This section contains 7 questions:
The organisms that cause plant diseases are called ‘pathogens’. They include all kinds of microorganisms such as fungi, bacteria and viruses and some species of protozoas and nematodes. Pathogenic organisms are usually a normal component of the soil population and they naturally exist in relatively low numbers. Some pathogenic species usually cause disease in only one species of plant, but others can cause disease in a wdie variety of plants that are not closely related to each other.
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 dependant 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. The factors affecting the suitability of soil conditions include:
Management practises that restrict the growth of pathogens by producing soil conditions unfavourable to their growth will decrease 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’. Monoculture can increase the probability of a disease outbreak occurring.
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 process to be successful:
This is called an agar plate. Agar is an artificial medium used to grow bacteria and fungi in the laboratory. On this plate each circle represents a colony of bacteria that was originally started by a single bacterium (which is a single cell). By looking at bacteria and fungi under the microscope it may be possible to identify the organism. Usually specific biochemical tests are necessary to identify bacteria.
In order to cause disease in plants, fungal and bacterial pathogens in soil must enter the plant roots. Most plants are resistant to the entry of most pathogens. 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 only a portion of the root.
The activity of nematodes is generally stimulated by the presence of host plants and disturbance of their environment. 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:
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.
Plant disease can be controlled in a number of ways. When chemicals such as fungicides are used to control plant disease this is called chemical control of disease. Biological control of disease refers to the use of living organisms to control the numbers and activity of a pathogenic organism. These 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.