Horticultural oils can be used to control a wide variety of insect pests and suppress the spread of both viral and fungal diseases. Many organic growers have begun to use horticultural oils because of the range of controls they provide and their lack of residual effects; however, it is imperative to follow labelled instructions to avoid any unintended effects on the plant and or environment.
Fungal diseases like powdery mildew and sooty mold can be controlled with horticultural oils in two ways:
- Controlling the vector insects.
- Trapping fungal spores disallowing continued spread of the disease.
If you have identified insects that are common vectors of disease (ie. leafhoppers, psyllids, etc.), you can apply a horticultural oil spray directly to the affected plants to suffocate the adults and disrupt the development of any eggs that may be present. Reducing the vector population will reduce plant infection chances. For fungal disease suppression when symptoms are present, coat both sides of the plant foliage with horticultural oil and make sure to spray areas that are already damaged. Thorough coverage is necessary as the oil will trap the spores and isolate the fungus to the affected areas. One addition that can be made to increase the effectiveness of horticultural oil sprays on fungal diseases is to mix a bicarbonate with the oil solution, which changes the pH of the environment on the leaves making it less optimal for fungal growth.
It is important to pay attention to temperature and humidity when applying horticultural oils as they are most effective when temperatures are 40-90° F and humidity is low. This allows the oil to evaporate quickly. While this temperature range for application is conservative, optimal results are achieved when applying horticultural oil when conditions fall in the range stated. Irrigation prior to horticultural oil applications will reduce water stress on the plants after applications are made. Extreme caution should be taken if you are planning on applying horticultural oil when temperatures are above 90°.