Elemental sulfur is a naturally occurring element widely used in agriculture and gardening as a horticultural treatment. This compound has a history stretching back thousands of thousands of years as a proven, low-toxicity compound for managing pests and diseases on a variety of crops and ornamental plants. When prepared into a fine dust or a wettable powder, it becomes a valuable tool for growers. Its elemental nature allows for relatively rapid breakdown in the environment.
The Primary Targets: Mites and Specific Arthropods
Sulfur is most recognized for its effectiveness against organisms that are not true insects. These primary targets include mites, which belong to the class Arachnida, making them relatives of spiders and ticks rather than insects. Sulfur is highly effective as an acaricide, a substance that kills mites.
Mites are particularly susceptible to sulfur because of their small size and delicate body structure. Specific pest mites controlled well by sulfur treatments include the two-spotted spider mite, European red mite, and rust mites. The application of sulfur powder or liquid acts as a contact killer and repellent for these tiny pests. The substance disrupts the mites’ cellular functions and can also dehydrate them on contact.
Sulfur’s Efficacy Against True Insects
While sulfur is a potent miticide, its action against true insects is more limited. Sulfur is not considered a broad-spectrum insecticide and typically provides control only against smaller, soft-bodied insect pests. It generally fails to penetrate the harder, protective exoskeletons of many common garden insects.
The compound is effective against specific types of insects, such as psyllids and thrips. Psyllids, sometimes called jumping plant lice, and thrips, which are very small, slender insects, are vulnerable to the contact action of sulfur. It can also be used effectively against the crawler stage of scale insects. This is the soft, newly hatched, and mobile phase before they develop their protective outer shell.
Controlling Plant Diseases (Fungicidal Action)
The most important function of elemental sulfur in horticulture is its role as a fungicide. Sulfur acts almost exclusively as a protective fungicide, meaning it must be applied to the plant surface before a fungal spore germinates. It prevents the development of fungal diseases by creating an inhospitable environment for the spores.
The sulfur residue deposited on the leaf surface prevents the germination of fungal spores, stopping the infection cycle before it begins. This protective action is different from a curative action, which would eliminate an infection already established within the plant tissue. Proper timing is essential for effective disease management, as the protective effect is lost once the residue is washed off by rain or irrigation, necessitating reapplication.
Sulfur is particularly useful for controlling common diseases like powdery mildew, a white, dusty growth on leaves, and various rust diseases. It is also utilized against fungal issues including:
- Black rot.
- Brown rot.
- Leaf spot.
- Apple scab.
How Sulfur Works and Safe Application Methods
Mechanism of Action
The mechanism by which elemental sulfur works involves its transformation into toxic compounds upon contact with the pest or pathogen. When applied to plants, the sulfur begins to volatilize, or turn into a gas, at warmer temperatures. This sulfur vapor is thought to enter the cells of fungi and mites, where it interferes with cellular processes, such as cell respiration and metabolism.
Application and Safety
For application, sulfur is commonly available as a fine dusting powder or a wettable powder that is mixed with water to create a liquid spray. Dusting is often preferred for crops like grapes, while liquid sprays offer better coverage on dense foliage. To avoid damaging the plant, sulfur should never be applied when temperatures are expected to exceed 90°F (32°C) within a few days of treatment.
Sulfur is chemically incompatible with horticultural oils, including dormant oils and neem oil. Applying sulfur within two to three weeks of an oil treatment can lead to severe plant burn. Users should wear protective gear, such as gloves and a dust mask, to prevent inhalation of the fine particles.