8.5: Plant Defenses, Management Practices and Pests

After discussing the key ecological principles and approaches to soil management, let’s do a deeper dive and see how amazing plants really are.

They use a variety of systems to defend themselves from attack by insects and disease-causing pathogens. Sometimes they can just outgrow a small pest problem by putting out new root or shoot growth. Many plants also produce chemicals that slow down insect feeding. While not killing the insect, it at least limits the damage. Beneficial organisms that attack and kill insect pests need a variety of sources of nutrition, usually obtained from flowering plants in and around the field. However, when fed upon, by caterpillars for example, many plants produce a sticky, sweet substance from the wounds, called “extra-floral nectar,” which provides some attraction and food for beneficial organisms. Plants under attack by insects also produce airborne (volatile) chemicals that signal to beneficial insects that the specific host it desires is on the plant. The beneficial insect, frequently a small wasp, then hones in on the chemical signal, finds the caterpillar and lays its eggs inside it (Figure 8.3). As the eggs develop, they kill the caterpillar. As one indication of how sophisticated this system is, the wasp that lays its eggs in the tomato hornworm caterpillar injects a virus along with the eggs that deactivates the caterpillar’s immune system. Without the virus, the eggs would not be able to develop and the caterpillar would not die. There is also evidence that plants near those with feeding damage sense the chemicals released by the wounded leaves and start making chemicals to defend themselves even before they are attacked.

Plant Pests

A variety of organisms can harm crops, from pathogens such as viruses, bacteria and fungi to nematodes to insects to weeds. Even larger animals such as deer (or, in Africa, elephants) can significantly damage crops. There is nothing inherently bad about these organisms we commonly call pests. They’re just doing what they naturally do in order to survive and reproduce. But when growing crops we need to minimize the damage done by such organisms. The key is doing so in environmentally sound ways: building healthy soils, using crop varieties with natural resistance, and using rotations and cover crops that suppress pests while providing many other benefits as well.

Figure 8.3. Plants use a number of defense strategies following damage by feeding insects. Modified from unpublished slide of W.J. Lewis. Illustration by Vic Kulihin.

Leaves are not the only part of the plant that can send signals that recruit beneficial organisms when under assault. When attacked by the western corn rootworm, a major pest, the roots of some varieties of corn have been shown to release a chemical that attracts a nematode that infects and kills rootworm larvae. During the process of breeding corn in the United States, this ability to signal the beneficial nematode has apparently been lost. However, it is present in wild relatives and in European corn varieties and is, therefore, available for reintroduction into U.S. corn varieties.

Plants also have defense systems to help protect them from a broad range of viral, fungal and bacterial pathogens. Plants frequently contain substances that inhibit a disease from occurring whether or not the plant is exposed to the disease organism. In addition, antimicrobial substances are produced when genes within the plant are activated by various compounds or organisms—or a pest—in the zone immediately around the root (the rhizosphere) or by a signal from an infection site on a leaf. This phenomenon is called “induced resistance.” This causes the plant to form various hormones and proteins that enhance the plant’s defense system. The resistance is called systemic because the entire plant becomes resistant to a pathogen, even far away from the infection site.

Plants have a number of defense systems that protect them from disease. Beneficial bacteria in soil surrounding the root (the rhizosphere zone) provide a first line of defense against soil-borne diseases by competition or antagonism. If the disease organism (let’s say a fungus like Rhizoctonia solani that causes root diseases in seedlings of diverse crops such as wheat, rice, potatoes, tomatoes and sugarbeets) makes it through the rhizosphere and contacts the root surface, beneficial organisms living inside roots provide another line of defense by producing chemicals that attach the fungus.

Then the plant itself also can produce chemicals that help it resist the attack. There are two major types of induced resistance that are induced in response to signals from microorganisms: systemic acquired resistance (SAR) and induced systemic resistance (ISR) (Figure 8.4). SAR is induced when plants are exposed to a pathogen or even to some organisms that do not produce disease. Once the plant is exposed to the organism, it will produce the hormone salicylic acid and defense proteins that protect the plant from a wide range of pathogens. ISR is induced when plant roots are exposed to specific plant growth, promoting rhizobacteria (PGPR) in the soil. Once the plants are exposed to these, hormones (jasmonate and ethylene) are produced that protect the plants from various pests. Some organic amendments have been shown to induce resistance in plants, and farmers who have very biologically active soils high in organic matter may already be taking advantage of it, as well as of other ways pests are controlled in such soils. However, there currently are no reliable and cost-effective indicators to determine whether a soil amendment or soil is enhancing a plant’s defense mechanisms. More research needs to be conducted before induced resistance becomes a dependable form of pest management on farms. Although the mechanism works very differently from the way the human immune system works, the effects are similar: the system, once it’s stimulated, offers protection from attack by a variety of pathogens and insects.

Figure 8.4. Types of induced resistance to plant diseases. Modified from Vallad and Goodman (2004) by Amanda Gervais. Illustration by Vic Kulihin.

Plant Defense Mechanisms

Plants are not passive in the face of attack by insects, nematodes or diseases caused by fungi and bacteria. Genes activated when plants are attacked or stimulated by organisms produce chemicals that

• slow insect feeding
• attract beneficial organisms
• produce structures that protect uninfected sites from nearby pathogens
• produce chemicals that provide a degree of resistance to pathogenic bacteria, fungi and viruses
• host organisms in roots that protect against pathogens

When plants are healthy and thriving, they are better able to defend themselves from attack and may also be less attractive to pests. When under one or more stresses, such as drought, nutrient limitations or soil compaction, plants may “unwittingly” send out signals to pests saying, in effect, “Come get me, I’m weak.” Vigorous plants, taller and with more extensive root systems, are also better competitors with weeds as they are able to shade them out or just compete well for water and nutrients.

Many soil management practices discussed in this chapter and in the other chapters in Part 3 help to reduce the severity of crop pests. Healthy plants growing in soils with good biological diversity can mount a strong defense against many pests (see text box). Ecological soil and plant management is so critical to plant health because it helps to suppress pest populations and also influences, as we have just seen, the ability of plants to resist pests. Developing optimal soil health is, therefore, the basis for management of crop pests on farms; it should be a central goal that underpins crop integrated pest management (IPM) programs.