Good Agricultural Practice

How to Catch Problems Before They Turn Problematic

By Danielle Fisher, Entomologist, CitChem

Agriculture has existed for the last 10,000 years, and we have been learning how to grow the best crops we can ever since. There are many factors that contribute to a productive crop, meaning there are many things that can go wrong.

Problem 1: Plant Stress

Plants have a natural immune response which helps protect them against attack. Safeguarding that plants are in their best condition assures that a crop is less affected by biotic and abiotic factors. There are many scientific studies that have proved that plants in their optimum health are less likely to undergo biotic attack than those that are stressed in some way.

Factors that cause plant stress include over- and under-watering, nutrient deficiencies, mechanical damage and high pest pressures. A good knowledge of what a plant requires, as well as understanding the factors that influence the performance of plants, will aid you in growing the best possible crops.

Problem 2: Pests

One word: scouting! All crops need consistent scouting to see what insects are in the field as well as numbers. This can be done either by hand; walking around the fields and noting any pests that are seen, or with pheromone traps. From here we look at the economic thresholds, the point at which pest numbers are high enough that control measures need to be taken. Scouting is still important after spraying, to confirm the effectiveness of the control measure taken. It is vital that scouting is done at least once or twice a week, the more frequent the better. If pests can be caught and controlled in the early stages of infestation, less crop damage will occur. Catching insects such as caterpillars in their early larval instars is crucial to effectively managing them. The larger the caterpillars get, the more difficult it is to kill them. The same concept applies for leaf miners such as Tuta absoluta, which becomes very difficult to control when the pest gets into the later stages of it’s life cycle and has burrowed the fruit and leaves.

Problem 3: Disease

Text Box: Managing caterpillars in the early stages is key to successful control.Most diseases can be avoided with a good preventative spray programme. This involves spraying products that protect the plant against pathogen growth and reproduction. Once a pathogen gets onto an economically important part of the plant, such as the fruit, nothing can be done to save it. Curative sprays may help, but the infection of those fruits cannot be reversed. A typical preventative spray programme involves using at least three different fungicidal groups (with a preventative or protective mode of action) and alternating their application every seven to fourteen days. An example may be:

Week 1: Copper Oxychloride

Week 2: Azoxystrobin

Week 3: Mancozeb

Week 4: Copper Oxychloride

Week 5: Azoxystrobin Week 6: Mancozeb

And this repeats until the plant no longer needs to be protected. In the rains or under pivots, it is recommended to spray every week as the combination of high humidity and cooler weather is very conducive to pathogen reproduction and spread. If disease occurs then a curative programme will need to be implemented.

Problem 4: Weeds

There is a two-part problem to weeds. The first is that, before the crop is even planted, weeds cause problems due to their affinity to host a large manner of organisms. Secondly, when weeds occur within a growing field, they compete with the main crop for elements such as nutrients and space. Some weeds grow so vigorously that they can choke and shade out the plants and take over.

Land that is overrun with weeds should be cleared at least two to three weeks before planting. One major reason for this, which is especially prevalent in open field horticulture, is that weeds provide food for cutworms. The cutworm is a major pest of most horticultural crops and can result in huge losses before germinated plants can even get to their true leaves. If weeds are cleared a few weeks before planting, the cutworm is starved of it’s food source and will move away or die. Weeds are hosts for all manner of other problems; nematodes, pathogens like Phytophthora, pests such as aphids and even viruses.

Problem 5: Nutrient Deficiencies

Nutrient deficiencies will massively inhibit a plant’s ability to grow. In the same way that we as humans need food to sustain us, so do plants. Nutrients are the basis of plant life, they provide the building blocks for growth, hormones, chemical signaling and so much more. Soil usually contains a fair amount of nutrients but seldom do they meet the plant’s requirements. It is thus important to do regular soil testing to ensure the plant’s nutritional demands are being met.

Problem 6: Spray Application

This problem is entirely human related. It is important to apply all products correctly for them work effectively. There are many factors that contribute to ineffective spraying. These include time, rate and method of application as well as selection of adjuvants, effective mixing and calibration of spray equipment. Results will be disappointing if spraying is not done properly.

There are many factors that contribute to a healthy crop. Being on the ground and monitoring certain factors regularly is a fundamental requirement to good agriculture. Identifying problems early on is much easier, and cheaper, than dealing with an epidemic.

We are committed to helping farmers effectively deal with their problems in the field. For more information contact us at or +263 772 235 554.

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