Vegetable IPM: Module 2

Module 2: “An Ounce of Prevention is worth a Pound of Cure”

Module Overview

Did you notice the title of this module? Most of you are probably familiar with this old proverb and what it means. For those of you more comfortable with metric measurements we should probably restate it as – “A gram of prevention is worth a kilogram of cure”. Whatever measurement system used the meaning is the same. Quite simply, it tells us that small inexpensive actions taken early can often mean not having to do major expensive interventions at a later date. More and more, farmers and agricultural professionals are learning that, if done properly, preventing pest problems in the first place can result in significant economic benefits and also minimize the health and environmental risks associated with chemical interventions that are often required to “cure” the problem later on.

For most crops, farmers and agricultural professionals have long been aware of the value of thorough, deep and timely soil cultivation in preventing pest and disease problems. Cultivation is still the most widely used natural way to kill weeds and an effective technique to control soil pests such as cutworms either by killing them or burying them to a depth where they cannot reach the surface. But there are many more preventative strategies that are also effective and which may not be as widely known or practiced. Some of these are general and others are specific to a crop or family of crops. On the following pages we present some of the most common and important pest and disease prevention practices associated with vegetable production. These include:

  • Growing varieties that are suitable to local conditions
  • Growing varieties resistant to common pests and diseases
  • Selecting seed material carefully
  • Planting disease free seed
  • Ensuring that plants are given optimal nutrition and water and are kept free of weeds
  • Avoiding soil-borne problems (Practicing crop rotations)
  • Destroying sources of infection

Lesson 2.1: Grow Varieties that are Suitable to Local Conditions

For almost any commercially grown vegetable, a farmer can choose from a range of varieties (also sometimes known as a cultivars). Although these varieties may appear similar, many have been selected to do best in particular growing environments. Temperature, daylength, rainfall, wind, pests and diseases and soil type are all things that some plants handle in different ways. If these conditions change or if a plant is grown in different conditions from those to which it is adapted, it will probably not do as well.

Farmers should always choose varieties carefully to ensure that they are well-matched to their farm environment. Such information can be found from seed companies, retailers, extension services, neighbours and other sources, but sometimes farmers may need to make comparisons of different varieties for themselves because no two farms are exactly the same.

A lot of current breeding and research work to come up with better varieties is now focused on genetic modification. This is a promising technology that has the potential to speed up the process and even provide types of resistance which could not be developed by a conventional breeding programme. It often involves implanting a gene from a different organism which produces chemicals toxic to pest or disease pathogens. While this approach to varietal development may offer some exciting advantages it is not universally accepted as a good thing.

Lesson 2.2: Grow Varieties Resistant to Common Pests and Diseases

If a plant has resistance to pests and diseases, the need for cultural, biological and chemical plant protection is reduced or even eliminated. This plant resistance might be developed by plant breeding – individual plants which show some resistance are selected for breeding and each succeeding generation, the resistant ones are selected again.

Plant breeders have developed varieties of some vegetables which are resistant to some important pests and diseases. Cabbages have been bred with inherent resistance to black rot and yellows disease. Tomato varieties designated V, F, and N are resistant to Verticillium wilt, Fusarium wilt and Nematodes respectively. There are examples in other vegetable crops too. Even when resistance is not specifically featured there can be some varieties which are less affected by specific diseases. Comparisons should be made on the farm to find out which types grow best. Experimentation by farmers is an essential component of IPM and we will be talking about this much more towards the end of the course.

Lesson 2.3: Select Seed Material Carefully

The previous section on suitable varieties refers primarily to seed obtained from others, whether from a commercial dealer or from friends and neighbors. But many farmers cannot afford to buy seed or may just prefer to save seeds from preceding crops. There is definitely nothing wrong with this practice and many farmer-selected varieties do much better than commercial F1 hybrid seeds. This is because carefully roguing out unsuitable individuals and only allowing the better ones to reproduce and set seed means that the seed will be quite suitable to the areas in which the parents were grown. On farm selections can be made over each successive crop and automatically produce varieties or selections suited to the local conditions.

There are, however, some things a farmer should be aware of when producing their own seed. One is that farmer selected lines may result in quite variable plants (days to maturity, size or shape of produce, taste, etc.). This may have implications for marketing. It must also be realized that plants selected in one area may not do well in distant areas (think back to the lesson on suitability). A third common problem with farmer-selected seeds is the potential for disease problems. The following true story will give some ideas about how this can happen.

