is also a root parasite that obtains its food from the host through the fungal hyphae it inserts into the host. Similarly, the world’s largest flower, Rafflesia arnoldii, and others in the genus are root parasites in the grape family (Vitaceae). Plants in five families have the capacity to trap and use insects to supplement their nutritional needs. These plants are described as carnivorous and include Venus-flytrap, pitcher plant, and sundew. Plants that live on the surfaces of other plants (not symbiotically but parasitically) are classifiable into three types: 1. Epiphytes. Epiphytes are plants that are autotrophic (photosynthesizing) but depend on others for support and some of their water and nutrient needs. These plants absorb water and minerals from the surfaces of their host plants and from the atmosphere. They are particularly adapted to the humid tropical forest; an example is Spanish moss. 2. Hemiparasites. Hemiparasites are equipped to photosynthesize but depend on the host species for water and nutrients. For example, mistletoe (Phoradendron spp.) is parasitic to many broadleaf trees including oak, birch, and huckleberry. Certain cultivars of these species are resistant to the parasite. Like mistletoe, the Indian warrior plant also produces haustoria. These plants are green plants and can therefore photosynthesize to some degree. 3. True parasites. Plants that are true parasites lack photosynthetic structures and are completely dependent on the host for all nutritional needs. An example is the dodder (Cuscuta spp.), which is parasitic to some vegetables and woody perennials. Epiphyte A plant that grows on another plant without deriving nutrition from the host. 7.3 SELECTED COMMON WEEDS The following are examples of common weeds that occur in the vegetable garden or landscape. A weed may be more of a problem in one part of the country than another because of adaptation. 1. Canada thistle (Cirsium arvense). Canada thistle is more of a problem in the northern states of the United States. It has well-branched, deep roots. A perennial that reproduces both sexually and asexually (by rhizomes), has disk flowers that may be white, lavender, or rose-purple. 2. Hedge bindweed (Convolvulus sepium). Bindweed, a perennial weed that reproduces by seed or rhizomes, is a twining weed with white or pinkish flowers. It occurs in different places, including cultivated fields, especially in the eastern half of the United States. 3. Field bindweed (Convolvulus arvensis). Field bindweed differs from the hedge variety by being more widespread and having extensive and deep root systems. It is also a perennial and reproduces by seed and rhizomes. It has twining habits and may also spread on the ground; its white or pink flowers are bell shaped. 4. Dandelion (Taraxacum officinale). Dandelion is widespread, occurring nearly everywhere in the United States. It is a perennial with a deep taproot system, and the crown is branched. Dandelion produces flower heads that are golden yellow and easily dispersed by the wind. This weed is particularly common in lawns. 5. Common milkweed (Asclepias syriaca). Common milkweed is a broadleaf weed with milky sap. It has sweet-smelling, pink-white flowers. This perennial has a well-branched root system with long rhizomes and reproduces by seed or rhizomes. It is more of a problem in fields in the eastern half of the United States. 6. Common lamb’s-quarter (Chenopodium album). Lamb’s-quarter is an annual weed that reproduces by seed. It has a taproot system and green flowers that are borne in spikes in a clustered panicle at the end of the branch and leaf axil. This weed is problematic, especially in the eastern half of the United States. 7.3 Selected Common Weeds 217
THE CURRENT APPROACH TO WEED CONTROL Department of Agriculture and Natural Resources Langston University RAYMOND FAUCETTE For the sake of clarity we will begin with the following three definitions from Webster’s Online Dictionary. Weed is the generic word for a plant growing in a spot where it is not wanted. The most prominent use of the word is in connection with farming, where weeds may damage crops when growing in fields and poison domesticated animals when growing on pasture land. Many weeds are short-lived annual plants, that normally take advantage of temporarily bare soil to produce another generation of seeds before the soil is covered over again by slower growth; with the advent of agriculture, with extensive areas of ploughed soil exposed every year, the opportunities for such plants have been greatly expanded. Pest is any animal or plant that is directly or indirectly detrimental to human interests, causing harm or reducing the quality and value of a harvestable crop or other resource. Weeds, termites, rats, and mildew are examples of pests. Integrated pest control is a pest control strategy based on the determination of an economic threshold that indicates when a pest population is approaching the level at which control measures are necessary to prevent a decline in net returns. In principle, IPM is an ecologically based strategy that relies on natural mortality factors, such as natural enemies, weather, and crop management, and seeks control tactics that disrupt these factors as little as possible. Also, a USDA/Environmental Protection Agency program aims to decrease pesticide applications by teaching farmers to use a variety of alternative control techniques to minimize pesticide use. These techniques include biological controls, genetic resistance, tillage, pruning, and others. (IPM) The current approach to pest control is Integrated Pest Management (IPM). As stated earlier IPM is an interdisciplinary approach that does not focus on eradication, but managing the pest population to keep it below a threshold that would cause economic loss to a crop. IPM combines legislative, cultural, biological, chemical, and physical methods of pest management. None of these methods are new, however they are used symbiotically to reduce our impact on the environment (Figure 1). FIGURE 1 Crop consultant scouting for both weed and insect pests. (Source: USDA) 218 Chapter 7 Biological Enemies of Horticultural Plants
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