Fungicides

Fungicides

• Ever since humans domesticated plants, crop plants have been targeted by fungi, insects, bacteria, and viruses. Growing the same kind of plant close together (a monoculture) made life easier for pathogens.

• Today, > 1/3 of all crop losses are due to fungal diseases.

• Trends in fungicides: toxins > less toxic substances that interfere with fungal growth or reproduction; external > internal treatments.

The First Generation: Inorganic Fungicides

• 1880s: Bordeaux in France was losing vineyards to Plasmopara viticola (Oomycota), which had been accidentally introduced from the U.S.

• Professor Millardet noticed that the vines near a walking path looked healthier than those farther

from the path.

• Millardet tried different substances and settled on a mix of copper sulphate and calcium hydroxide, which became known as the Bordeaux mixture.

• The Bordeaux mixture is still used as a wide- spectrum fungicide, but has largely been replaced by copper oxide, copper hydroxide, and copper oxychloride.

More Inorganic Fungicides

• This and all other fungicides developed before 1960 are called protectants.

• They are only toxic to fungi if they intercept the fungi outside the host plant.

• The plant’s surface must be coated with the fungicide.

• Inorganic fungicides can damage the plant itself, especially if it gets inside the plant.

• They can be washed off by rain.

• This, plus the new plant growth, means that growers must spray plants multiple times per growing season.

• After many years, toxins can build up in the soil.

The Second Generation: Organic Fungicides and Seed Dressings

• Organomercurials were developed in response to problems with mercurous chloride. They were as toxic to fungi as mercury, but were less poisonous to animals.

• Too toxic to spray, but were used as seed dressings.

• Organotin fungicides were similar to organomercurials.

More Organic Fungicides and Seed Dressings

• Dithiocarbamates were introduced in the 1930s.

→ Very low phytotoxicity.

• Phthalimides

→ Captan is the most famous. Captafol and Folpet were also

widely used.

The Third Generation: Systemic Fungicides

• 1960s: benzimidazoles arrived.

• Benomyl (Benlate) was the first systemic or eradicant

fungicide. It gets inside the plant to kill the fungus.

• Nontoxic

• Effective against many ascomycetes and their conidial

anamorphs in very low doses

More Systemic Fungicides

• Some ambimobile systemic fungicides can move into new plant growth, which is tissue that was not treated with the fungicide.

• Some are selective, killing some fungal groups but not others. Might allow us to treat fungal diseases without disrupting the soil mycota or mycorrhizal fungi so much.

• Sterol inhibitors

→ Systemic but nonselective

→ Fairly new (1970s)

→ Prevent biosynthesis of ergosterol, the major sterol in many fungi. It’s a basic component of fungal membranes, so fungal growth is stunted.

→ Some pathogens are already developing resistance.

An Unexpected Fourth Generation: the Strobilurins

• 1980s: Strobilurus tenacellus, a small agaric that fruits on pinecones, produces a fungicidal substance: Strobilurin A (β-methoxyacrylic acid).

→In the lab, effective against almost all fungi tested, but sensitive to light and oxygen, and not effective in fungal-infected plants.

• Strobilurin fungicides were sold starting in 1996. By 2006, strobilurin fungicides made up > 20% of the global fungicide market.

• Also marketed as improving yield through their growth regulatory activity and are being investigated as antibiotics and anticancer agents.

Serenade Garden

• This is a formulation of Bacillus subtilis strain QST 713.

• It attacks and controls powdery mildew fungi on squash, cucumber, melons, and pumpkins.

• Also controls anthracnose, Botrytis blights (gray mold), downy mildew, head and leaf drop, leaf spot, pink rot, and Sclerotinia stem rot.

• EPA approved, can be used in organic gardening, seems to be nontoxic to plants and beneficial

insects.

• Problems might develop or be discovered in the future.

Resistance to Fungicides

• A broad-spectrum fungicide like mercury poisons so many enzyme systems that only an absolutely inconceivable number of simultaneous genetic alterations could confer resistance on a pathogen.

• But systemic fungicides act on single sites, so repeated exposure prompts evolution of resistant strains.

• We now realize that it is often best to use mixtures of unrelated fungicides or to apply a sequence of different fungicides as part of an integrated pest management scheme.