Example of disease problems caused by improper seed selection practices:

A group of farmers were involved in growing Irish potatoes. Each year they sold or ate the large tubers and saved the small tubers for planting next season as seed material. Eventually all the harvested potatoes were small. They were puzzled and wanted to have good crops again, and large tubers. Unknowingly by saving the smallest potatoes they were saving diseased potatoes. The whole crop next year was raised from diseased tubers. Their way of selecting planting material helped the disease to continue into the following crop. When researchers looked at the crop and identified the presence of disease, they informed the farmers that they should start with clean healthy disease-free planting material the next season. Fortunately crop yield was restored. After that they saved healthy tubers but they did not like to hold on to large tubers for seed next year, because they could be sold. And a large tuber is not really good as a seed potato – you need many kilos to re-plant the crop next year. So special small potatoes suitable for seed were produced specifically for seed, by growing the plants close together. In this way the small tubers were disease-free.

Lesson 2.4: Plant Disease Free Seed

A very common problem with vegetable seeds is that they carry diseases. If seeds contain disease pathogens before they are planted, plants are unhealthy from the start. These diseases can be fungal, bacterial or viral. For example bacterial leaf spot of peppers, bacterial canker of tomatoes and black rot on brassica crops. Farmers should always try to ensure that they are using healthy planting material, either by buying certified disease-free seed or harvesting seed only from healthy plants if they are collecting and saving their own seed selection. Some seeds can be given treatment to rid them of pests and diseases. The treatment can be a chemical treatment or hot water can be used.

Lesson 2.5: Ensure that Plants are Given Optimal Nutrition and Water and Competing Weeds Controlled

An often quoted first rule of IPM is “Grow a Healthy Crop” and proper nutrition and water are the foundations of crop health and therefore its pest and disease status. If there are shortages of particular nutrients, or the soil pH (acidity/alkalinity) is wrong, the crop may not be able to grow well. Also, if weeds are not controlled they tend to outcompete vegetable crops for these valuable resources. Farmers should enquire whether there is a government or private scheme for testing soils for acidity and nutrients and then take steps to correct deficiencies and problems. An excess of nutrients can also have unwanted effects including stimulating weed growth. For example, if too much nitrogenous fertiliser is used on brassicas, they can become very soft and sappy and susceptible to sucking pests. Water can also have a major influence. Heavy rainfall or overhead irrigation can have positive effects in washing off pests such as caterpillars and aphids, but can have negative effects in encouraging fungal and some bacterial diseases.

Below are some excellent resources for learning more about soil, water and weed management in vegetable production.

Lesson 2.6: Avoid Soil-borne Problems (Practice Crop Rotation)

Many of the pests and diseases that plague vegetable plants live in the soil. A common source of soil borne problems is the seedbed. If a farmer has a problem with his or her seedbed it may mean that plants are infected/infested very early and never have a chance to thrive. It is possible to drench seedbeds with pesticide to prevent this.

Another and more common source of soil-borne pest and disease infection is the main production field itself. If a farmer grows crops in the same category year after year on the same land an almost certain result is a build up of pest and disease problems.  A much better practice is to rotate different crops or crop types. And rotation has other positive results as well. Although the main basis for rotation is avoidance of an accumulation of pests and diseases in the soil, it also helps with effective cycling of nutrients. Some crops remove different plant foods, and beans or other legumes add nitrogen.

The table below, taken from a University of Vermont Extension website dealing with Insect & Disease Control In the Home Vegetable Garden, indicates which crops are in the groups that suffer from the same pest and disease problems and which should therefore not be planted in rotation.

Group 1

Group 2

Group 3

Group 4

Group 5

Group 6

Group 7

Summer squash
Winter squash

Brussels sprouts


Spinach beet




Rotations to manage weeds

Rotations are also used to control weeds. Crops such as Irish potatoes and other root crops may be generally more weedy than cereal crops because there are good selective herbicides for cereal crops, so rotating with a crop which allows easy weed control gives farmers an opportunity to reduce the build up of weeds. Interestingly, before herbicides were introduced the cereals were generally troubled by weeds more than the root crops because mechanical weeding was easier.

Rotations to manage nematodes

A special example of rotation is used to manage nematodes, which can be a problem in light sandy soils. Some crops are less suitable hosts for the nematodes. Nematode numbers actually reduce through the season if such crops are planted. This allows the susceptible crop to be grown again in the soil after one or two years.

