Write short notes on the following:

The control measures of plant diseases may be divided into two main groups-

(a) Prophylaxis: It includes the protection of the host from exposure to the pathogen, from infection or from the environmental factors favourable to disease development.

(b) Immunization or disease resistance: It implies the improvement of resistance of the host to infection and to disease development. This method is generally used as a means of control by the development of strains of the host through hybridization, which are more resistant to one or more pathogens.

The different control measures to check the infection are as follows:

Eradication: Eradication is concerned with elimination of the disease agent after it has become established in the area of the growing host or has penetrated the host. Such measures include crop rotation, destruction of the diseased plants or alternate host plants, pruning, disinfectants and heat treatments.

Protection: It involves placing a protective barrier between the pathogen and susceptible part of the host. This can be accomplished by regulation of the environment, cultural and handling practices, control of insect carriers and application of chemical toxicants (poisons).

Host resistance and selection: Ideal control is normally provided by plant varieties with good genetic resistance to most common diseases. Resistant or immune varieties are critically important for low value crops in which other controls are unavoidable or their expense makes them impractical. Much has been accomplished in developing disease-resistant varieties of field crops, vegetables, fruits, turf grasses and ornamentals.

Among symbiotic fungi, those that enter into mycorrhizal relationships and those that enter into relationships with algae to form lichens are probably the best known. A large number of fungi infect the roots of plants by forming an association with plants called mycorrhiza. This association differs markedly from ordinary root infection, which is responsible for root rot diseases. Mycorrhiza is a non-disease producing association in which the fungus invades the root to absorb nutrients. Mycorrhizal fungi establish a mild form of parasitism that is mutualistic, meaning both the plant and the fungus benefit from the association. About 90 per cent of land plants rely on mycorrhizal fungi, especially for mineral nutrients and in return the fungus receives nutrients formed by the plant.

There are two main types of mycorrhiza- ectomycorrhizae and endomycorrhizae. Ectomycorrhizae are fungi that are only externally associated with the plant root, where as endomycorrhizae form their associations within the cells of the host.

Mycotoxins are toxic compounds that are naturally produced by certain types of moulds (fungi). Moulds that can produce mycotoxins grow on numerous foodstuffs such as cereals, dried fruits, nuts and spices. Mould growth can occur either before harvest or after harvest, during storage, in the food itself often under warm, damp and humid conditions. Most mycotoxins are chemically stable and survive food processing.

Several hundred different mycotoxins have been identified, but the most commonly observed mycotoxins that present a concern to human health and livestock include aflatoxins, ochratoxin A. patulin etc. Mycotoxins appear in the food chain as a result of mould infection of crops both before and after harvest. Exposure to mycotoxins can happen either directly by eating infected food or indirectly from animals that are fed contaminated feed, in particular from milk.

After the pathogen has landed on the plant surface, it has to penetrate, grow/ multiply and invade the plant cells or tissue for establishing a successful infection. Thus infection is the process of establishment of contact (by the pathogen) with host cells or tissue and procure nutrients from them. Infection thus includes penetration (entry), invasion (colonisation of plant) as well as growth in some (fungal pathogens) and reproduction in others (bacteria, mycoplasma, viruses, viroids).

Penetration : Pathogens penetrate plant surfaces in three different waysdirect penetration, through natural openings, or through wounds.

(i) Direct penetration through intact plant surfaces: This is perhaps the most common way of penetration in fungi and nematodes and the only method in parasitic higher plants. None of the other pathogens can enter plants in this way.

(ii) Through natural openings: Most fungi and bacteria enter plants through stomata, and some enter through lenticels, hydathodes and nectarthodes.

(iii) Through wounds: All bacteria and most fungi enter plants through various types of wounds. In nature, all viruses, viroids and mycoplasmas enter plants through wounds made by their vectors.

Viruses, viroids and mycoplasmas: They enter through wounds (caused by vectors or other means). Their entry depends on the deposition of their particles/ cells by their vectors in fresh wounds created at the time of inoculation. In most cases they are carried by specific vectors.

Nematodes: They penetrate directly by means of repeated back-andforth thrusts of their stylets. This finally creates an opening in the cell wall and the nematode sends its stylet into the cell or the entire nematode enters the cell. Some nematodes enter through stoma.

Parasitic higher plants: They also form an appressorium and penetration peg at the point of contact of the radicle with the host plant, and penetration is similar to that in fungi.

Fungi are non chlorophyllous, eukaryotic organisms. They are a large and successful group. They are universal in their distribution. They resemble plants because they have cell walls. But they lack chlorophyll which is the most important attribute of plants. They are ubiquitous in habitat which ranges from aquatic to terrestrial. They grow in dark and moist habitat and on the substratum containing dead organic matter. Mushrooms, moulds and yeasts are the common fungi. They are of major importance for the essential role they play in the biosphere and for the way in which they have been exploited by man for economic and medical purposes. The study of fungi is known as Mycology. It constitutes a branch of microbiology because many of the handling techniques used, such as sterilizing and culturing procedures are the same as those used with bacteria.

They have a definite cell wall made up of chitin. They exhibit heterotrophic mode of nutrition. They may be saprotrophic in their mode of nutrition or parasitic or symbiotic. They are usually non-motile except the subdivision- Mastigomy cotina. They reproduce mostly by spore formation. However sexual reproduction also takes place.

