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What's Wrong with the Plants?
Test/Worksheet Key
1. A. Since the whole field was treated uniformly you could rule out nutrient deprivation or drought symptoms. There is no indication that there is a viral or fungal infection on the leaves. The affected plants are localized on the sandy soil which is the preferred type of soil for nematodes. The plants also seem to follow furrows which carry water and nematodes simultaneously. It is possible that students may consider alternative hypotheses. For example the clay soil may be absorbing the water better than the sand and the plants in the soil are wilting. The important factor is that they can support their hypothesis.
B. Talk to the grower about the history of the field (did he have nematodes previously?) method and mode of cultivation, (tractors can move nematodes), make an external examination of the plant; look for galls on the roots; take soil samples; etc.
C.The female root-knot nematodes penetrate the root cells with their stylets. As the plant responds to the invasion, giant cells in the root develop which act as nutrient sinks from the plant to the nematode. As more galls form in response to the infestation, root tissues are altered so they cannot absorb water adequately (wilting) and this with the other effect diverts energy for growth, fruit production and quality.
2. Chemicals can be harmful and toxic to animals, humans, and can pollute water systems (lakes, ponds, ground water, etc.). Pathogens resistant to the chemicals may also develop but, for the most part, economical chemicals can be dispersed over large areas and are very effective. They can increase yields and profit. Biological control methods generally are non-toxic, less stressful to the environment, and more available to farmers that depend on their farms for sustenance. But, they are not as effective, especially for large farms. They may take more manpower to treat crops, are not always predictable, and yields are usually lower. Products are usually smaller and may not be as aesthetically pleasing as chemically treated plants. Therefore, less profit may result.
3.Possibly let it stay fallow for a year; rotate crops to less susceptible plants; lay down plastic to solarize soil which can kill some parasitic nematodes by increasing soil temperature; plant at a cooler time of the year; plant nematode-resistant cultivars.
4.Farming comprises 50% of the world’s occupations. All people are dependent on plants for sustenance. Anywhere from 20% to as much as 50% of crops around the world are destroyed by pests or disease. Years of widespread pesticide use has produced pathogens which have become resistant to chemical control so alternate means have to be developed, etc.
Extra Credit:
IRISH POTATO FAMINE 1845--caused great suffering and economic impact
BENGAL FAMINE 1943--brown spot epidemic of rice
DUTCH ELM DISEASE
Answers will vary
Department of Biochemistry and Molecular Biophysics
General Biology Program for Secondary Teachers
warder@email.arizona.edu
http://biology.arizona.edu/sciconn/lessons2/lessons.html
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