FIGURE 7–12 Corn earworm damage. (Source: Photo provided courtesy of Oklahoma Cooperative Extension Service, Oklahoma State University) FIGURE 7–13 Leaf miner damage on spinach. (Source: Photo provided courtesy of Oklahoma Cooperative Extension Service, Oklahoma State University) 7. Leaf miners. Leaf miner damage is visible as tunnels between leaf surfaces (Figure 7–13). These tunnels are caused by larvae of flies such as the serpentine leaf miner (Liriomyza spp.). When a leaf is heavily infested, it yellows and eventually drops. Leaf miners attack a variety of garden plants such as tomato, pepper, eggplant, and especially lettuce and melon. 8. Spider mites (Tetranychus spp.). Mites are piercing or sucking insects that live on the undersides of leaves. They vary in color from reddish-yellow to light green. They attack a variety of garden crops, especially tomato and cucurbit. After feeding, the leaves appear bronzed or discolored. Severely attacked plants whither and drop. Spider mites have the capacity to produce six to ten generations each year. They devastate ornamentals, fruits, nuts, vegetables, and numerous other species. 9. Thrips. Sucking insects, thrips are very common in the garden. Plants attacked have distorted leaves. Onion thrips (Thrips tabaci) and western flower thrips (Frankliniella occidentalis) are two of the most common thrips in the garden and landscape. 10. Aphids. Also called plant lice, aphids are common on young leaves and growing tips of stems. One of the most common of the aphid species is the green peach aphid (Myzus persicae), which attacks many garden crops. Aphid populations are effectively held in check where natural enemies such as lacewings occur. This sucking insect affects many ornamentals, orchard crops, and vegetables. They are important because they are able to transmit a number of viruses that cause devastation to horticultural crops, examples being cucumber mosaic, bean mosaic, and lettuce mosaic. Aphids cause plants to grow with less vigor. 11. Ants. Ants are common in the garden and the landscape. One of the most common is the Argentine ant (Iridomyrmex humilis). It feeds on honeydew secreted by aphids. The leaf cutter ant (Acromyrmex spp.) cuts pieces of leaves for food. 7.4.7 CONTROL Insects are commonly controlled by the use of chemicals. However, they may be controlled by cultural, legislative, and biological methods. Control methods are discussed in detail in Chapter 8. 7.4 Insects 229
7.5 DISEASES Pathogen An organism that causes disease. Saprophyte An organism that derives its nutrients from the dead body or the nonliving products of another plant or animal. Cultivated plants are usually more susceptible than their wild relatives to diseases. This susceptibility is due in part to the fact that many different cultivars have uniform genetic backgrounds as a result of the activities of plant breeders using the same breeding stock in developing new cultivars. Further, modern cultural practices allow plants to be grown in dense populations (as in monoculture), which facilitates the spread of disease. Horticultural plants are attacked not only by insects but also by a large number of microorganisms that cause infectious diseases. These parasitic pathogens (diseasecausing organisms) can be placed into four categories—fungi, bacteria, viruses, and mycoplasma-like organisms. Like all living things, these organisms have their own life cycles. Since they are parasitic, they need a susceptible host on which to survive. Plant diseases may be classified on the basis of the causal organism, such as fungi, bacteria, viruses, and microplasma-like organisms. 7.5.1 FUNGI An estimated 75 percent of all seed plant species live in some form of association with fungi in their roots, which is known as mychorrizae. This association is called mutualism, where both host and fungus benefit, and is similar to symbiosis, a mutually beneficial plant-bacteria association found in legumes only. Mychorrhizal fungi are known to be more efficient than plant roots in absorbing phosphorus. They are also essential for the growth and development of forest trees and herbaceous species. Acid rain destroys michorrhizae. The reproductive structures from which fungi develop are called spores. Spores occur in a tremendous variety of shapes, sizes, and colors. A few fungi have no spore stage. When spores germinate, they produce hyphae, which grow and branch to form the fungus body called mycelium. Certain spores are visible as mold growth on the leaf surface. Fungal diseases such as Helminthosporium leaf spots, rusts, powdery mildew, and cercospora leaf spots are examples of this mold growth form. Other fungi occur as tiny dark fruiting bodies that are embedded in the tissue of the diseased plant. Examples of fungi are Septoria and Ascochyta. Fungal spores are transported in a variety of ways—by wind, water, birds, insects, spiders, slugs, and mites. Some of them have a protective covering that enables them to survive adverse environmental conditions. In order to infect, hyphae of fungi gain access to the host through wounds (caused by equipment, pests, hail, ice, and so forth) or natural openings such as stomata. Sometimes the pathogen penetrates the epidermal layer by direct action on that layer. Most of the infectious plant diseases are attributed to fungi. Fungi are either unicellular or multicellular (mostly) plants that lack chlorophyll. More than 250,000 species of fungi have been described, of which about 22,000 are known to cause plant disease. Some of them can live only on dead tissue (saprophytic), such as organic matter, while others live on living tissue (parasitic). Some of them are restricted (obligatory) to one host type (dead or living), while others have flexibility (facultative). Those that feed on dead and decaying matter are beneficial to plants because they aid in the decomposition of organic matter or compost to release nutrients for plant use. In the lawn, they aid in the decomposition of thatch (accumulated dead grass on the surface of the soil). All fungi are not pathogenic. Many are useful to humans and plants. Penicillin (from Penicillium) is one of the most important antibacterial drugs. Mushrooms used for food are fungi; fermented beverages and foods (e.g., bread, wine, cheese, and beer) depend on fungi (yeast) in their production. Even though most plant diseases are caused by fungi, they are usually relatively easy to control. Methods of disease control are described later in this chapter. Table 7–1 shows some important plant fungal diseases, their symptoms, causal organisms, hosts, and methods of control. 230 Chapter 7 Biological Enemies of Horticultural Plants
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