Pathogen-Home

Plant pathogenic organisms are viruses, viroids, bacteria, mollicutes, fungi, nematodes and plant parasitic higher plants.

Pathogens are identified by morphology (shape) and size of reproductive structures. Some reproductive structures also serve as survival structures that reside in an array of sites. If pathogens successfully infect a host, varying types and degrees of symptoms develop during the life of the host plant. Symptoms are changes in plant appearance that offer clues to cause of abnormal plant health. Symptoms are accompanied frequently by signs, pathogen structures, of plant disease.

Characteristics of Plant Pathogenic Bacteria - Agrios pp. 407-415

Morphology	Extremely small, one-celled, prokaryotic microorganisms. Exception: filamentous actinomycetes.
Reproduction	Cell division by binary fission. Bacteria reside in spaces between plant cells or vascular system. Plant pathogenic bacteria do not form spores.
Sites of pathogen survival	Plants, seeds, storage organs, plant debris (original host plant), equipment and in the bodies of insect vectors.
Symptoms caused	Slimy soft rots, leaf spots, leaf and stem blights, decay of fruits, pods and tubers, stem cankers, vascular wilts and galls.
Signs of disease	Bacterial ooze from symptomatic tissue.

Characteristics of Plant Pathogenic Fungi - Agrios pp. 245-259

Morphology	Eukaryotic microbes that form filamentous structures called hyphae. In mass, the hyphae are referred to as a mycelium. An array of spores and spore bearing structures are formed and are the basis of taxonomic classification of fungi.
Reproduction	Spores formed sexually (meiosis) or by an asexual (mitosis) process. Spores are produced on hyphae or in specialized reproductive spore bearing structures called sporocarps. Other specialized asexual reproductive structures are formed. Fungi reside in spaces between plant cells or in plant cells.
Sites of pathogen survival	Dormant survival propagules form in colonized host debris, soil, vectors, seeds and as active pathogens in living plants.
Symptoms caused	Leafspots, blights, chlorosis, vascular wilts, root rots, cankers.
Signs of disease	Signs are reproductive structures of the pathogen.

Characteristics of Plant Pathogenic Oomycetes - Agrios pp. 266-270

Morphology	Eukaryotic microbes that form filamentous structures called hyphae. In mass, the hyphae are referred to as a mycelium. Oomycetes reproduce sexually by oospores and asexually by zoospores formed in sporangia. Zoospores are motile one celled spores that are a product of mitosis and are analogous to conidia formed by fungi in form and function. Reproductive structures the basis of taxonomic classification of Oomycetes.
Reproduction	Other specialized asexual reproductive structures are formed. Oomycetes reside in spaces between plant cells or in plant cells.
Sites of pathogen survival	Dormant survival propagules form in colonized host debris, soil, vectors, seeds and as active pathogens in living plants.
Symptoms caused	Leafspots, blights, vascular wilts, root rots, cankers.
Signs of disease	Signs are reproductive structures of the pathogen.

Characteristics of Plant Pathogenic Mollicutes - Agrios pp. 457-462

Morphology	Prokaryotic, pleiomorphic microbes that lack a cell wall. Phytoplasma and spiroplasma are most common plant pathogenic mollicutes.
Reproduction	Cell division by binary fission. Mollicutes reside in phloem tissue.
Sites of pathogen survival Sites of pathogen survival	Perennial plants serve as hosts and sources of inoculum. Mollicutes are transmitted by many species of leafhoppers.
Symptoms caused	Chlorosis, stunting, brooms, distortion of flowers, necrosis.
Signs of disease	Cells of mollicutes are observed in host tissue, but not observed outside host tissues.

Characteristics of Plant Pathogenic Viruses - Agrios pp. 479-490

Morphology	Submicroscopic molecules, consists of nucleic acid (RNA or DNA) surrounded by a protein coat.
Reproduction	Viruses are obligate parasites that take control of host cells to replicate new virus particles (virions).
Sites of pathogen survival	Viruses survive in living plants, especially perennials, vectors (organisms that transmit viruses) and seed.
Symptoms caused	Yellows, mosaics, stripes, anthocyanescence and stunting.
Signs of disease	Virus virions are observed via electron miscroscopy.

Characteristics of Plant Pathogenic Viroids - Agrios pp. 556-560

Morphology	Submicroscopic molecules, consists of nucleic acid RNA only; a protein coat is absent.
Reproduction	Viroids are low molecular weight RNAs that take control of host cells to replicate new RNA.
Sites of pathogen survival	Viroids survive in living plants, especially perennials, outside of hosts for varying lengths of time. Most hosts are perennial or vegetatively propagated annuals.
Symptoms caused	Yellows, mosaics, stripes, anthocyanescence, necrosis, plant decline, distorted flower and fruit, and stunting.
Signs of disease	Virus virions are observed via electron miscroscopy.

Characteristics of Plant Pathogenic Nematodes - Agrios pp. 565-571

Morphology	Microscopic round worms (non-segmented) that are closely related to parasitic roundworms in animals. Member of some genera change shapes during maturation.
Reproduction	Mature females form eggs. Reside outside or inside plant tissues.
Sites of pathogen survival	Eggs and larvae are present primarily in soil, but may also survive in living host tissues, especially perennial hosts and seed.
Symptoms caused	Stunting, chlorosis, lesions on roots, galls on roots, general decline in plant health.
Signs of disease	Nematode larvae and eggs are microscopically observed within and outside host tissues.

Characteristics of Plant Pathogenic Plants - Agrios pp. 471-478

orphology	Sometimes small, but macroscopic plants. All form stems, flowers and seed, but all lack roots; May or may not form leaves (chlorophyll).
Reproduction	Flowers and seeds; conductive system of the parasite penetrates and connects to the vascular system of the host.
Sites of pathogen survival	Seed of parasitic plant may be present in soil or mixed with seed of host plant. Perennial host plants infected by parasitic plants.
Symptoms caused	Brooms and stunting.
Signs of caused	Stems, leaves, flowers and seeds of parasitic plant.