In plant pathology, the following procedures are used in diagnosing plant disease,
Pattern of occurrence: Pattern of occurrence enables the pathologist to know the agent spreading the disease, for example, if the occurrence is by the road side, the pathologist will understand that the disease might have been spread by trespassing animals; he could control the spread by applying appropriate pesticide.
Disease symptoms: The symptoms of a disease could be of immense value in suspecting a particular outbreak, Cassava leaf mosaic disease, is observed with the symptoms of chlorosis of the leaf, this can be easily controlled by the elimination of the host plant.
Disease type: The plant pathologist should be able to link diseases symptoms with its cause,
· Bacteria causing disease
· Fungi causing disease
· Viral causing disease
To control abnormal growths in plants caused by microorganisms like the bacteria, we can decide to visit the laboratory to establish the disease by using Koch’s Law. Fungi diseases can be easily controlled through the use of fungicides, while viral infection causing organisms can be controlled by removing the host.
Establishing the Cause of a Disease
The cause of a disease could be established in the laboratory using Koch’s postulate, He designed four criteria to determine the causal relationship between microorganism and diseased plant.
· The microorganism must in every way be associated with the cause of the disease or the infection must not occur in the absence of the causative agent.
· The microorganism must be isolated in pure culture and special characteristics studied.
· When a healthy susceptible plant is inoculated with the pathogen under favorable condition, the symptoms of that disease must appear.
· The microorganism must be re-isolated and identified as a first isolate.
Factors That Influences Disease Continuity
There are two growth period in plant namely, the
· The growth season
· Permanent season
As a result of growth discontinuity, pathogen must ensure they survive unfavorable conditions; a number of factors help the success of the pathogen,
· The amount of inoculum produced
· The libration of inoculum from the substrate
· The resistance of the inoculum to unfavorable condition
· The abundance and timeliness of the inoculum
· Factors affecting the germination of the inoculum
Amount of Inoculum Produced and Abundance
Bacteria and viruses do not produce the same type of inoculu, the individual bacteria cells and virus particles can act as inoculum, However fungi produces several types.
Inoculum produced by fungi
Mycelium: Fungi mycelium can act as inoculums that can be dormant or active depending on the environment.
Sclerotium: This consists of a resting body of certain fungi that vary in sizes, it consists of a hard mass of hypae from which fruiting bodies like the mycelium and candiospore can develop. It can remain dormant for a long period of time.
Rhziomorph: The rhizomorphs is a rootlike structure or cord made up of congregated hypae in some fungi.
Spore: The most important type of inoculums is the spore; they are usually tiny and can be disseminated by the wind, Spores are produced differently in different classes of fungi and are sometimes used as bases for classification and identification.
Libration of Inoculums
Passive libration: Passive liberation occurs when the spore is produced within the tissue of the host; it thus takes place when the host ruptures. Passive libration is common in bacteria disease, sometimes, the host epidermis ruptures as a result of pressure, however, in the case of externally produced spores, they are easily blown by the wind and thus disseminated.
Resistance of the inoculum
During adverse environmental conditions such as dry season, the pathogen must be maintained or survived, among the fungi, the spore of the mycelium can carry the spore of the pathogen from one season to the other for many years.
Ostilago megalis: This is a cotton pathogen that is known to survive inside cotton seeds for more than 13years. A motile spore (200spores) is not viable for more than 10yers. Bacteria do not have spore inoculums but remains dormant in the plant for many years. On the other hand, viruses are transferred by plant’s insect and as a result, they are not exposed to adverse weather condition. A few viral pathogens like the tobacco mosaic virus (TMV) have been known to survive in dry plants material for several years.
The abundance and the timeliness of the pathogen
The more abundance the pathogen, the more wide spread the disease and the more timely the inoculums, the more successful the pathogen and hence the more economically destructive.
If the spore of the particular pathogen is available when the host is not available, they perish, for example, for the maize rust to be very effective the mature spore has to be available around the month of March and April, at this particular time, the inoculums is very abundant because this when the host is also very abundant. There are cases where pathogen affects seedling, example, is the damping of pathogen, therefore the inoculums has to be abundant when the young plant is available, not when the plant is matured, in other words, a host should not only be available but a susceptible stage of the host should also be available.
The Followings are Agents of Disease Dissemination,
Wind: Any pathogen using wind as an agent of dissemination must be large in number; this large number takes care of wastage, the spores are always lighter and many are turned to resistance hazards when blown about.
Insect: Insects are able to transport inoculums and sometimes acts as agents themselves especially in the case of virus; they may act as a medium for the multiplication of the pathogen
Man and other animals: Man and other animals transport pathogen by carrying the spores in their hair or fur respectively, a process similar to pollination in various countries of the world; there are rules to safeguard the induction of foreign pathogens into our plants hence quarantine centers are established in our various airports and seaports. Plants are restricted from certain areas that are critical in transmitting pathogen, only some plants maybe imported into the country from a specified country and the plant must be screened by the quarantine service to ensure they do not carry a destructive pathogen.
The following factors influences the continuation of a disease
1. Optimum temperature
2. Optimum light
3. Sufficient moisture