The microclimate (i.e. temperature, humidity, wind speed) in the crop can be manipulated by using different planting densities. The distance between plants in a field is thus a factor which will have influence on the development of pests and diseases.
If plants are sown or planted close together, they will cover the soil rapidly. This can be an advantage as it will delay the developments of weeds. It has also been observed that certain insects (for example aphids) are less abundant in a densely planted crop.
Close spacing will increase the humidity in the crop. This may be an advantage, as in a dense vegetation, more fungal pathogens will develop that can reduce the pest populations (see also: Pathogens).
For example, in dense planted beans, more aphids get killed by fungi. In many situations, biological control is favored by high humidity.
On the other hand, a higher humidity will also favor the development of plant diseases. Several fungal diseases are more serious in densely planted crops.
For example sheath blight in rice is more common when rice is planted close together. Farmers can avoid this disease to some extent by planting the rice farther apart. By allowing the sun and wind to penetrate deeper in the crop, the humidity will be lower and the disease will be less serious.
Also some insect pests prefer higher humidity. For example Brown Plant Hopper populations will develop faster when rice is planted at high density.
The optimum distance between plans thus depends on many factors. The farmer will have to consider all these factors before making a decision. Experimentation in his own field will help to increase his knowledge on the effects of different spacing.
Generally it is recommended to plant in lines. By planting in lines is it easier to maintain uniform distances between plants. Another advantage of line planting is that it facilitates most field work such as weeding, regular inspection and harvesting.