Lesson 2.7: Destroy Sources of Infection

On a vegetable crop, pests and diseases may survive on volunteer plants (crop plants which come up on their own after the main crop has been harvested). These volunteer plants should be destroyed. The same applies to some weeds which are also hosts of pests. If an old diseased or pest infested crop remains nearby when a new one is planted, it is likely that the pests or diseases will move over to the new crop. For example, if spider mites are present on a crop they will surely move to the new one. If the old infested crop is upwind the pests will almost certainly be blown onto the new crop. If possible, get rid of the old crop first. If the sticks or canes used to support a crop such as tomato are used for a second time, disease pathogens and pests may be present on them, so they should be washed thoroughly with hot water, dilute bleach or strong soap before being re-used.

Lesson 2.8: “Farmers’ Friends”

Well, we certainly talked a lot about prevention in the previous lessons but missed one of the most important approaches. No discussion of preventative strategies would be complete without discussing the role and importance of the various beneficial organisms that can help the farmer to keep pests (and some diseases) under control and prevent them from causing economic damage.

These organisms certainly deserve their title of “farmers’ friends”. Although large outbreaks of plant-eating pests do sometimes occur in natural systems, any one particular species is less likely to build up a large population if the organisms which feed on it are also present – in other words, its natural enemies or farmers’ friends. Predators are one type of natural enemy which tend to keep the population of their prey in check. They catch and eat other insects and mites, including pest species. Parasitoids are another type of natural enemy. They lay eggs in or on other species of insect (called hosts) and the larval stage kills the host as it feeds on it and develops. The third major group of friends is pathogens. These are fatal or debilitating diseases and include fungi, nematodes, bacteria, viruses, and other microbes. Fungi, particularly Deuteromycetes, can infect pests externally under favourable conditions, but other pathogens must be ingested to be effective as control agents. Pathogens are very specific to their hosts. Pathogens are often referred to as biopesticides because they can be applied in similar ways to chemical interventions. We will be revisiting these non-insect friends later on.

Insect farmers’ friends are usually present in any crop unless broad spectrum pesticides (which kill a wide range of arthropods) have been used too much. They can be protected or conserved by taking care with farming practices so that they are not killed or (better still) are actually encouraged. Farmers who collect ladybird beetles in field margins and release them on their crop are practicing augmentation (of beneficial natural enemies of pests). Alternatively, if suitable types of beneficial organism are not present in the crop, they can be introduced. Where introduction involves a local beneficial organism which has simply not yet reached a particular crop, this is known as inoculation. If the introduced beneficial organism is from outside the area (typically from the country or area where the troublesome pest originated) and then becomes established as the controlling factor for the pest in the new area, it is known as classical biological control. The pest stops having the crop to themselves, and a new balance is created so that the pest becomes less important.

An example of how farmers can help to keep the balance in their favour is to try not to harm predatory insects such as ladybirds, spiders and hover fly larvae which feed on plant-eating pests such as aphids and caterpillars. Some of the ways to do this are:

  • use control products only when necessary and then not broad spectrum ones (note that most botanical pesticides are broad spectrum)
  • if control products are used – using them selectively
  • growing flowering plants which provide nectar and pollen to farmers’ friends such as adult parasitoid wasps, hover flies and ladybird adults
  • having living fences (hedges) around the crop to provide shelter and refuge for farmers’ friends. These are sometimes called refugia, and examples include beetle banks (grassy areas near crops) and unsprayed field edges
  • mulching to provided refuges for ground-living farmers friends such as predatory beetles (more on this in the next lesson)

For more information on farmers’ friends interested participants should click the links below.

For more information on using pesticides selectively.

A booklet can be downloaded at the following site, describing, and identifying natural enemies of pests

Lesson 2.9: Importance of Identification

A key factor in protecting and conserving farmers’ friends is being able to tell friends from enemies. Various studies have shown that many farmers (and also many agricultural professionals?) are not good at insect identification and know little about insect biology and behaviour. In vegetable pest management, if farmers cannot distinguish between pests and friends, they will be likely to control anything, whether helpful or a genuine pest. They do not want to risk having living things crawling over their crop and so resort to spraying.

Although proper identification of pests and beneficials is important it is not always easy. Take for instance ladybird beetles which are one of the most talked about and valuable farmers’ friends because of their ability to eat large numbers of plant sucking aphids. But some ladybird species do eat plants and can cause crop damage! These species usually look dusty or slightly hairy – as compared with the very shiny species which are usually farmers friends. To make matters more difficult, most insects look very different at different stages in their development. For example, ladybird larvae look like tiny spotted crocodiles and are very different from the adults, but they also eat many aphids. This highlights the importance of being able to identify the living things on the crop no matter what life cycle stage is observed. Interested participants should check out the following article for additional information on this topic.

