started as buds break in the spring and repeated at 10–14 day intervals. Recommendations will vary by disease and fungicide used. Have the disease identified before purchasing a control product. 6. Replace the plant. Though a drastic measure, many gardeners find it less bother and more rewarding to replace a plant that is continually plagued with leaf spot diseases. Either replace with a different kind of plant or a variety that is more resistant or tolerant of disease. A nursery can help you in your selection.

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2017年09月19日

Chlorosis is a general term that refers to the symptoms of uniform yellowing of leaves. It may be caused by any number of stresses including: nutrient deficiency, root damage, temperature extremes, herbicide misapplication, too much light, too little water or too much water, insect feeding, or disease pathogens.[图片]Iron chlorosis is a common form of chlorosis. It is the result of a lack of iron in the new growth of the plant. Iron is not necessarily deficient in the soil—it may be there, but just in an unavailable form for absorption through the root system. Symptoms and Diagnosis The leaves of affected plants turn uniformly yellowish-green; or very commonly, they turn yellow between the veins, but the veins remain green. Yellowing is most common on new growth. The new growth may be stunted. In severe cases, leaves may die and drop prematurely. Twigs and branches may experience stunting and dieback, and plants may fail to produce flowers or fruit.[图片]Yellowing is fairly certain to be due to iron chlorosis if the leaf veins remain green on the otherwise yellow leaf, and the chlorosis appears first on the younger or terminal leaves, spreading later to the lower parts of the plant. Iron chlorosis is most common on pin oak, sweet gum, and river birch trees as well as rhododendrons and azaleas. Integrated Pest Management Strategies 1. Monitor the problem. Many factors can cause leaves to become chlorotic. Therefore, before assuming the problem is lack of iron, have the problem diagnosed. Weather conditions and extreme changes in soil moisture may induce temporary symptoms of yellowed leaves. These symptoms may disappear as conditions normalize. Persistent chlorosis, however, needs attention. Plants commonly associated with chlorotic symptoms include holly, hydrangea, apple, blackberry, blueberry, cherry, grape, arborvitae, birch, boxwood, lilac, rhododendron, azalea, sweet gum, pin oak, and other oaks.[图片]2. Use a foliar feeding for temporary results. Spraying the foliage with an iron compound will correct leaf chlorosis temporarily. Iron sulfate, iron chelate, and soluble organic iron complexes are used. Spraying will not affect leaves produced later in the season. Several sprays at intervals of 2 or 4 weeks may be needed to keep developing foliage green. The corrections will last only one season and will not change the underlying deficiencies that are soil-related. 3. Give your plants good growing conditions. Longterm correction of iron chlorosis requires good soil. Improve the soil surface by mulching with 2–4 inches of organic materials such as leaf mulch or compost. Also, ensure good drainage. The soil pH should be measured with a soil test. An alkaline soil is often the cause of iron chlorosis because it limits the solubility of iron in soil water.[图片]4.Adjust the soil pH to make the iron available.The best long-range solution for correcting iron chlorosis in soils that are too alkaline is to make the soil more acid by lowering the soil pH. A pH of 6.0 to 6.5 is desirable for most plants. As low as 4.5 is recommended for azaleas, rhododendrons, and blueberries. The soil can be made more acid by the addition of sulfur, aluminum sulfate, or iron sulfate. Iron chelates, ferrous sulphate, aluminum sulphate, or sulfur can simply be spread on the soil surface and allowed to dissolve into the soil by watering and rainfall. This method requires more material, but is practical and feasible for the home gardener and may last several seasons, though the initial results are slow. Quicker effects may be seen if the chemicals are worked into the soil taking care not to cultivate too deeply so as to damage plant roots. Another method is to use water-soluble materials which can be injected into the soil with a root feeder.[图片]An excellent technique for solving chlorosis problems has been developed by Dr. Carl Whitcomb of Lacebark Inc in Stillwater, Oklahoma. His directions from his website are included here with his permission Solving Chlorosis Problems and Total Nutrition for Trees. 5. Trunk implantation and injection. Implanting or injecting an iron chelate or iron sulfate directly into the trunk of the tree through holes is often effective in 10-30 days and may be effective up to 4-5 years, for iron sulfate some arborists report, before retreatment is needed. This method is employed by professional arborists and is quick, neat, and safe. However, it is only used when the plant has little or no dieback as a result of iron chlorosis.

