| Chapter 6 Rhubarb Pests & Problems | | | --- | | Rhubarb has relatively few problems, but occasionally they do appear. Most often pests will attack other plants in your garden and move on to your rhubarb when they are done chomping on your carrots and tomatoes. This section will prepare you for dealing with weeds, bacteria, fungus, viruses, insects, larger pests, and plant interactions. | | | | | --- | --- | | 6.1 - Weeds | | | A weed is simply an unwanted plant. A corn plant, for example, is a weed if it is in a rhubarb patch. Most weeds are plants that are generally considered undesirable by gardeners. They are not eaten and not considered particularly attractive. They are frequently invasive and difficult to control. Weeds compete with crop plants for moisture, nutrients and light. They may also harbor insects and diseases, promote other diseases by increasing humidity, decrease vegetable quality and make harvests difficult. A weedy garden or flower bed is also unattractive. Weeds are highly adapted to where they grow. They have large, efficient root systems, grow rapidly and frequently produce tremendous amounts of seeds. Most weeds tolerate drought and low fertility. Little of their energy is used producing lush foliage, large seed or fruit; thus, they often thrive where vegetables struggle to produce. Many spread rapidly by vegetative structures as well as by seed. 71 | weed.dandelion.gif (51787 bytes) Dandelion (a common weed) | | Rhubarb plants grown under weedy conditions will have reduced yields or they may not survive to produce at all (if the weeds introduce a virus, bacteria or fungus). An established rhubarb plant will have few weed problems, the agressive roots and large leaves are suffient to choke out all but the most stubborn weeds. Weeds in other areas of your garden can also harbor diseases and problems taht can damage your rhubarb. The principal methods of weed control fall into three categories: - mechanical - cultural - chemical | | | | --- | | mini76.gif (138 bytes) Identifying Noxious Weeds of Ohio mini76.gif (138 bytes) Web Resources for Plant âWeedâ Identification A large collection of web sites that can aid you in figuring out what that odd plant is in your garden. | | | 6.1.1 - Mechanical weed control | | | Mechanical weed control involves removing weeds while they are small and preventing them from producing mature seed. It consists principally of mowing, plowing, rototilling, hoeing and/or hand pulling. These procedures give immediate results, are chemically safe and require little, if any, specialized equipment. Their main disadvantage is that they work only on growing weeds and may need to be repeated frequently. You should remove weeds while they are small because it is quicker, easier and does less damage to crop plants. Hoe or till shallowly (less than 2 inches deep) to avoid damage to desirable plants and to minimize moisture loss from the soil. The deeper the soil is disturbed, the more weed seed will be brought to the surface where it will grow. Weed seed may survive many, many years in the soil. Hand pulling if often easier when the soild is wet just aster watering or rain, or in the early morning. Gardeners often discontinue weeding as their crops mature. Weeds do not affect crop yields as much at this stage, but they will produce seed. A single weed of some species can produce hundreds of thousands of seed. Weed removal should continue until the vegetable crop is completely harvested. Gardens may then be kept tilled, mowed or heavily mulched while cops are not growing to prevent weeds from propagating. Frequent tillage has the additional advantages of turning organic material under where it will add to soil organic matter and mechanically destroying any insects present. 71 | | | 6.1.2 - Cultural weed control | | | Most cultural methods of weed control emphasize prevention. These include mulching, solarization and multiple or wide-row planting. 71 | | | Mulch Any layer of material spread over the soil surface may be considered to be a mulch. There are two classes of mulch, organic and inorganic. Both reduce weed growth, retain soil moisture, improve soil s tructure and moderatesoil temperature. Hay, grass clippings, straw, sawdust, ground bark, leaves, compost and even newspapers are among the many substances used as organic mulches. All reduce weed emergence and make it easier to pull those weeds that do emerge. | mulch.gif (71701 bytes) Use mulch to control weeds | | Solarization Solarization kills weeds by elevating the soil temperature. The process involves pre-soaking an area to be treated to 6 to 12 inches, cultivate the soil to a 4 to 6 inch depth. Then lay out clear plastic sheet (3 or 6 mm plastic sheeting is commonly available in garden supply stores or hardware stores in the paint section for covering furnature while painting) over the area to be treated. Lay soil over the edge of plastic to seal in heat. Allow seeds time to germinate (2 to 4 weeks) with the solar heat. After several days of sunshine, soil temperatures rise to as high as 140 degrees F (60 deg C) at the surface and well offer 100 degrees as far down as 18 inches. It takes four to six weeks of sunny weather to pasteurize the soil. After seedlings have been killed, remove the plastic and dead growth and work the soil for planting wildflowers, vegetables, or annuals and perennials. 