Pathogenic agents . A pathogenic biological agent is an element or medium capable of producing some type of disease or damage in the body of an animal , human being or plant , whose conditions are predisposed to the aforementioned occasions.
Agents that cause plant diseases are characterized as infectious (biotic or living) and noninfectious (abiotic or nonliving). Infectious agents include bacteria , fungi , mycoplasmas , nematodes , and viruses . Noninfectious agents include nutritional imbalances, environmental stress , and chemical toxicity caused by pesticides and air pollutants .
The most common pathogens in plants are fungi, although bacteria and nematodes are also important. Diseases caused by mycoplasmas and viruses are not often recorded, mainly because they are very difficult to detect.
Fungus
Fungi are simple filamentous unicellular organisms . They do not have chlorophyll and depend on a host plant for food . They are larger than bacteria and are more easily identified. Some of the structures they produce can be seen with the naked eye and are useful for identifying them.
Fungi attack susceptible host plants through the movement of their reproductive structures, such as sclerotia and spores . Spores are easily disseminated by mechanical means and by wind and water. In addition, fungi are easily transferred in pots with contaminated soil and on plants or plant parts that harbor the pathogen.
Fungicides are used to control fungal diseases. There are specific and broad spectrum fungicides, contact fungicides and systemic fungicides (which are translocated through the interior of the plant). The host’s nutrition, temperature , humidity and safe use of pesticides must also be managed in order to achieve an integrated control of diseases.
Bacteria
Plant pathogenic bacteria are unicellular, microscopic and do not produce spores . They cannot produce their own energy and depend on a host plant for sustenance. There are five main genera: Agrobacterium , Corynebacterium , Erwinia , Pseudomonas and Xanthomonas . Most plant diseases are caused by the last three.
Bacteria enter plants through natural openings such as stomata and hydathodes or through wounds . They survive inactively in plant tissue
for a time and affect the plant when environmental conditions are favorable. Bacteria are transferred from one plant to another by mechanical means (on hands and cutting instruments) and are commonly spread in splashing water. Some may be spread by nematodes and insects .
An important characteristic of bacterial diseases is that they require the presence of moisture on the plant surface to begin the infection process and disease development.
The use of commercially available bactericides for plant disease control is generally sparing, and they can be phytotoxic in some crops. Therefore, cultural control is very important in the management of bacterial diseases.
Virus
Viruses are the smallest organisms known as plant pathogens. They are very simple in structure and usually consist of a single strand of DNA (deoxyribonucleic acid) or RNA (ribonucleic acid) with a protein coat .
They are not capable of developing outside the host plant and act by interfering with the normal production of materials in the nucleus of the host cells. Viruses are spread mostly by mechanical means and insects and are disseminated in propagules of plants that are infected. There are no commercially effective chemicals for controlling diseases caused by viruses. Control must be based on sanitation, removal of diseased plants and control of insect vectors.
Nematodes
Nematodes are the largest organisms causing plant diseases. They are non-segmented roundworms and are usually microscopic, although some can be seen with the naked eye. Nematodes move to plant roots or can be spread in pots, soil or water and on parts of the plant that are contaminated.
Most plant pathogenic nematodes feed on plants by penetrating the root surface and absorbing the cell contents. Some travel from one location to another in the root to feed, while others, such as root-knot nematodes, attach to a specific location on the root permanently and feed and reproduce there. Many insecticides-nematicides have been developed to aid in the control of plant parasitic nematodes.
Identification and diagnosis
There are many factors that influence the development of diseases. However, diseases cannot occur unless the following elements are present: a susceptible plant, a pathogen, and a favorable environment. Some diseases do not occur if there is no vector to transmit them, which is common with diseases caused by viruses, which can be transmitted by insects or nematodes.
Diagnosis of diseases cannot be made based on symptoms alone, although some generalizations can be made. Symptoms caused by infectious agents ( fungi , bacteria , viruses , nematodes ) and non-infectious agents (nutrient deficiencies, toxicities, excess or shortage of water, environmental contaminants, soil acidity or alkalinity) are similar.
An accurate diagnosis can only be made after evaluating the affected plant by direct observation or by culturing the pathogens on specific media.