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2017年09月19日

Guignardia blotch is a foliage disease of many Aesculus species, including California, Ohio, red, and yellow buckeye and common, red, and Japanese horse chestnuts. Bottlebrush buckeye (A. parviflora) and some varieties of Ohio buckeye (A. glabra varieties arguta, monticola, and sargentii) may be resistant.[图片]The leaves of affected plants develop large reddish brown blotches surrounded by yellow tissue, often causing the foliage to curl and brown. In severe cases, leaves may fall prematurely in late summer. Since the foliage is usually not badly damaged until after the tree has completed much of its annual growth, the disease is primarily aesthetic. Wet weather promotes the growth of Guignardia blotch. In the United States, this fungal disease only occurs east of the Great Plains, perhaps because of the drier conditions of the West. This disease may become severe in nurseries or in plantings where tree crowns are close together.[图片]Symptoms and Diagnosis Guignardia blotch first appears as water-soaked irregular areas. These enlarge quickly and in a few days are reddish-brown to brown leaf spots with clear bright yellow margins. The blotches vary in size and may frequently grow together, covering large areas of leaf tissue. This causes the leaf to curl and brown, becoming dry and brittle, and may cause early leaf drop. Fruiting bodies of the fungus, seen as black pinhead-sized specks, may be visible in the lesions. Occasionally petioles and immature fruit may also have small reddish brown lesions. Symptoms of this disease are similar to those of environmental leaf scorch. Leaves affected by scorch will be on the sunny or windy side of the tree while Guignardia blotch may affect most leaves. Scorched leaves will not have the black fruiting bodies. See also “Scorch of Trees and Shrubs”.[图片]Life Cycle The Guignardia fungus overwinters on fallen Aesculus leaves. In early spring, its fruiting bodies mature and during wet weather, they release spores into the air. If the spores land on newly developing susceptible leaves that remain wet for several hours, the leaves become infected, resulting in blotches within 10–20 days. New fruiting bodies develop in early June and in wet weather, more infections may continue throughout the summer.[图片]Integrated Pest Management Strategies 1. Sanitation. Rake and dispose of infected leaves. 2. Pruning. Thin the tree canopy to improve air circulation and to speed drying of leaves. 3. Fungicides. Chemical treatment is usually not necessary or recommended in the home landscape. Serious damage may be controlled by applying a fungicide containing chlorothalonil or mancozeb at bud break and then repeating the application at 10 to 14 day intervals as long as wet conditions continue.

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Dummer. ゛☀

2017年09月19日

Dutch elm disease is a wilt disease caused by the fungus, Ceratocystis ulmi. It was described in Ohio in 1930. By the 1980's, it could be found in most of the U.S. It is a serious and fatal disease of American elms. Even after years of study, there is no effective cure for the disease.[图片]Symptoms and Diagnosis Symptoms develop rapidly during a period of 4–6 weeks after leaves reach full size. The first visible symptoms are yellowing of foliage followed by wilting and browning, a condition called flagging. Usually a single branch is affected first; wilted branches die rapidly and leaves brown, curl, and often drop prematurely. The symptoms spread to nearby branches and then to one whole part of the tree. The entire tree finally wilts and dies. This progression of symptoms may develop in one season or may take several years. Positive diagnosis of the disease requires a laboratory test to culture the fungal pathogen.[图片]Life Cycle The fungus is primarily dependent on insectvectored transmission from tree to tree over long distances. There are 2 vectors for the fungus in North America, the native American elm bark beetle and the less prevalent European elm bark beetle. Both of these beetles feed and breed under the bark of living or recently dead elm trees or logs. They carry the spores of the fungus from infected trees and innoculate healthy trees as they feed. Both species of elm bark beetles are effective carriers. The cycle of infection by the fungus is tied to the life cycle of the vectors. The beetles breed in recently dead elm wood or weakened living trees. If the fungus is present in breeding sites, emerging beetles will carry spores to healthy elms and introduce the fungus in feeding sites on young twigs. The beetles can fly up to 1/4 mile in search of feeding or breeding sites, but they may be blown many miles by winds. The disease may also spread by root grafts from a diseased elm tree to a healthy elm tree provided that the root systems overlap.[图片]Integrated Pest Management Strategies 1. There is no cure for the disease. If your elm tree has leaves yellowing or wilting on one or more branches, cut off several small branches and look for brown streaking in the sapwood. If brown streaking is evident, a laboratory test is suggested for positive identification. This disease may be confused with other canker and wilt pathogens of elm. 2. Maintain plant health. Provide adequate amounts of water and fertilizer. Mild cases of Dutch elm disease may respond to pruning of diseased wood if less than 5% of the tree is infected. Control the insect vectors. Systemic fungicides injected into the tree at 1–3 year intervals have proven beneficial in providing protection. An arborist should be consulted to discuss costs and potential benefits.[图片]3. Remove diseased trees. Severe cases require complete removal of the tree. Wood should not be saved for firewood. It should be chipped to destroy the beetle’s breeding place. Nearby American elms may also be infected through root grafts. Destroying root grafts by mechanical severing can slow the spread of the disease, if done before the fungus has moved into healthy elm trees. This may not be practical in a home setting. 4. Replace diseased trees with varieties resistant to Dutch elm disease. The true Chinese elm, Ulmus parviflora, a tree with multi-colored bark, is one choice. A related tree with a vase shape similar to American elm is the Japanese zelkova, Zelkova serrata. Both are resistant to Dutch elm disease but not immune.

