Chapter 24

Greenhouse Pesticides

Fungicides

Probably the largest numbers of vine diseases are caused by fungi, and the most common chemical tools for the vine disease control are fungicides. Fungicides are classified in a number of different ways, which involve the following categories: 1) mobility, 2) mode of action, 3) breadth of activity, and 4) general function. These classes are not discreet—some active ingredients are both preventative and curative. In addition, some fungicides have two active ingredients, one preventative and one curative.

Mobility in the Plant

Contact Fungicides

Contact fungicides adhere to the leaf surface but do not penetrate the tissue; these fungicides have no after-infection activity. These products are most susceptible to removal by rain and other weather effects.

Systemic Fungicides

Systemic fungicides are absorbed into the leaf tissue, and then translocate from their point of entry to other tissues. In addition, fungicides vary as to how quickly, and to what extent, they are taken up by the plant.

Mode of Action

One way to classify fungicides is by their chemical structures, or their mode of action (MoA). MoAs serve to describe how a particular chemical or chemical group acts to kill or disable fungi. Fungicides kill fungi by damaging their cell membranes, inactivating critical enzymes or proteins, or by interfering with key processes such as energy production or respiration. Others impact specific metabolic pathways such as the production of sterols or chitin. For instance, the demethylation inhibitor (DMI) fungicide group (contains the triazoles) inhibits a specific enzyme in fungi that plays a role in sterol production. Sterols are necessary for the development of cell walls in fungi.

Target Site

Single-Site

Site-specific fungicides react with one very specific, very important biochemical process, called the target site. For example, a fungicide target site could be the specific proteins involved in cell wall biosynthesis, RNA biosynthesis, or cell division.

Multi-Site

Multi-site fungicides have multiple modes of action, so they affect multiple target sites, and simultaneously interfere with numerous metabolic processes of the fungus.

Role in Protection

Protectants

These fungicides are applied to form a protective barrier on the plant surface preventing the fungi from successfully penetrating host tissue. Protectant fungicides are active on the plant surfaces where they remain after application. There is no movement of the fungicide into the plant. Because they remain on the plant surface, protectant fungicides loose activity after being washed off the plant and must be re-applied to new growth that develops after application.

Penetrants

Penetrant fungicides are absorbed into plants following application. Because these fungicides are absorbed into plants, they are generally considered systemic fungicides. However, penetrant fungicides have different degrees of systemic movement once inside the plant. Some fungicides are “locally systemic”, only moving a short distance such as through a few layers of plant cells. Highly mobile fungicides are either “xylem-mobile” or “true systemics.” Xylem-mobile fungicides move upward in plants and outward to the periphery of leaves with water through the xylem, the water conducting tissue of the plant.

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