This summer we are discussing the many tools available to control insect pests and diseases in a series on the essentials of integrated pest management (IPM). So far, we have discussed monitoring pest populations, action thresholds, prevention, and cultural controls. Another aspect of an IPM program involves taking advantage of adaptations that many plants have to avoid or tolerate insect or disease pests. Host plant resistance (HPR) makes use of the fact that all wild plants have many adaptations to protect themselves from insect herbivores and diseases. These adaptations can be used to protect crops from herbivores and diseases in agricultural systems. As cultivars are developed, plant breeders often focus on a narrow range of desirable traits, such as flavor, color, and storage potential. Unfortunately, many cultivars with exemplary flavor and other marketable traits end up being especially susceptible to disease and insect pressure. In order to implement HPR, plant breeders find cultivars or wild relatives of crop plants with certain resistant characteristics, then cross those genes with other cultivars to try to develop a plant with resistant traits along with the other desirable traits.
There are several ways a plant can show HPR, each with advantages and disadvantages:
Antibiosis occurs when feeding on a resistant plant has a negative effect on the pest’s health or fitness. In general, this is caused by chemicals in the plant tissue which can either directly kill, slow the development of, or reduce the reproductive capacity of a pest.
Antixenosis occurs when a pest is less likely to find or feed on a resistant plant. This can be in the form of physical characteristics (such as dense hairs or a waxy surface) or chemical characteristics that deter feeding or disease infection.
Tolerance occurs when a plant is able to continue to thrive despite being attacked. This does not decrease the likelihood of a pest to attack a resistant plant, but rather indicates an ability of the tolerant plant to continue to thrive despite being attacked.
Some HPR breeding programs have been extremely successful. For example, a cooperative apple breeding program from Purdue, Rutgers, and the University of Illinois have developed a number of apple cultivars which are resistant to apple scab, rust, fire blight and/or powdery mildew. In other cases, instead of specifically breeding for resistance, growers can simply make use of naturally resistant cultivars. A classic example of insect HPR comes from the European grape industry, where the introduction of resistant American rootstocks saved the industry from grape phylloxera root damage. In a more passive form of HPR, summer-bearing raspberries show some HPR to spotted wing drosophila in Wisconsin simply due to a temporal disjunction (called phenological resistance) between when spotted wing traditionally is present and when the fruit is ripe.
Putting it into Practice
Host plant resistance can be a very economical control method, since resistant cultivars often cost the same or only slightly more than susceptible cultivars. It is also generally easy to combine with other control methods, such as biological and chemical controls, and can avoid some of the environmental and health worries that can be associated with pesticide use.
Host plant resistance in perennial crops can be hard to keep up with, since the decision to implement this tactic takes place before planting and can’t be changed unless you are willing to tear out and replant a crop block. Of course, that’s easier to do with something like strawberries or raspberries than with apples or cherries. For that reason, using HPR in your IPM program requires a good deal of research into the susceptibilities of various cultivars to the insects and diseases most prevalent in your area PRIOR to planting a new block.
Finally, as with other control tactics we’ve described in this series, HPR is meant to be a single tool in the IPM toolbox and should never be relied on for complete insect and disease management. It will always be necessary to continue to scout and monitor (as discussed in previous issues), and may be necessary to apply cultural, biological or chemical controls as needed, even when making use of host plant resistance.This article was posted in Insects and tagged Host plant resistance, HPR, IPM.