Plant Pathogens and Pests

Plants, being immobile, are unable to escape pests (herbivores) that eat them, or microorganisms (pathogens) that cause plant diseases. On a global basis, it is estimated that plant diseases annually cause an 11 to 16 percent reduction in the value of rice, wheat, corn, and potato harvests. Additional losses to these major food crops as a result of pests (primarily insects and mites) are estimated at 9 to 21 percent. The magnitude of these losses, despite peo­ple’s best efforts at prevention, is of major concern.

Diseases

Diseases of plants are caused by fungi, bacteria, viruses, mollicutes, and ne­matodes. Fungi are eukaryotic, spore-bearing, heterotrophic organisms that produce extracellular enzymes to break down plant or animal products to small molecules (for example, sugars and amino acids), which they absorb as nutrients. Fungi may grow as unicellular yeasts, but more commonly they grow as multicellular chains of elongated cells that form threadlike struc­tures collectively called mycelium. Important diseases caused by fungi in­clude Dutch elm disease, apple scab, and wheat stem rust. Fungi like protists (primitive eukaryotic microorganisms) also cause many serious diseases, of which the best known is the late blight of potato, which caused the Irish potato famine in the 1840s. Since the 1980s, late blight (caused by Phytophthora infestans) has become the single most important biological con­straint to global food production and the cause of one of the biggest uses of pesticides.

Bacteria, although differing from fungi in being unicellular prokaryotic organisms, also cause disease by extracellular digestion of plant tissues. These bacteria are highly infectious and are easily spread on seed and by wind­blown rain, irrigation water, and insects. Fire blight of apple and pear, caused by Erwinia amylovora, is a serious problem because the bacterium is so eas­ily spread by rain and insects. Most bacterial plant pathogens survive as saprophytes living on crop debris and in soil. Mollicutes, which can be de­scribed as prokaryotes lacking cell walls, cause diseases of plants by living within the phloem cells from which they obtain their nutrients. Mollicutes are very effectively carried from plant to plant by insects in which they can also reproduce.

The viruses that cause plant diseases are also often carried by insects and other pests, as well as by grafting and on cutting tools, machinery, or in seed. Most such viruses consist of ribonucleic acid (RNA), surrounded by a protein coat (the capsid). A few plant pathogenic viruses contain de­oxyribonucleic acid (DNA) rather than RNA. All viruses are intracellular and are obligate parasites as they are dependent on the plant cell for their reproduction.

Plant pathogenic nematodes are small (approximately 1-millimeter long) wormlike animals that live in soil and feed on plant roots by piercing the cells with a needlelike structure called a stylet through which they suck up the cell contents. Nematodes, which may feed from outside or inside the root, cause enormous damage to roots, thus reducing nutrient and water up­take. Root knot nematodes, one of the most damaging pathogens, stimulate division and expansion of root cells to create “galls” in which the female ne­matodes remain to feed and produce eggs.

­ Pests

Plant pests include the arthropods (such as insects and mites), slugs, snails, sowbugs, and pillbugs. Only a small proportion of insects are plant pests with the most conspicuous being the butterflies and moths. The larvae (caterpillars) of butterflies and moths cause severe damage by feeding on fo­liage until they pupate. The adults rarely feed on foliage. The most com­mon butterfly pest in North America is the cabbage white, which is seen in great numbers in the summer. Beetles also damage plants as both larvae and adults chew on plant tissue. The Colorado potato beetle is the most noto­rious of these pests. Juvenile (nymphs) and adult grasshoppers are also fo­liage-eating insect pests. Larvae of flies feed and burrow into roots, bulbs, and stems of plants and thus cause considerable damage.

The least conspicuous insect pests are those that pierce the stem or leaf and suck nutrients from the plant. Nymphs and adults of aphids, leaf hop­pers, stink bugs, and plant bugs cause extensive damage in this manner and, as well, they carry plant pathogens, especially viruses, from plant to plant. Insects called thrips also pierce plant parts and are important in transmit­ting viruses.

Mites differ from insects, as the adults have four pairs of legs (versus six for insects) and lack an antennae. Larvae of mites feed and molt to form six­legged nymphs before becoming adults. The mites that feed on plants have rasping and sucking mouth parts that damage plants and they also transmit plant pathogens as they feed. Both thrips and mites are very small and, as a result, often avoid detection until the plant growth is visibly affected.

Damage to plants caused by slugs and snails is very obvious, but is gen­erally limited to crops growing in very damp situations and those, such as strawberries, in contact with the soil. Slugs and snails glide on an obvious slime trail of secreted mucus and feed at night, or on very cloudy days, to avoid drying out. Also at home in damp environments are the sowbugs and pillbugs. These oval (pill-sized) bugs have a small head, two pairs of anten­nae, and seven pairs of legs. These species are more important as decom­posers of rotting vegetation than as plant pests.

Control

Crop management to reduce damage by diseases and pests is based on in­tegrated control strategies involving exclusion, eradication, and protec­tion. Whenever possible, growers attempt to exclude the pathogen or pests from their land by purchasing pathogen- and pest-free planting material (seeds, seedlings, grafting material, tubers, and bulbs). When a pathogen or pest is present in fields or orchards, every effort is made to eradicate it by cultivation practices designed to “starve” the organism, for example, by planting a crop on which it cannot obtain nutrients. When such methods fail, pesticides may be required to reduce pathogen populations; for ex­ample, nematocides to kill root-knot nematodes. Many pests and pathogens (for example, apple scab and wheat stem rust fungi, fire blight bacterium) are, however, so widespread and so readily distributed from field to field that exclusion and eradication are impossible. Ideally, for these problems, plant varieties that are genetically resistant to the pathogen or pest are available.

Alternatively, growers may be able to reduce crop losses by cultural prac­tices that make the environment unfavorable for the agent; for example, spacing plants to prevent the high humidity conducive to plant disease. If such methods are unsuccessful, the grower may be required to use biolog­ical control (for example, the bacterium Bacillus thuringiensis for moth and beetle control) or chemical pesticides (fungicides to control late blight of potato, or insecticides to control grasshoppers). Bioengineering techniques are enhancing researchers’ ability to produce genetically resistant crop plants, and this technology will eventually decrease reliance on chemical pes­ticides.

References

Borror, Donald Joyce, and Richard E. White. Peterson Field Guide to Insects. Boston: Houghton Mifflin, 1998.

Insect Biology and Ecology: A Primer. http://www.nysaes.cornell.edu/ent/biocontrol/ info/primer.html.

Plant Disease Lessons. http://www.apsnet.org/education/phi_index.html.

Schumann, Gail L. Plant Diseases: Their Biology and Social Impact. St. Paul, MN: The American Phytopathological Society, 1991.