Development and Prevention of Plant Disease

What kinds of organisms cause plant disease?

Organisms that cause plant diseases are called pathogens. They include microorganisms such as fungi, bacteria and viruses, some species of protozoas and nematodes. Pathogenic organisms are usually a normal component of the soil population and naturally exist in relatively low numbers. Some pathogenic species only cause disease in one species of plant, but others can cause disease on plants that are not closely related to each other.

How is a pathogen identified?

Koch was a scientist who, in the 1880’s, proposed a method for identifying the organism responsible for a disease. When his procedure is applied to identifying plant diseases the following must occur for the identification to be successful:

• An organism is isolated from a plant showing symptoms of the disease.
• The organism is grown separately from other organisms and the host (on an artificial food source).
• The organism is placed into contact with a healthy plant and the plant develops the same symptoms of the disease.
• The organism is isolated from the second diseased plant.

What soil conditions allow plant disease to develop and how can outbreaks be prevented?

Pathogens can exist in the soil for long periods of time without causing an outbreak of disease in plants. Disease outbreaks are either caused by an increase in the population of the pathogen or by an increase in the susceptibility of the plant. The population of the pathogen is dependent on whether the soil conditions are favourable for its growth and survival. The conditions that are favourable for the growth and survival of pathogens are different for each species of pathogen but are related to:

• soil pH,
• water content,
• oxygen level,
• nutrient level and
• the activities of other soil organisms.

Management practises that produce soil conditions that are unfavourable to pathogen growth will reduce the likelihood of disease outbreaks. The susceptibility of the plant to disease is affected by factors such as its age and nutritional status. Outbreaks of disease are also more likely in agriculture and horticulture than they are in natural systems. In agriculture and horticulture similar species are planted together in what is called a ‘monoculture’. Monocultures can increase the probability of a disease outbreak occurring.

How do fungal and bacterial plant pathogens enter plant roots and how do plants respond?

To enter a plant root the bacteria or fungi must first be present in the rhizosphere of the plant. It then uses molecular signals to recognise whether the plant root is susceptible to entry or not. The pathogen attaches to the root surface possibly by the use of hair-like structures and enters the root. Some pathogens enter through areas that have been damaged by animals and some fungi and bacteria produce enzymes that dissolve the chemical compounds that make up the cell wall. Once the pathogen is in the plant cells, the plant may try to prevent its spread by producing chemical or physical barriers. These procedures may confine the pathogen to a portion of the root.

How do nematodes attack and kill plants and how are they controlled?

The presence of host plants and disturbance of their environment generally stimulates the activity of nematodes. Parasitic nematodes prefer to attack the young growing roots and any areas of roots already damaged by other organisms.

Plants react in three different ways to nematode attack:

1. By production of a local swelling and ‘stubby root’ symptom and the immediate suppression of root growth
2. By cell death at sites where wounds and lesions have occurred
3. Through the formation of root galls resulting from cell hypertrophy

Once nematode populations have developed at a site where a particular host plant has been cropped, they may survive as resistant cysts for periods of up to 10 years. Population density generally increases steadily for five years and then stabilises. If the numbers attained at that stage are detrimental to crop yields, the usual practice is to stop growing that particular crop and either leave it to fallow or grow non-susceptible crops until the cysts are eliminated (Lavelle and Spain, 2001 p. 443).

What is biological control?

Plant disease can be controlled in a number of ways. Chemical control uses fungicides and other pesticides. Biological control of disease refers to the use of living organisms to control the numbers and activity of a pathogenic organism. These beneficial organisms may occur naturally in the soil. The objective is to manage soils so that the soil environment is suitable for growth of the beneficial organisms that naturally control plant pathogens.