78, 79 | | | | --- | | mini76.gif (138 bytes) Organic Weed Control | | | Wide-row planting Wide rows are formed by broadcasting the vegetable seeds in a band 10 to 30 inches wide rather than in a single row. The resulting spacing between plants is much closer. Wide-row gardening offers two advantages: Two to three times more food can be produced in the same amount of space for the work performed. The close spacing of the plants allows them to serve as a living mulch canopy that shades the soil, thus discouraging weeds, keeping the soil cool and conserving soil moisture. Wide-row planting is not a good solution for rhubarb but crops such as radishes, carrots, beets, onions, leaf lettuce, peas, and beans are well suited to planting in wide rows. 77 | | | 6.1.3 - Chemical weed control | | | Prior to planting a field of rhubarb, the field can be treated with Glyphosate (Roundupâ˘). There are few herbicides for established beds. 32 Chemical herbicides are only occasionally used by home gardeners to control weeds. The main reason why herbicides are not used as much as mechanical or cultural weed control methods in home gardens is that suitable herbicides are seldom legally available in small economical amounts. Several other difficulties also exist with the use of herbicides by home gardeners: 71 Most herbicides are non-selective and will kill both vegetables and weeds. Others are selective and can be used only with certain vegetables or control only certain weeds. Home gardens generally contain many vegetable species and weeds, which makes using selective herbicides difficult. Some herbicides may also damage nearby vegetables or remain in the soil and damage future plantings. Many herbicides may not be legal (or safe) for use on a specific vegetable or at the time when they are needed. The herbicide application rate may be very low and extreme accuracy in application may be absolutely essential. Herbicides may also be effective only for a short period or produce results slowly. | | | | --- | | mini76.gif (138 bytes) Chemical Control of Weeds in the Flower Garden | | | | | | --- | | rwd.book.gif (11790 bytes)Applied Weed Science Merrill A. Ross, Carole A. Lembi buy-the-book-cart.gif (2893 bytes)This introduction to the principles of weed science prepares readers to analyze real-life weed control problems and to develop integrated, practical approaches to solving them. Key Topics: Comprehensive in coverage and unique in presentation, it blends basic information on plant systems, soil systems, control methods, and management systems, and discusses various plants and herbicides by groups to provide an integrated framework from which to extend information to many different situations. Visit The Compendium Bookstore for more books on Botany, Gardening, Herbs & Medicinal Plants, Poisonous Plants, and Cooking & Recipes. right.gif (913 bytes) | | | | | | | --- | --- | | 6.2 - Bacterial Diseases | | | 6.2.1 - Bacterial Soft Rot | | | Bacterial Soft Rot (Red Leaf) is caused by Erwinia rhapontici, a bacterium. Observations are that the organism builds up in soil and may contaminate it for many years. Other crops are reportedly susceptible to the bacterium, although there is no evidence the disease spreads from one to another. Symptoms: A brownish rot begins just below a bud near the soil level. Rot gradually extends to tissue above the infection point until it reaches the rootâs center. The entire plant crown turns black. The rootâs center may become hollow. Decay sometimes extends up into the base of the stalk. Leaves may be dull red or yellow, and stalks eventually collapse. It may be several months after initial infection before aboveground symptoms are apparent. 