Modern fungicides have become critical components in the prevention and treatment of plant diseases. Ever since the beginning of agricultural production, plant diseases have had a devastating impact on the human population. In the 1840s, potato blight caused by ooymcete Phytophthora infestans was responsible for the Irish Potato Famine. It is estimated that 1.5 million people starved to death when this critical food staple was wiped out. More recently, in 1970, the Southern Corn Leaf Blight Epidemic in North America caused more than $1 billion in crop losses. In most fields, the crop loss from this blight was between 80% to 100%. Therefore, as the world’s population is expected to increase by more than 1 billion people in the next 15 years, it is critical for farmers to have the needed fungicides in order to maximize crop production and protect against crop losses.
Fungi in a nutshell
In order to simplify things, plant pathologists group all fungal diseases as “fungal pathogens.” Actually, there are two distinct types of fungal pathogens: the true fungi and the fungal-like pathogens know as the Oomycetes. The cells of all true fungi are surrounded by cell walls made up of the polymer chitin. Chitin also is the main structural component of the exoskeletons of insects and arthropods. Most of the true fungal diseases are caused by either ascomycetes or basidiomycetes. Ascomycetes are responsible for common diseases such as southern corn leaf blight, peanut leaf spot, gummy stem blight, and powdery mildew. Basidiomycetes are famous for producing the white button mushroom; however, they are also responsible for a number of devastating plant diseases such as white mold in peanuts, corn and bean rust, southern blight, and rhizoctonia.
Oomycetes, also known as water molds, closely resemble the true fungi, however, they are totally unrelated to them. The cells of the oomycetes like the true fungi are surrounded by a cell wall. However, their cell wall is primarily made up of cellulose, the same polymer found in the cells walls of both plants and algae. Oomycetes are distantly related to brown algae such as kelp. Common diseases caused by oomycetes include downy mildew, late blight, pythium root rot and tobacco black shank. In this article, the terms fungal pathogens and fungal body will refer to both the true fungi and the oomycetes.
Types of Fungicides
Fungicides can be grouped together according to how they interact with fungal pathogens. Protectant fungicides or contact fungicides were the first type of fungicides to be discovered. These fungicides are not absorbed in to the plant tissue and can be washed off by rain or irrigation. They form barriers around the plant cells. These barriers kill the fungal spores and hyphae, the thread-like strands of the fungal body. Since protectant fungicides are not absorbed into the plant tissue, they can only prevent fungal infections. They cannot treat fungal diseases that have already penetrated into the plant tissue. As a result, to achieve good disease control, these fungicides must be applied before the onset of disease. Sulfur and copper were some of the first protectant fungicides discovered. Bordeaux mixture, for example, was accidentally discovered by grape farmers in the Bordeaux region of France. The farmers sprayed the grapes near the roadside with a mixture of copper sulfate and hydrated lime to deter passersby from helping themselves to the ripe grapes. The farmers then observed that the vines which had been sprayed with the mixture remained free of downy mildew. Other common contact fungicides discovered in the 1930s and 1940s include, chlorothalonil, mancozeb, thriam and captan.