Lecture Notes

Lecture Notes For Level-3 Semester-2 Prof. Dr. Md. Rashidul Islam Department of Plant Pathology Bangladesh Agricultural University Mymensingh

A plant parasite

An organism that lives on or in some other organism and obtains its food from the latter is called a parasite. A plant parasite A plant parasite is an organism that becomes intimately associated with a plant and multiplies or grows at the expense of the plant. Obligate parasite or Biotrophs

The parasites that can grow and reproduce in nature only in living hosts are called obligate parasites. e.g. Viruses, viroids, mollicutes, some fastidious bacteria, protozoa, and fungi causing downy mildews, powdery mildews, and rusts, are biotrophs. Necrotrophs The parasites that can live and thrive well on only dead organic matter are called necrotrophs.

Saprophyte

Saprophyte An organism that uses dead organic material for food. Semibiotrophs/ Facultative saprophytes Some nonobligate parasites live most of the time or most of their life cycles as parasites, but, under certain conditions, may grow saprophytically on dead organic matter; such parasites are semibiotrophs and are called facultative saprophytes. Facultative parasites

Some necrotrophs live most of the time and thrive well on dead organic matter (necrotrophs) but, under certain circumstances, may attack living plants and become parasitic; these parasites are called facultative parasites.

Pathogenicity

Pathogenicity Parasitism is intimately associated with pathogenicity, i.e., the ability of a pathogen to cause disease where the ability of the parasite to invade and become established in the host generally results in the development of a diseased condition in the host. Parasitism The relationship between host and parasite in which a parasite obtains food from its host is called parasitism. Plant Parasitism

The relationship between plant and parasite in which removal of nutrients and water from the host plant by the parasite takes place that usually reduces efficiency in the normal growth of the plant and becomes detrimental to the further development and reproduction of the plant is called plant parasitism.

Pathogenesis

Pathogenesis The word “Pathogenesis” comes from the Greek pathos, "disease", and genesis, "creation". The entire chain of events leading to disease development.

The sequential chin of events leading to development of a disease during a compatible interactions between host and pathogens through a series of changes in the structure and /or function of a cell/tissue/organ being caused by a pathogen. The sequence begins with initial contact between the pathogen and host and ends when the pathogen is no longer associated with that host (i.e. when the host/pathogen dies or the pathogen moves to another host).

The pathogenesis of a disease is the mechanism by which an etiological factor causes the disease.

Chain of events

Chain of events The primary events of pathogenesis or in a disease cycle are: 1. Inoculation 2. Penetration 3. Establishment of infection 4. Colonization (invasion), growth and reproduction of the pathogen, 5. Dissemination of the pathogen, and 6. Survival of the pathogen in the absence of the host, i.e., overwintering or oversummering (overseasoning) of the pathogen.

In some diseases there may be several infection cycles within one disease cycle.

1. Inoculation is the initial contact of a pathogen with a site of plant where infection is possible. The

1. Inoculation is the initial contact of a pathogen with a site of plant where infection is possible. The pathogen(s) that lands on or is otherwise brought into contact with the plant is called the inoculum. The inoculum is any part of the pathogen that can initiate infection.

For example, in fungi the inoculum may be spores, sclerotia (i.e., a compact mass of mycelium), or fragments of mycelium. In bacteria, mollicutes, protozoa, viruses, and viroids, the inoculum is always whole individuals of bacteria, mollicutes, protozoa, viruses, and viroids, respectively. In nematodes, the inoculum may be adult nematodes, nematode juveniles, or eggs. In parasitic higher plants, the inoculum may be plant fragments or seeds.

Attachment of pathogen to host

Attachment of pathogen to host Pathogens such as mollicutes, fastidious bacteria, protozoa, and most viruses are placed directly into cells of plants by their vectors and, in most cases, they are probably immediately surrounded by cytoplasm, cytoplasmic membranes, and cell walls.

However, almost all fungi, bacteria, and parasitic higher plants are first brought into contact with the external surface of plant organs. Before they can penetrate and colonize the host, they must first become attached to the host surface(Figs.2-3–2-6).

Attachment takes place through the adhesion of spores, bacteria, and seeds through adhesive materials that vary significantly in composition and in the environmental factors they need to become adhesive.

Spore Germination and Perception of the Host Surface

Spore Germination and Perception of the Host Surface It is not clear what exactly triggers spore germination, but stimulation by the contact with the host surface, hydration and absorption of low molecular weight ionic material from the host surface, and availability of nutrients play a role.

Spores also have mechanisms that prevent their germination until they sense such stimulations or when there are too many spores in their vicinity. After attachment of the propagule to the host surface, as spores and seeds germinate, germ tubes also produce mucilaginous materials that allow them to adhere to the cuticular surface of the host, either along their entire length or only at the tip of the germ tube.

In regions of contact with the germ tube, the structure of the host cuticle and cell walls often appears altered, presumably as a result of degradative enzymes contained in the mucilaginous sheath.