Fungicides and Other Plant Disease Management Approaches
by Ethel M. Dutky
Department of Entomology, University of Maryland
Plant disease is caused by a diverse collection of microbes including fungi, bacteria, viruses, viroids, nematodes and parasitic plants. Probably the largest numbers of plant diseases are caused by fungi, and the most common chemical tools for the plant disease control are fungicides. In addition to the application of a fungicide, plant diseases are controlled by breeding for resistance, vector control, sanitation, crop rotation, eradication and quarantine.
I – Early fungicides
The first fungicides were broad spectrum surface protectants. After observing that wheat seed salvaged from sunken ships had less smut, farmers (in the late 1600’s) treated wheat seed with brine to control the seed born smut fungus. By the mid 1700’s lime sulfur was substituted as a wheat seed treatment. In 1882 Millardet noticed that grape vines in rows near the road which had been sprayed with a mixture of copper sulfate and lime (to deter pilferage) were protected from the disease grape downy mildew. This mixture became known as Bordeaux mixture, and is still used today. Bordeaux mixture is considered to be the first fungicide used to protect a crop from disease through regular applications to foliage.
In 1913 organic mercury compounds were introduced as seed treatments, and remained a widely used method to protect seedlings until the 1960’s when their use was banned because of the high toxicity to humans of the mercury compounds. Many incidents of starving people being poisoned from eating treated seed had happened. In 1934 the first dithiocarbamate fungicide, thiram, was introduced as a seed treatment. Many other protectant fungicides were developed (ferbam, zineb, maneb, captan, moncozeb, chlorothalonil, coppers) and used widely. These compounds have multiple sites of toxicity to the fungal cell = multi-site toxicants.
Advantages and disadvantages of multi-site toxicant fungicides
Most multi-site toxicant fungicides provide broad-spectrum control and different types of fungi (Oomycetes, Ascomycetes, Basidiomycetes). Once on a plant surface, these compounds are re-activated with each wet period. These compounds kill spores on plant surfaces. The development of resistance is not a serious problem with these fungicides.
These compounds are applied at high rates (pounds per acre or per 100 gallons). They leave a visible residue (a problem for ornamentals). They remain as a residue on the crop, possibly preventing use of the crop residues for forage. They are often phyto-toxic. They must be used to prevent infection, and are not effective after plant infection is present. The high application rates raise concerns about environmental effects and chronic low dose health effects on humans.
Several of these fungicides remain a useful part of crop protection programs, and are important in programs to reduce the development of fungicide resistance.
II – Single-site Fungicides
In 1965 the first systemic fungicide, carboxin, was discovered, and was soon followed by the introduction of other systemic fungicides such as the benzimidazoles (benomyl, thiophanatemethyl). These compounds acted primarily on a single metabolic process in growing fungal cells = single-site toxicant. They were taken into the plant, and in some cases could eradicate the fungus after infection was present.
Advantages and disadvantages of single-site toxicant fungicides
They are applied at relatively low rates (ounces per acre). Most leave no visible residue. Because they are applied at such low rates, most leave little if any detectable residue in plant parts at harvest or in the environment. However, most of the single-site fungicides have a more narrow spectrum of activity. Some compounds control only Basidiomycete fungi, others control only Oomycetes (water molds), etc. Most single-site toxicants do not kill spores on plant surfaces. They become active only in a growing cell.
Unfortunately, resistance to these new “single-site” systemic fungicides soon appeared. Within any population of fungi, there are individuals that have alternate metabolic methods to do ordinary metabolic tasks. If these alternate pathways are not damaged by the fungicide, the fungus will be resistant. When most of the susceptible (normal metabolic pathway) fungi are eliminated, these resistant ones soon predominate in the population. Spontaneous mutations may also play a small role in the development of fungicide resistance. If the resistant type is also fit, it can remain a major part of the population even after exposure to the fungicide is stopped. Fitness refers to the ability of the fungus to reproduce rapidly, infect the plant, tolerate cold, etc. Resistant fungi which also have a high degree of fitness result in “stable resistance”.
III – New Approaches to plant disease management
A – Biological fungicides
In nature most plant diseases are under some “natural” or biological control. Examples abound showing that biological control exists. Some soils can be shown to have biological components that are suppressive of disease. That disease “Take All” or wheat is known to decline over time if wheat is successively planted for several years. Investigation into these natural examples of biological control give rise to identification of specific microbes associated with disease suppression.
Today there are a number of products that are biological fungicides. These include preparations of fungi (RootShield, SoilGard, Trichodex, AQ10) or bacteria (Mycostop, Galltroll-A, Companion, Serenade, Kodiak, Deny). These biological fungicides have many types of activity, most have multiple methods including competition, antagonism, antibiosis (antibiotic compounds excreted), stimulation of plant systemic resistance reactions, and attack and consumption of fungal resting structures.
Some plant extracts are available for disease control. Several neem-based products are on the market (Triact-70). An extract of the giant knot-weed (Milsana bioprotectant) is said to act as an inducer of systemic acquired resistance. Other plant-derived products are being investigated: essential oils, ascorbic and citric acids, mint oil, chili pepper extracts and others.
