Using soil management to control pests and diseases
Tuesday, March 4, 2014
Managing your fields effectively can help you reduce or even eliminate the need for pesticides altogether
by KEITH REID
Many chemical compounds are available to control pests and diseases in your crops, so it's easy to assume that cultural controls are less important. But paying attention to how you manage your fields can not only reduce or delay the need for pesticides; in some cases, it can eliminate it altogether.
Effectively managing the field can reduce insect or disease pressure in three ways:
- Reduce the pest population or disease inoculum;
- Create an environment that is less favourable for infection;
- Create conditions that allow the plant to outgrow the infestation.
Crop rotation is the most obvious way to reduce pest or disease pressure, as long as the crops in the rotation are not susceptible to the same disease or pest. Corn rootworm larvae that hatch in a soybean field will starve to death because there isn't a host crop for them to feed on. White mould spores from soybean residue
released in a following corn crop won't do any damage there although, if the following crop were sunflowers, the white mould could cause severe damage.
The impact of tillage can vary depending on the particular pest or disease. In some cases, leaving crop residue on the surface exposes the fruiting bodies of the disease or the insect pupae to freezing temperatures and predation by birds and rodents, so no-till can reduce, for example, white mould inoculum.
Leaving crop residue on the surface can also reduce the transmission of diseases like bacterial blight, which are carried by soil splashed up onto the plants. In other cases, burying the residue places the inoculum in an environment where it will rot quickly, or at least where the spores cannot reach the top part of the plant. Often, however, burying or not burying the residue is of relatively minor importance, so clean plowing corn stalks to reduce corn borer pressure is never very effective.
Most plant diseases are most infective under moist conditions, so a lush, humid canopy can increase the amount of disease pressure. Excess soil fertility, particularly of nitrogen (N), can create these conditions, so the incidence of white mould in soybeans can be greater following manure application, for example.
It has also been shown that wheat can produce higher yields with high rates of N, but only if accompanied by a fungicide to control the extra disease.
Plant nutrition can also affect how easily an infestation can occur. Soybeans deficient in potassium have higher levels of sugars and amino acids in their sap, making those plants highly attractive to soybean aphids. Maintaining adequate potassium in the soil won't eliminate aphid infestations, but it will make them less severe. Adequate potassium will also help maintain strong cell walls and cell turgor, which can make plants less susceptible to disease.
Plants can tolerate the loss of part of their roots or leaves, as long as new tissue is growing faster than it is disappearing. This is part of the reason insects or disease cause much greater losses in crop productivity when the plant is exposed to additional stresses, like adverse weather.
Maintaining adequate, but not excessive, fertility and good soil tilth will go a long way towards making plants more tolerant of low levels of disease or insects.
One specific example of using nitrogen to reduce the impact of diseases is in carrots. Carrot leaf rust is seldom severe enough to hurt the growth of the carrot roots, but it does cause the top of the plant to die back late in the season. Since mechanical carrot harvesters operate by pulling the carrots out of the ground by their tops, this can reduce the harvestable yield. Extra nitrogen can keep the carrot tops growing actively longer so, even though it does not increase the growth of the carrot, it does result in more carrots making it onto the grocery shelf.
Managing your fields to reduce insect and disease impacts falls right in line with the management practices for good agronomy and fertility. It does not necessarily mean doing more, or differently, from what you already are doing, but it does give one more good reason to pay attention to this basic management. BF
Keith Reid is a Soil Scientist, Agriculture and Agri-Food Canada, Guelph