There are over 50,000 destructive diseasesthat affect the growth of their respective plant species.  As agriculture struggles to support the rapidly growing global population, plant disease reduces the production and quality of food, fiber, and biofuel crops.

To ensure the safety of our food, fiber, and biofuel supply plant diseases must be controlled. The most frequently used disease control solution is the application of a protectant pesticide. The use of surfactants helps improve control.

What is a Plant Disease?

Plant disease is the impairment of the normal state of a plant that interrupts or modifies its vital functions. All species of plants, wild and cultivated alike, are subject to disease. Plant disease may be caused by a living microorganism or virus (pathological) or by the continued deficiency of a nutrient, such as a lack of available iron in soil (physiological).

Injury to plants is similar to a disease in some respects, but is caused by the momentary or transient impact of some agent. For example, an injury that is caused by machinery/tools, insects, or environmental stress such as hail, wind, or drought.

How does aplant disease develop?

For a disease to develop, three factors must be present: a susceptible host, a disease-producing agent, and an environment favorable to disease development. Together, the three factors are commonly referred to as ‘the disease triangle’.

The disease triangle

1.     Susceptible host

A plant that has little to no resistance to the disease-producing agent.

2.     Disease-Producing Agent

Three common disease-producing agents are fungi, bacteria, and viruses.

• Fungi – simple plants that lack chlorophyll (the green coloring found in seed-producing plants) and therefore cannot manufacture their own food.
• Bacteria – very small, single-celled organisms.
• Viruses – organisms that are smaller than bacteria. Viruses are often transmitted from plant to plant by insects.

3.     Environment Favorable for Disease Development

Disease development is favored when the temperature and moisture are most conducive for the particular disease-producing agent.

Steps in Disease Control

1. Understand the disease triangle and how plant disease develops.
2. Identify the disease
   • Causal agent – fungi, bacteria, virus, or other microorganisms.
   • Symptoms displayed – rotted fruit, blight, canker, spots, shrunken fruit, yellowing of foliage, dying plant, or other symptoms.
     • Symptoms by themselves may not lead to an accurate diagnosis of a plant disease because several different casual agents may produce the same symptoms.
     • However, displayed symptoms used with other evidence, plus experience, often will produce a satisfactory diagnosis.

3. Correctly diagnose infected plants
   • Identify the affected plant species.
   • Carefully examine the diseased area in the field and note how the diseased plants are distributed over the affected area. Obtain a record of environmental conditions preceding the appearance of the disease.
   • Describe the primary symptoms of the diseased plant while comparing it to a healthy plant.
   • If unable to correctly diagnose the disease, additional specialized techniques need to be used to isolate and identify the causal agent in the laboratory.
4. Use one or more of the following disease control strategies:
   • Exclusion – using measures to prevent a disease organism from being introduced and established in an area where it does not occur. For example, plant quarantine.
   • Eradication – the elimination of a pathogen from an area, usually when it has limited or restricted distribution.
   • Protection – consists of the placement of a protective barrier – usually a chemical – between the plant and the pathogen.
   • Resistance–involves the use of plants that are not susceptible to disease.

Chemical control of plant diseases

Chemicals used to protect plants from diseases are generally called fungicides. However, a fungicide is used specifically to control fungi. To control bacteria a bactericide is used, whereas a virucide is used to control viruses. Collectively, they are all a form of pesticide.

Any pesticide used to protect plants from disease must have the following attributes:

• Remain active for a relatively long period.
• Have good adhesive properties and not be easily washed off plants.
• Has good spreading properties.
• Is stable in the sun and won’t be easily deactivated.
• Is toxic to plant pathogens but nontoxic to the plant and non-target organisms.
• Is active against a wide range of plant pathogens.
• Is compatible with other pesticides, e.g., insecticides and herbicides.
• Is relatively easy to apply and does not present an undue hazard to the applicator or the environment.
• Is non-corrosive to the application equipment.

How do surfactants improve disease control in plants?

Surfactants are additives commonly applied with pesticides to improve spray performance, including coverage, persistence on plant leaves, and absorption. Compounds used as surfactants include petroleum and crop-based oils, inorganic salts, and organic compounds.

Surfactants can:

• Activate pesticides to improve pest control, for example:
  • Brewer 90-10™
  • Cide-Kick™
• Help pesticides adhere (stick) to leaf surfaces, for example:
  • Poly Control II™
  • Poly Film R™
• Enables pesticide to spread on leaf surfaces, example:
  • Brewer 80-20™
  • Silnet 200™

Read the Label

For best results, make sure to read and understand label instructions before mixing surfactants with pesticides.

Further Reading

• Everything You Need to Know About Agricultural Wetting Agents
• Limonene: An Environmentally Safe Compound for Pest Control
• How Surfactants Affect Spray Droplet Size
• Use of Silicone Adjuvant in Agriculture
• Postemergence Spraying and Tips for Application
• Adjuvants 101: Nonionic Surfactants
• Adjuvants 101: Agricultural Surfactants Yesterday, Today, and Tomorrow

References

• Bohmont, B.L. 2007. The Standard Pesticide User’s Guide – 7^th Pearson Prentice Hall. Pp. 62-79.