Types and diagnosis of plant diseases

1. Concept of plant diseases

Plant disease is a phenomenon in which the normal physiological functions of a plant are seriously affected and it shows abnormalities in physiology and appearance due to continuous interference by pathogenic organisms or adverse environmental conditions, the intensity of which exceeds the degree that the plant can tolerate. This deviation from the normal state of the plant is the occurrence of disease. The effects of plant diseases on plant physiological functions are mainly reflected in the following seven aspects:

Absorption and channeling of water and minerals: diseases may prevent the plant root system from absorbing water and minerals, affecting the normal transportation of water and nutrients.

Photosynthesis: diseases may affect the photosynthetic efficiency of plant leaves and reduce the production of photosynthetic products.

Nutrient transfer and transport: diseases may interfere with the normal transfer and transport of nutrients in the plant.

Growth and development rate: diseases may inhibit the normal growth and development rate of plants.

Accumulation and storage of products (yield): diseases can reduce the yield of the plant and affect the economic return.

Digestion, hydrolysis and reuse of products (quality): Diseases can affect the quality of plant products, making them less valuable in the marketplace.

Respiration: diseases may increase plant respiration and consume more organic matter.

2. Types of plant diseases

There are many types of plant diseases with different etiologic factors that cause a variety of diseases. Plant diseases can be categorized into invasive and non-invasive diseases according to the type of cause.

Infectious diseases

Invasive diseases are caused by pathogenic microorganisms, which can be transmitted through plant-to-plant contact, insects, and other vectors. Such diseases include the following:

Fungal diseases: diseases caused by fungi, such as gray mold of tomato. Fungal diseases are often characterized by necrosis, rot, and mildew on plant tissues.

Bacterial diseases: diseases caused by bacteria, such as watermelon bacterial fruit spot disease. Bacterial diseases are often characterized by watery spots, rotting, and pus spillage.

Nematode diseases: diseases caused by nematodes, such as tomato root-knot nematode disease. Nematode diseases are often manifested as galls on the roots, plant dwarfing, and so on.

Virus diseases: diseases caused by viruses, such as tomato yellow leaf curl virus disease. Virus diseases are often manifested as leaf blossoms, dwarfing, etc.

Parasitic plant diseases: diseases caused by parasitic plants, such as dodder disease. Parasitic plant diseases are often characterized by the parasitic plant wrapping itself around the host plant and sucking up its nutrients.

Non-infectious diseases

Non-invasive diseases are caused by adverse environmental conditions or problems with the plant itself. Such diseases include the following:

Hereditary or physiological diseases: diseases caused by the plant’s own genetic factors or congenital defects.

Diseases caused by deterioration of physical factors: Diseases caused by physical factors such as high or low atmospheric temperatures, wind, rain, lightning, hail, and so on.

Diseases caused by deterioration of chemical factors: Diseases caused by excessive or insufficient supply of fertilizer elements, pollution of the atmosphere and soil with toxic substances, improper use of pesticides and chemicals.
Notes
Infectious diseases: diseases caused by pathogenic microorganisms (such as fungi, bacteria, viruses, nematodes, parasitic plants, etc.), which are infectious.

Non-infectious diseases: Diseases caused by adverse environmental conditions or the plant’s own problems, which are not infectious.

3. Diagnosis of plant diseases

After the occurrence of plant diseases, the first thing to do is to make an accurate judgment of the diseased plant, in order to propose appropriate control measures to minimize the losses caused by plant diseases.

Diagnostic procedure

The procedure of plant disease diagnosis generally includes:

Recognition and description of plant disease symptoms: Observe and record the disease symptoms shown by the plant.

Questioning of disease history and review of relevant records: to learn about the disease history of the plant and relevant information.

Sampling and examination (microscopy and dissection): Collect samples of diseased plants for microscopic examination and dissection.

Perform specific tests: Perform specific tests, such as chemical analysis or biological tests, as needed.

Draw conclusions using step-by-step elimination: determine the cause of the disease step-by-step by elimination.

Koch’s Law.

Diagnosis of invasive diseases and identification of pathogens should be verified by following Koch’s Law, which is described below:

The presence of a pathogenic microorganism often accompanies the diseased plant.

This microorganism can be isolated and purified on isolated or artificial media to obtain a pure culture.

The pure culture is inoculated onto a healthy plant of the same species and a disease with the same symptoms appears.

A pure culture is obtained by further isolation from the inoculated diseased plant with the same traits as the inoculum.

If this four-step identification process is carried out and solid evidence is obtained, the microorganism can be confirmed as its pathogen.

Notes

Koch’s law: four criteria for identifying pathogens proposed by the German microbiologist Koch, used to prove that a microorganism is the pathogen of a particular disease.

Plant disease control strategies

Plant disease control is to change the interrelationship between plants, pathogens and the environment through human intervention, reduce the number of pathogens, weaken their pathogenicity, maintain and improve the disease resistance of plants, optimize the ecological environment, in order to achieve the purpose of controlling diseases.

Comprehensive control measures

In integrated control, we should take agricultural control as the basis, and reasonably and comprehensively apply measures of phytosanitary, utilization of disease resistance, biological control, physical control and chemical control according to the time and place, and treat multiple pests at the same time. These measures include:

Phytosanitary: preventing the spread of pathogens with seeds, seedlings, etc.
Disease resistance utilization: selecting and promoting disease-resistant varieties.
Biological control: utilizing natural enemies or beneficial organisms to control diseases.
Physical control: control the disease by physical methods such as regulating temperature and humidity.
Chemical control: rational use of pesticides to control diseases.

Through the comprehensive use of these control measures, the disease can be effectively controlled, reducing the loss of plants due to disease epidemics.

Notes
Phytosanitary: Measures to prevent the spread of pathogens with seeds, seedlings, etc., aiming at protecting plant resources and agricultural production safety.