B – Hypovirulence
The reduction of the virulence or aggressiveness (ability to cause disease) caused by infection of the fungus by a virus or virus like agent is termed hypovirulence. This was observed in the early 1970’s in Belgium and France in chestnut plantations where chestnut blight cankers were spontaneously “healing”. Inoculation of cankers with hypovirulent chestnut blight is now utilized in Europe to control chestnut blight in European chestnut plantations. It is being developed to control chestnut blight on American chestnuts, and may some day permit the reestablishment of the American Chestnut. Hypovirulence has also been seen in other major plant pathogenic fungi (Rhizoctonia solani, Gaeumonnomyces graminis, and others).
C – Systemic Acquired Resistance (SAR)
The induction of plant defenses by artificial inoculation with microbes or by treatment with chemicals is termed “systemic acquired resistance” (SAR). This plant response acts nonspecifically throughout the plant to reduce the severity of a wide range of diseases caused by fungi, bacteria and viruses. Early research used inoculation of plants with a fungus or bacterium to prevent later infection by a virulent pathogen.
At present there are two SAR products on the market: Actiguard and Messenger. These compounds are applied to the crop on a regular schedule, and can be highly effective in preventing bacterial, fungal and viral infections. Although presently not registered on ornamentals, as more SAR products are registered, some should have ornamentals on the label. Note – these products do not have any toxicity to the pathogens (bacteria, fungi, virus, etc.), rather they act on the plant to produce toxic or inhibitory chemicals. Research has shown that some biological fungicides also have SAR activity.
D – Genetic modification of the crop
Genetic engineering permits introduction of resistance genes from diverse plant sources, and also from pathogen sources. This technology greatly expands the possibilities for effective resistance.
The use of resistance genes from pathogens has historical roots in plant-virus systems. In 1986 it was shown that tobacco plants transformed to express the coat protein gene of tobacco mosaic virus (TMV) showed resistance to subsequent inoculation with TMV. In addition, once the TMV coat protein gene was integrated in the tobacco genome, it was carried through the seed and behaved like any other tobacco gene. Since 1986 many other crop plants have been transformed with coat protein genes and one or more of the viruses that infect them. These transgenic plants usually show a very high level of resistance to the virus from which the coat protein gene was derived, and in addition, often to other more or less related viruses as well. Transgenic plants have also been transformed with viral genes other than the coat protein gene, with excellent resistance resulting. The use of inducing plant resistance through genetic transformation with pathogen-derived genes could provide a powerful tool to control diseases that are otherwise very difficult to control.
Fungicide Resistance Management
For many years pesticide resistance was seen primarily in insecticides and miticides, but not in fungicides. This was because the early types of fungicide acted on many life processes in the fungal cell (multi-site activity), and it was very unlikely that cells could exist with resistance to the many forms of toxicity. However, with the advent of fungicides that act primarily on one specific cellular function (single-site activity), resistance was soon seen. When a pest population is exposed to a poison over many generations the individuals that are sensitive die, leaving individuals that are less sensitive to reproduce. This is possible because within a population there may be individuals that have the ability to tolerate the poison, perhaps because of alternative metabolic pathways, or the ability to metabolize the poison, or an altered receptor site to which the poison cannot attach. Whatever the mechanism, resistance is a serious risk in the use of “single-site” pesticides, including fungicides. Some examples of plant diseases where resistance is now a common problem in the field and greenhouse include Botrytis, powdery mildews, and Pythium root rot.
What Can the Grower do to Prevent Fungicide Resistance?
Several strategies exist to cope with the risk of fungicide resistance. One strategy is to rotate the chemicals you use, so the pest is not continuously exposed to the same type of poison. Rotation usually consists of a few (2 to 4) consecutive applications, then switch to a different mode of action. This is the primary strategy used for insecticides, miticides and herbicides. Another strategy is to use a mixture of two fungicides, one component of the mixture is a “single-site” and the other is a “multi-site” fungicide. Some examples of this type of mixture are Zyban, ConSyst, Sprectro 90 and Stature. This strategy is unique to fungicides as we still have several types of fungicide that have “multi-site” activity. Some examples of these multi-site toxicant fungicides are chlorothalonil, coppers and mancozeb. It is also important to apply the fungicide at the label rate, and not to apply at reduced rates. Repeated applications of single site compounds at reduced rates will promote resistance development.
The fundamental basis for disease prevention relies on modification of the environment to reduce the risk of disease. Once the crop is exhibiting symptoms, it may not be possible to avoid losses. Powerful non-chemical tools to reduce diseases include sanitation and environmental management.
Some fungi and bacteria are now available to provide biological control on crop diseases. These products work using a variety of mechanisms including: competition for food sources and infection sites, stimulation of plant defenses, production of antibiotics, and attack and consumption of fungal resting structures. They are broad spectrum. These products work best when incorporated into the production system from the start. They do not provide control once the disease is established. Using biologicals can permit the grower to reduce the number of chemical applications to the crop. Some examples of biological fungicides are: RootShield, SoilGard, Trichodex, AQ10, Mycostop, GalltrollA, Companion, Serenade, Kodiak and Deny.
The following table lists some fungicides labeled on ornamentals by their mode of action class. You can use this information to assist you in developing a rotation program.