74 Control: - Plant disease-free roots in soil without a history of the disease. - Rogue out affected plants and the soil adjacent to the plants. | | | | | | --- | --- | | 6.3 - Fungal | | | 6.3.2 - Leaf Spot | | | Leaf Spots are caused by Ascochyta rhei and Ramularia rhei, fungi that can overwinter on old rhubarb leaves and stalks. Leaf spots are common on field-grown rhubarb in areas with frequent rains in the early spring when leaves and stalks are actively growing. Symptoms: Ascochyta infections appear as small, yellow-green infections on the upper leaf surface. Later, spots merge to give a mosaic appearence. The spotsâ centers turn white and fall out, giving the leaf a tattered appearence. Ramularia leaf spots begin as small red dots which enlarge to become white to tan circular spots with purple borders. Similar spots commonly occur on the plantâs stalks. Some spots appear as irregular, circular spots, less than 1/2 inch in diameter, on the leaves. The white spots are surrounded by a red band bordered by light green. I am not certian if this is a different species or what.  76 Control: - Harvest infected plants first to help reduce the source of inoculum. - Destroy any infected plant debris especially after first harvest. - Remove and destroy rhubarb leaves and stalks after first frost. | R007.jpg (18073 bytes) Leaf Spots on rhubarb | | 6.3.1 - Violet Root Rot | | | Violet Root Rot is caused by Rhizoctonia crocorum, a soil borne fungus. The fungus reportedly has a wide host range including asparagus, artichoke, alfalfa, bean, beet, cabbage, carrot, celery, clover, dandelion, fennel, mangel, parsley, potato, rape, rhubarb, sugar beet, sweet potato, and turnip. The fungus may be spread as a contaminant on seed, plant parts, soil, tools, flood water, and equipment. The fungus remains alive in the soil at least 1 year. The mycelium grows through the soil from plant to plant. Optimum temperature for infection is 59 - 61ďż˝F (15 - 16 ďż˝C). The disease is most destructive on heavy, unlimed soils. Symptoms: The disease attacks the plantsâ underground parts, causing stunting, yellowing, and wilting of aboveground parts or the plantâs death. The fungus produces a loose mat of reddish-purple mycelium over the underground plant portions. Embedded in the mat are numerous small dark sclerotia. A dry rot occurs which may be followed by a wet rot caused by secondary organisms. 75 Control: - Use long rotations (4 to 5 years). Include cereals and grasses (nott really something your going to do with rhubarb). - Provide good weed control and drainage. - Maintain good fertility, and lime acid soils to increase pH. | | | 6.3.3 - Honey Fungus | | | Honey Fungus is the soil borne fungus Armillaria mellea, prominent among the wood decaying fungi in rotting these down and returning their nutrients to the soil to assist the new growth of future generations of trees. Other species include Armillaria obscura, Armillaria ostoyae, Armillaria bulbosa and Armillaria tabescens. Honey fungus is so called because the caps of the sporophores (mushrooms) are scaly like an armadillo and honey coloured. Honey Fungus in a garden originates from a dead stump or the buried lateral roots from a tree that has died from Honey Fungus or been felled some years before. Unfortunately when a tree is removed from a garden, more often than not, it is sawn off across the base rather than pulled over which sprags up the lateral roots - the roots left being the food supply for the pathogen. 19,27,32,80 | honeyfungus1.jpg (38591 bytes) Honey Fungus | | Control: - Infected plants should be dug up and discarded or destroyed (do not add them to your compost pile, some folks recommend burning them). - Remove dead wood or trees from completly or avoid placing a new garden in a previously wooded area. - Treating prophylactically with Armilatox (Armillaria is where âArmillatoxâ gets its name) all your favourite and valuable subjects in the area of risk and continue every year until the Honey Fungus in the source is starved out. | | | 6.3.4 - Gray Mold | | | Gray Mold, Botrytis cinerea, is a fungus that overwinters on decayed rhubarb plant parts. Spores also are abundant on decayed parts of other plant species and in the air. The disease can be a problem in the field but is more destructive in forced crops. Symptoms: On older leaves and stems, a powdery gray mold develops under moist conditions. During harvest, packing, and shipping, stems are further infected. Reddish-brown, water-soaked areas develop on stems as the disease advances, producing additional gray mold spores. Control: - Remove all broken root and crown pieces from forcing house. - Circulate air through the forcing house to keep leaves dry. - Proper plant spacing ensures good air circulation and drying. - Remove leaves from stalks before packing and shipping. - Refrigerate harvested stalks. - Chemical control: For Greenhouse only. - Botran 75WSB at 1.33 lb/100 gal water. Do not use within 3 days of harvest. For greenhouse use only. 12 hr reentry. - Captan 50W at 2.25 to 3.25 kg/1,000 l water. Do not use within 2 days of harvest. For use in forcing houses only. 24-hr reentry. 