Teacher Guide - Background Information

What's Wrong with the Plants?

Teacher Background Information

(*TOM=Teacher Overhead Masters)

The study of plant disease is an important branch of Biology. The science of Plant Pathology examines the cause and development of disease in plants and, with this knowledge, develops methods to diagnosis, treat, and prevent plant disease. The practical application of the science is to improve the production of food and plant products and protect the health of the natural environment for people in all parts of the world.

Plant disease occurs when certain factors interfere with the normal physiological functions of a plant (TOM-1). Essential plant functions include photosynthesis, absorption of water and minerals, translocation of organic substances, cell division, differentiation, and development, storage, and reproduction. Factors that may interfere with these functions are environmental conditions such as light, temperature, pH, or moisture, and pathogenic organisms such as bacteria, viruses, viroids, fungi, mycoplasmas, protozoa, and nematodes (TOM-2).

Whenever environmental factors or pathogenic organisms interfere with the normal functions of the plant beyond tolerance, the plant reacts. Reaction to the disturbance initially may be evident only at the cellular level but eventually the disease will spread through plant tissues preventing the normal physiological processes to take place at the site of interference. If there is no control or treatment of the of the disease, the plant cannot perform the necessary functions to survive and it ultimately dies.

In this lesson, students will examine some principles of plant pathology by designing experiments that attempt to identify the cause of disease symptoms in a given plant sample. The diseased plant samples will be infected with root-knot nematodes. Although the intention of the lab is for students to observe the nematodes and their eggs in the infected plant and deduce that these may be the cause of the disease, it is possible that they may suggest other hypotheses and these alternatives should be examined if students can support their proposals and offer an effective experimental design. During the initial observations of the "healthy" and "unhealthy" plants, it should be stressed that both plants were treated exactly the same (i.e. water, nutrients, light, temperature, etc.) This explanation is necessary to rule out environmental factors that may be the cause of the disease.

Nematodes are among the top five plant pathogens worldwide. They can be found in any habitat capable of supporting plants and are responsible for an estimated 5% of worldwide crop loss annually. The average may not seem significant, but crop losses are not evenly distributed around the world. In underdeveloped countries where farmers are totally dependent on their crops for survival and cannot afford modern methods of control, nematodes may destroy 25%-50% of their food crops.

Nematodes are roundworms that are sometimes referred to as eelworms. Most species are free-living in fresh or salt water or in the soil, but there are many parasitic nematodes that can infect plants, animals, and humans. Plant-parasitic nematodes cause a variety of plant diseases. They can affect the roots, (TOM-3) bulbs, seeds, and stems of plants. They are small, 300-1000 m m (some 4 mm), by 15-35 m m wide, are only visible microscopically, and appear more or less transparent.

This lab examines the effects of the root-knot nematode Meloidogyne spp. on plants (TOM-4). The genus name Meloidogyne, from the Greek "honey" and "female", refers to the shape of the female. The name was given by Goeldi in 1887 to describe nematodes that were causing galls on coffee plants grown in Rio de Janeiro, Brazil. The root-knot nematode invades the roots of plants and is the commonest root-damaging type of nematode in the west (TOM-5). Many species prefer temperate climates with short or mild winters but are not limited to these environments. They can also be found in non-sterilized soil in green-houses. They are major pests to cotton, citrus, grapes, tomatoes, eggplant, squash, cucumbers, ornamentals, and many other plants.

Root-knot nematodes damage the roots of plants either by stopping the growth of the root or stimulating the abnormal growth of root tissue. The roots also develop characteristic swellings called galls. As a result, normal absorption and translocation of water and nutrients in the plant is diminished. Galls can also stunt or disfigure root-type crops decreasing their market value or destroying them entirely. Young seedling plants that are infected have less chance of survival than older plants. Nematode infection may not destroy a mature plant but above ground symptoms include stunted growth, reduced number and yellowing of leaves, abnormal wilting, and reduction or lack of flower and fruit production (TOM-6). The roots develop the characteristic root-knot galls, localized abnormal thickening, and occasionally the roots may rot. In many cases, the primary nematode infection causes decreased resistance, and as a result the plant will be attacked by secondary opportunistic pathogens such as bacteria, fungi, and/or insects. The overall effect of nematode infection is decreased plant growth and production and in severe cases, plant death.

The infection of a root begins when young first-stage larvae in the nematode egg molt to become a worm-like second-stage larvae which emerges from the egg (TOM-7). This is the only stage that can infect a root. After it enters the root it grows and begins to appear sausage-like. It inserts its feeding apparatus, the ‘stylet´ into the surrounding cells to feed. At the same time saliva is secreted which stimulates the enlarged growth of plant cells called ‘giant cells’ around the head of the larvae. The saliva also liquefies the giant cells’ contents and the worm sucks the material through the stylet. This creates a type of nutrient ‘sink’ for the worm at the expense of the plant. The sex of worm can be identified at the fourth larval stage (TOM-8: F&G). The male becomes long and slender and emerges from the root into the soil as a free-living adult. The female continues to grow and develops an enlarged pear-shaped body but remains in the root for the rest of her life.

Many of the Meloidogyne species reproduce by parthenogenesis. The female can produce eggs with or without fertilization from the male. The eggs are laid in a gelatinous sac which can be found on the inside or outside of the root. As many as 500 eggs can be laid in one sac. Females may produce eggs for 2-3 months, but can live in the root for some time after they cease egg-laying. From that point the whole cycle begins again and larvae can infect the same root or other plant roots. One root can contain many females.

The duration of the life cycle for the root-knot nematode is dependent on temperature and host plant suitability. The average is about 25 days at 27° C, but it can take longer at lower temperatures depending on the species.

Parthenogenic species of Meloidogyne produce males in response to environmental factors and availability of food. If food is readily available most larvae will develop into females, but if the roots are heavily infested or unhealthy, many of the larvae will become males because the plant cannot support the nutritional needs of many females.

The root galls develop from the affected tissues around the giant cells. Old galls can contain many egg-laying females. Commonly, many galls may coalesce forming thickened areas on the roots.

The nematodes have very limited mobility and can only move short distances in the soil. They are spread passively by anything that will move soil. Irrigation, farm equipment, and/or natural flooding are methods of dispersion.

There are several methods of controlling root-knot infection. The most successful of these is rotating root-knot susceptible crops with immune or resistant varieties of plants. This allows the nematodes to die out in the alternating years. Weed control is also very important. Meloidogyne can reproduce on weeds and if present the new crop can become infected.

Another possible control method is application of organic material to the soil before planting. The breakdown of organic matter produces compounds such as acetic or proprionic acids which may be toxic to the nematodes. These are referred to as natural nematicides. The organic material also encourages the growth of microorganisms such as species of bacteria and/or fungi that act as parasites, predators, or toxin-producers which can kill nematodes and/or eggs.

Chemicals that are used to control nematodes are called nematicides. Most nematicides are used as soil treatments before the crop is planted. They have been used very successfully to control infection, but as with many other pesticides, they are strictly regulated and have restricted use because of their potential environmental impact. Unfortunately, farmers that have the greatest need for nematode control live in underdeveloped countries where they do not have the economic means to acquire the nematicides.

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The University of Arizona
Department of Biochemistry and Molecular Biophysics
General Biology Program for Secondary Teachers
warder@email.arizona.edu