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Dummer. ゛☀

2017年09月19日

Crown gall is a plant disease caused by the soil-inhabiting bacterium, Agrobacterium tumefaciens. The bacterium causes abnormal growths or galls on roots, twigs, and branches of euonymus and other shrubs primarily in the rose family. The bacterium stimulates the rapid growth of plant cells that results in the galls. In addition to being unsightly, the galls weaken and stunt the growth of the plant. Although galls can disrupt the flow of water and nutrients up the roots and branches, they usually do not cause total plant death. The disease can spread to other susceptible plants through contaminated soil and tools. Most chemical treatments are not effective.[图片]The galls on forsythia, viburnum, highbush blueberry, American elm, hickory, maple, oak, and privet are believed to be caused by a fungus, Phomopsis sp. Since its cultural controls are the same as for bacterial crown gall, both are treated together herein. Chemical treatments, however, will differ. Chemical treatments for both are of very limited value to the home gardener. Only those relating to bacterial crown gall have been addressed.[图片]Symptoms and Diagnosis Galls are most commonly found near ground level on the roots and lower branches of the plants. As the galls enlarge, they become woody and hard. The outer layer turns brown and corky. The plant may be weakened and stunted with some branch or tip dieback. Symptoms may not develop immediately after infection. Galls grow most rapidly during the warm months of the year.[图片]Life Cycle Crown gall forming bacteria inhabit the soil and survive for many years. The bacterium can initially be brought in on the roots of infected plants. It then spreads by soil and water movement or contaminated pruning tools. Bacteria enter the plant through wounds possibly caused by chewing insects, cultivation damage, or from grafting and pruning tools. The crown gall bacterium has been known to survive more than two years in the soil in the absence of susceptible plants. It can live for several years in decomposing galls buried in the soil. Crown gall is likely to be more serious in limed soil than in acid soils so soil pH could be important in limiting the disease.[图片]Integrated Pest Management Strategies 1. Prune out infected material. Crown gall cannot be eliminated from a shrub even though the infected plant may live for many years. To improve the appearance of the plant, prune out and destroy infected stems below the galled area. Sterilize the pruning shears after each cut with a mixture of one part bleach to nine parts water. 2. Destroy infected plants. Destroy the infected plant. The bacterium will remain in the soil so it is important to plant a resistant plant species. If the same species needs to be planted in the area, remove and replace the soil or consider soil sterilization.[图片]3. Sterilize the soil. Soils known to be infected with crown gall bacteria can be sterilized using chemicals, heat, or antibiotics. This is not practical for most home gardeners. A biological control has been introduced using a bacterium, Agrobacterium radiobactor strain 84. This bacterium was discovered to be antagonistic to crown gall bacterium. It is available for use as a preplant treatment by dipping nursery stock in a suspension of the live bacteria in water. 4. Exclude the problem. When purchasing forsythia and euonymus plants, inspect them carefully for signs of galls. Do not purchase plants that show gall-forming symptoms.[图片]5. Replant with a more resistant plant species. The following plants recommended by the University of Illinois show greater resistance to bacterial crown gall: barberry, hornbeam, true cedars, ginkgo, golden raintree, tulip tree, mahonia, spruce, linden, boxwood, catalpa, beech, holly, larch, magnolia, black gum, pine, Douglas fir, bald cypress, hemlock, birch, firethorn, redbud, smoke tree, sweet gum, deutzia, serviceberry, yellowwood, yew, and zelkova.

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