82,83,84 | conidia.jpg (6238 bytes) Gray Mold, Botrytis cinerea | | 6.3.5 - Crown rot | | | Crown rot is caused by Phytophthora cactorum, a soil-borne fungus (also known as Leather Rot in fruit trees) causes lesions at the base of the stalks resulting in a sudden collapse of the entire leaf stalk and leaf. The infection penetrates the crown and secondary organisms can then increase the rapid deterioration of the plant. This disease is common in heavy, moist and poorly drained soils. Chemical control measures have not been shown to be effective. Crown rot has been reported in many regions throughout the United States. In many areas, it is considered a minor disease of little economic importance. However, excessive rainfall in the spring can lead to severe losses and quality reduction. Several different species of phytophthora can cause crown rot, but the most common in is Phytophthora cactorum. The fungus survives the winter as a thick-walled oospore in the soil. It is also found in the silt in canal bottoms and is carried to the orchard in irrigation water. In wet conditions, the oospores germinate to produce sporangia, which release zoospores into the water in the soil. Wet conditions favor the dispersal of these swimming spores and increase the likelihood that they will infect crops. 72,73 Symptoms: include oozing and gumming at the base, lesions at the base of the stalks and a sudden collapse of the entire leaf stalk and leaf. Control: - Avoid overwatering. - Fumigate before replanting new crops will kill the fungus if it is in the soil. | | | | | --- | | 6.4 - Viruses | | Several viruses can infect rhubarb including Arabis Mosaic Virus, Cherry Leafroll Virus, Cucumber Mosaic Virus, Strawberry Latent Ringspot Virus, and Turnip Mosaic Virus. Reports are that Turnip Mosaic Virus is the one most common in British Columbia. Mosaic symptoms as described below have been observed. Many of these viruses can be found on weed hosts, and some are transmitted by nematodes. Symptoms: Pale yellowish or dead spots or rings on leaves, usually with pale green mottle. Young leaves show well-defined to severe light and dark green mosaic mottle. Leaves often are crinkled and distorted. Symptoms may disappear in hot weather. 81 Control: - Assay soil for nematode vectors before planting. - Plant virus-free stock. - Dig up and destroy symptomatic plants. - Remove and destroy adjacent weeds. | | | | | --- | | rwd.book.gif (11790 bytes)The Ortho Home Gardenerâs Problem Solver Cheryl Smith buy-the-book-cart.gif (2893 bytes)With descriptions of problem-solving techniques, as well as solutions for hundreds of specific problems, The Ortho Home Gardenerâs Problem Solver helps home gardeners identify and solve problems both indoors and outdoors. A wide range of possible solutions is represented. You choose the method that suits you best â from hand picking to spraying. Hundreds of full-color illustrations. Visit The Compendium Bookstore for more books on Botany, Gardening, Herbs & Medicinal Plants, Poisonous Plants, and Cooking & Recipes. right.gif (913 bytes) | | | | | | --- | --- | | 6.5 - Insects and Bugs | | | 6.5.1 - Rhubarb Curculio | | | Description: The rhubarb curculio, Lixus concavus, is a large (about 1/2 inch, 13 mm), dark gray, snout beetle dusted with a yellow powder that easily rubs off when the insect is handled. The head has a long, curved snout at the end of which are the mandibles. It damages rhubarb by puncturing stalks and laying eggs into them. However, the larvae feed and develop on curly dock, which is also sometimes known as wild rhubarb. The eggs are oblong and yellowish-white in color, while the mature larva is a 3/4-inch legless grub with a brown head capsule. Symptoms: Rhubarb curculio injury may be either feeding injury (which appears as notches in the rhubarb stem and on the leaf edges) or egg-laying punctures in the stem. Feeding injury often leads to stem decay as other organisms invade the injured tissue. Sap exudes from wounds of either type and collects as glistening drops of gum when fresh. Fortunately the eggs of this insect do not hatch when deposited in rhubarb. The leaf stalks of the rhubarb may show exuding sap and partial decay from late spring through early summer due to the feeding and egg-laying punctures of the rhubarb curculio. | BollWeevil.jpg (10449 bytes) Rhubarb curculio (photo credit) | | | | Life Cycle: The curculio overwinters as an adult is piles of debris or in other protected places. The adults appear mid summer and are seen resting on the stalks and leaves of rhubarb, dock, thistle, or