So how to make a correct identification? One good source of information is the local extension office which should have field guides and pest and disease identification guides on local problems. Sometimes such guides are also available at universities or colleges and more and more such information is available through the Internet. If you know of any good ones be sure and share these with the rest of us. Here are some we have found.

Lesson 2.10: Composting and Mulching

Compost is plant material which has been piled up and left to rot. This process releases nutrients and kills most pests and pathogens which may be present (for example it is know that bacterial canker (Cornebacterium michiganense) survives up to 2 years in compost). Once completely broken down it can be added to the soil to improve soil structure and fertility. When compost, fresh crop residues or plastic sheeting is used to cover the surface of the soil the practice is referred to as mulching. Mulching reduces weeds, water loss, splashing and can also help to break some pest life cycles. These practices are some important additional ways to help prevent pest and disease problems. As mentioned in the last lesson, mulching provides a home for ground-living farmers’ friends such as predatory beetles.

These techniques are valuable contributors to maintaining crop health through provision of nutrients, conservation of water and control of weeds. Agricultural research shows clearly the value of these practices for vegetable production. In this lesson we would like you to visit the following selected Websites to learn more about these techniques. You will need to have a good understanding of the information they contain to do well on the next assignment coming right after this lesson.



Soil health:

Lesson 2.11: Isolation

The last technique we will focus on in this module on prevention is that of isolation. Although not strictly a preventative strategy, it does have some things in common with the approaches presented earlier. By isolating a crop, even though pests may still be present and their numbers high enough to pose a risk, these pests can be kept from doing any serious damage.

There are many ways to help a crop avoid pests. Some of the most common include:

  • Physical barriers
  • Growing the crop far away from other infected crops.
  • Intercropping
  • Adjusting time of planting

Physical barriers

Probably the most straightforward isolation technique is the use of physical barriers. In using physical barriers, like all forms of avoidance, the objective is to keep pests and diseases from reaching the crop. Covering vegetables with a fine mesh will stop them being attacked by flying pests. This works well for carrot root fly and pea moth. Fine mesh is also an all inclusive way of protecting brassica and other crops from flea beetles, leaf weevils, birds, moths of pest caterpillars and white fly. Please note, however, that it only works if it is put on before any of these pests have entered. Netting can also be very useful at preventing bird damage to fruit and vegetables. However, mesh may be too expensive for very small-scale farm use.

Spatial separation

Planting in an area physically distant from the pressures of pests and diseases is one good way to avoid pest and disease problems but is not always practical. In a newly cropped area, insects can only invade if they can travel successfully from an infested area. Flying pests can migrate most easily – examples are moths of cutworms, bollworms and other pest caterpillars, winged aphids, whiteflies and thrips. Even pests without wings such as spider mites can travel through the air by spinning a long thread of silk which can catch the wind and lift them to new areas. Pests can also be spread by human activity. For example, spider mites can be introduced to clean crops on people’s clothes if they come into it after working in an infested field.

If diseases are not already in the soil or in the planting material, which they often are, they can reach a crop in several natural ways: either on the wind (for example spores of fungal diseases), in water (for example bacterial pathogens in water splashes) or by insect vectors of disease (for example virus diseases spread by aphids, whitefly and thrips). Diseases can also be spread from other diseased plants by man’s activities – either on clothing or on cultivation tools such as hoes and pruning knives. Further information on diseases and the interactions between diseases, plants and humans can be found at the following Website.


Even in areas where pests are present close by, there are some methods which can help prevent them reaching or finding the crop. Intercropping can help to confuse some pests – for example planting a tall crop like maize next to a short crop like cabbage can help to prevent aphids finding the cabbages.

If the inter-cropped plants are strong smelling like onions and garlic, the odours can also discourage pests from lingering in the area and so help to protect the cabbage. Perhaps surprisingly, aphids can be confused by covering the soil with mulches, and are much less able to find the crop.

Time of sowing

Another method of avoiding problems is to plant the crop at a time when pest and disease pressure are low. For example, tomatoes and potatoes are particularly susceptible to blight during the rainy season. That changing the season in which a certain vegetable is grown can lower pest risks may be as a result of seasonal factors or because neighbours are not growing that crop at that time so are not harbouring pests and diseases which can migrate in.