The asexual reproduction takes place by zoospore and conidiospore formation, which are formed inside the conidiosporangium. In this case beneath the epidermis the mycelium gives rise to compact clusters of erect conidiophores. These are formed on ends of short sympodially branching which arise from a mass of mycelium gathered in a limited area underneath the epidermis of host. They lie parallel to one another and perpendicular to the leaf or the stem surface. They are sporangiophores or conidiophore. The thin walled apical end of the sporangiophore enlarges and it contains dense cytoplasm with 5 to 6 nuclei. A depending constriction then appears below the swollen end. It results in the formation of the first conidiosporangium. Just below this another conidiosporangium is cut off in the same manner. This process may be repeated a number of times, forming a chain of conidiosporangia on the above side of each conidiophore.

The conidiosporangia are small smooth spherical multinucleate structures. They are arranged in a basipetalous sequence in the chain. The chains of conidiosporangia grow and press epidermis above. This causes the leaf surface to bulge out and the overlying epidermis eventually bursts over the growing conidiosporangial sorus. This results in the exposure of white conidiosporangial masses causing the characteristic pustules.

The conidiosporangium are now liberated in the presence of moisture by the disintegration of gelations pads. They are then blown away in the air by wind or washed away by rain water.

The conidosporangium at the high temperature and unfavourable periods behave like conidiospore and directly infect the host by the formation of germ tube. But in the presence of water and favourable periods the conidiosporangium behaves as zoosporangium. It multinucleate protoplast undergoes division and it divides to form a number of biciliate kidney shaped or heart shaped uninucleate zoospores. The sporangial wall ruptures and the zoospores are liberated. They swim in water for a while and finally they settle down in the host and round off. It germinates and each zoospore puts out a germ tube which enters into host through stomata. The germ tube within the host tissue develops into a mycelium.

Puccinia graminis tritici produces five different types of spores in its life cycle. These are –

Uredospore: The uredospores are borne on uredosporus. The uredosporus develops on wheat plant from the dikaryotic mycelium produced by germination of aeciospore, which comes from barberry plant generally through wind dissemination.

Teleutospore: At the end of wheat season, the uredosori also produce teleutospore along with uredospore. At that time the sorus is known as mixed sorus. The mixed sorus gradually converts solely into teleutosporus.

Basidiospore: It develops on germination of teleutospore. The teleutospore germinates by producing one germ tube from each cell. The germ tube has limited growth and is called promycelium, probasidium or epibasidium. The diploid nucleus passes inside the probasidium and undergoes meiosis to form four haploid nuclei, two (+) and two (-) type. The probasidium now becomes four celled by a transverse wall. Each cell then develops small projection, the sterigma through which nucleus and cytoplasm push outside and develop basidiospores of which two are (+) and two of (-) type.

Pycniospore: During favourable condition, the basidiospore germinates on contact with barberry leaflet towards the supper surface by producing germ tubes. The germ tube penetrates the epidermis and grows there intercellularly. The nature of the mycelium depends on the nature of basidiospore, either of+ or – type. Within few days, the growing mycelium becomes aggregated under the epidermis and forms a yellowish flask – shaped structure, called Pycnium or Spermatogonium. The bottom of the inner side of pycnium is lined by many uninucleate tapering cells, the spermatophores, which develop many small oval uninucleate cells, called pycniospores.

Aeciospore: The acciospores are unicellular, binucleate, thin-walled and orange in colour. The young aeciospores are polyhedral in shape, but becomes globose with maturity. They are dispersed by air current and can infect only the graminaceous host i.e., wheat plant.

In order for a disease to develop, a pathogen must be present and successfully invade plant host tissues and cells. The chain of events involved in disease development includes inoculation, penetration, infection, incubation, reproduction and survival.

Inoculation: This describes the introduction of the plant pathogen to the host.

Penetration : Wound sites and natural plant openings, such as stomata and hydathodes, facilitate the entrance of some plant pathogens, others have evolved unique mechanisms for direct penetration.

Infection: This occurs when the pathogen invades the plant tissue and establishes a parasitic relationship between itself and the plant. Viruses, bacteria and phytoplasmas are not able to actively penetrate or enter plant host tissues. Therefore they must rely on other methods to infect plant tissues and cells.

Incubation : Once inside the plant, pathogens may undergo an incubation period and remain latent for a period of time before initiating disease. Reproduction: Plant pathogens can reproduce sexually and asexually. It is dependent on the pathogen.

Survival: Plant pathogens have evolved so they can survive prolonged periods of unfavourable weather conditions. For example, brown spot is a fungal pathogen that produce spores that are dark in coloration which reduces the amount of UV light penetrating and preventing cell death.

Knowing and understanding the disease cycle for a particular disease is very helpful in managing the disease.

The most common method of plant disease control in the field and in greenhouse and sometimes in storage is through the use of chemical compounds. The application of chemicals to plants for control of insects and fungi has been practised for many years. Sulphur may have been used as a fungicide in ancient times followed by copper, arsenic, zinc and mercury compounds within the last more than 150 years or so. The production of plant protection chemical has made great strides since the mid-1930’s when organic fungicides were developed, particularly so since World War II. Chemicals used in plant protection are termed as pesticides, which include fungicides, bactericides, nematicides, insecticides, acaricides, rodenticides, herbicides and so on. Most of the chemicals are used to control diseases of the folliage and other above ground parts of plants. Others are used to disinfect and protect seeds, tubers and bulbs from infection. Some are used to disinfect the soil, others to disinfect warehouses, to treat wounds, or to protect stored fruit and vegetables from infection. Still others (insecticides) are used to control insect vectors of some pathogens.

The great majority of chemicals applied on plants or plant organs act as protectants which can protect them from infection and can not stop or cure a disease after it has started. Most of them are effective only in the plant area to which they have been applied (local action) and are not absorbed or translocated by plant. Some chemicals have therapeutic (eradicant) action and several are absorbed and systemically translocated by plants (systemic fungicides and antibiotics).