sunflower. They soon begin laying eggs. Eggs are deposited singly in cavities about 1/8-inch deep in the stalks. Hatching occurs in 7 to 10 days in all plants except the rhubarb. Eggs deposited in rhubarb are crushed by the actively growing plant tissue. The newly hatched larvae burrow their way down through the weed stalk, reaching maturity in 8 to 9 weeks at the bottom of the stalk just below the soil surface. Usually only one grub reaches maturity in a host plant. Pupation occurs in a cavity at the base of the host plant. Within a few weeks, the adult beetles emerge and feed for a short time before seeking out protected places to pass the winter. There is only one generation a year. The rhubarb curculio completes its life cycle in weeds in or near the garden. It damages rhubarb only through feeding and oviposition. Control: - Handpicking the beetles from the plants during early summer and destroying them is the only direct method of control. When the beetles first emerge, they are easily picked from the vegetation on which they are resting. Their large size aids in finding them and helps make them easy to handle. - The removal of all wild plants in which the beetles breed (curly dock, thistle, and sunflower) growing in or near the rhubarb planting during summer (while the curculio larvae are still in them) will also be helpful. 18,32,85,86,87,88 | | | 6.5.2 - Aphids | | | I have had a minor problem with aphids (Genus Aphis and Macrosiphini) on a few of my rhubarb plants. Most aphid species are relatively host-plant specific, although they may occasionally be found colonizing certain plants or âreserve hostsâ with which they are normally not associated. The Aphidoidea are predominately a north temperate group (North America, Europe, Central Asia). Almost all major pest aphids are introduced species. Description: Aphids are small, soft-bodies insects about the size of rice grains (typically range in size from 1/16 - 1/8 inch or 1.5 - 3.5 mm in length). They are usually light green or black with a white wooly coating. Aphids survive by sucking the juices of soft new growth which damages a plantâs ability to properly process food, reducing the plants vigor and causing distorted growth in severe cases. Symptoms: Check newly forming leaves and buds for aphids and also look for distorted yellow leaves with a grayish cast. Aphids also leave a sticky coating on leaves from an excretion called âhoneydewâ. | insect2_7.jpeg (52958 bytes) Aphid | | Life cycle: The life cycle of aphids is rather unusual and can be complex. Most aphids reproduce sexually and develop through gradual metamorphosis (overwintering diapause egg, nymphs and winged or wingless adults) but also through a process called âparthenogenesisâ (the production of offspring without mating, asexually). Sexual reproduction involves gene recombination while parthenogenesis does not. The life cycle (development to sexual maturity) for most aphids is 5 to 6 days. Under greenhouse conditions the cotton aphid may complete a maximum of 51 generations a year, with each adult producing about 85 young. Sexual maturity is reached in 4 to 10 days and the reproductive period is about three weeks. The average length of life of an adult is about one month. Parthenogenic green peach aphid females, Myzus persicae, can produce three to six fully formed young per day for several weeks. Host alternation is common in aphids. Often, aphids use one host plant as the âprimary hostâ for reproduction and another plant (perhaps a distantly related plant) as a âsecondary hostâ for parthenogenetic reproduction. 89,90,91,92 | | | | --- | | Aphid References mini76.gif (138 bytes) A huge document on Aphid Management (or here) | | | Control: - Aphids are attracted to the color yellow, so set out a yellow bowl full of soapy water or buy a sticky yellow color trap to lead the aphids away from the plants. - Control ant population. Ants rub the bellies of aphids causing them to excrete honeydew, which the ants eat. So, control the aphid infestation by setting ant traps or destroying ant colonies. Spray fruit trees in the spring with dormant oil. - Add natural predators, such as ladybugs. They will eat the aphids and other pests. - Destroy the aphids by hand. Rub the infested leaf between the thumb and forefinger. Wash them away with a stream of water or a natural insecticidal soap, if desired. - Treat with insecticides. Materials recommended for control of aphids include Ambush*, Baythroid*, Diazinon, Dimethoate, Furadan*, Lannate*, Lorsban, Malathion, Penncap-M*, Pounce* or Supracide*. - Treat with natural insecticides, such as pyrethrum or rotenone. | R009.jpg (18263 bytes) Aphids under a rhubarb leaf | | | --- |