Host pathogen interaction

Host pathogen interaction

In pathogenesis, relation between pathogen and host factors is an important phenomenon. This relationship determines the intensity of the disease. In addition with this the influences of environmental factors are also effective. For the process of establishment of disease the following steps are required.

Host + Pathogen + Environment → Host-Pathogen complex →Pathogenic interactions

When a pathogen spread to and affects may individuals with in a population over a relatively large area and within a short time, the phenomenon is called epidemic. An epidemic is simply defined as the increase of disease in defined as the increase of disease in a population. For epidemism the necessary factors must be exists in their optimum form. The overall factors of pathogenesis are classified into 3 categories-

(i) Host factors, (ii) Pathogen factors, (iii) Environmental factors.

(i) Host factors :-

Several internal and external factors of a host plant play important roles in the development of disease with in the host.

a) Level of genetic resistance:-

Host plants carrying vertical resistance do not allow a pathogen to become establish with in them and thus no epidemic can develop. Host plants carrying horizontal resistance will probably become infected but the rate at which the disease and the epidemic will develop depend on the resistance and the environmental conditions. In the presence of virulent pathogen and favourable environment susceptible host favour the development of disease epidemics.

b) Types of crop :-

In disease of annual crops such as corn, vegetable, rice, cotton, fruit etc. Generally develops more rapidly than the diseases of branches and stem of perennial woody crops. Herbaceous stems are also favours the development of disease.

c) Age of host plant :-

Plants change in their susceptibility in disease with their age. In some plant pathogen interaction like Pythium and root rots Downy Mildews, Rust, Bacterial Blights and viral infections, the growth period and become resistant during the adult period. In some cases such as late blight of potato (caused by Phytophthora infestans) and early blight of tomato (caused by Alternaria solani) the plants remain susceptible during the growth period and become relative resistance in the early adult stage and again become susceptible after maturity.

(ii) Pathogen factors :-

Some pathogen factors also play an important role in the development of plant disease. These are-

a) Levels of virulence :-

Virulent pathogens are capable of rapid infection of the host than the pathogens of less virulent.

b) Quantity of inoculum :-

The greater number old inoculums with in or near fields of host plant the more inoculums reaches the host and thus there is a chances of appearance of an epidemic.

c) Mode of spread of pathogen :-

Pathogens are spreaded by several way, like spores, vegetative parts etc. the spores of many pathogenic fungi like rust fungi, mildew causing fungi, leaf spot causing fungi etc are released into the air and can be dispersed by air current from a short distance or a long distance. This kinds of pathogen are responsible for the frequent and the wide spread epidemic. Many pathogens are dispersed through aphids, white flies, leaf hopper, and other insects. Their pathogenicity depends on the factors of vectors also.

d) Type of reproduction of the pathogen :-

All pathogens produce many offspring but some of them produce more offspring others. An important fact is that, some pathogens have short reproductive cycle and therefore can produce many generations in a single growing season. These ate the polycyclic pathogens and leaf spot. For this these pathogens have the capability to cause wide spread epidemics in a single season.

(iii) Environmental factors :-

The role of environmental factor is also important in the development of disease. The most important environmental factors that affect the development of plant disease epidemics are as follows-

a) Moisture :-

Abundant prolonged or repeated high moisture is the dominant factor in the development of most diseases caused by fungi, bacteria and nematodes. In case of fungal diseases moisture facilitates the production of spores and their dispersal. For example high moisture content in the soil favours the dispersal of zoospores of Synchytrium endobioticum (causal organism of wart disease of potato). On the contrarary condition. Eg. Fusarium.

b) Temperature :-

Diseases are sometimes favoured by the temperatures higher or lower than the optimum for the plant because they reduce the plants level of horizontal resistant. Low temperature reduces the amount of inoculums of fungi bacteria and nematodes but they survive in cold condition. High temperature reduces the amount of inoculums of viruses where as they are survived at hot ewnvironment.

c) Light :-

There is a little effect of natural light up to four thousand to 8000 A° on spore germination. However intense light may cause heating and inhibit germination of spores. In prolonged bright light a number of fungal spores are survived. Eg- Alternaria, Cladosporium. However the oospores of Plasmopara viticola, spores of Colletotrichum lindemuthianum, uredospores of Puccinia dispersa germinate equally well in light and dark.

On the other hand, germination of uredospores of Puccinia graminis tritici is checked by day light and germinates more abundantly and rapidly in dark.

d) pH :-

Most of the fungal spores germinate well under slightly acidic conditions, almost at a range of pH 3.0-8.0. However, there are some fungi which have an alkaline optimum pH for their colonization. pH is not considered to be an important factor in limiting spore germination because pathogen may change the pH of its habitat.

e) O₂ and CO₂ :-

Fungi are aerobic organisms and hence require an O₂ concentration up to 50% for its spore germination of spore germination. For the germination of spores of Puccinia graminis tritici, Botrytis cineri, and Ustilago zeae 30-38% of O₂ seems to be sufficient.