Plants make up the majority of earth's living matter. All
animals, including humans, depend on plants for survival. Hence humans developed
a fundamental interest in plant health. The science of plant pathology deals
with the identification and treatment of factors affecting plant welfare.
The origin of human interest in plant pathology dates back to the early days
when our ancestors foraged the plains for nutrients as nomads, since diseased
plants usually produce less fruit. The field of plant pathology itself is
very wide and ranges from fundamental basic research to applied assistance
to farmers in the field. It covers abiotic stress factors like humidity and
salinity as well as biotic stress caused by the action of animals, especially
humans. In a more narrow sense, however, plant pathology deals with disease
caused by the action of ever present microorganisms, like viruses, bacteria,
and fungi.
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| Wheat harvest in the Midwest http://waltonfeed.com/rahn/farming.html | Release of massive amounts of teliospores
of the smut fungus Ustilago nuda during wheat harvest http://www.apsnet.org/education/LessonsPlantPath/ StinkingSmut/text/fig5.htm |
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| Apple orchard in Upstate New York http://www.nyapplecountry.com/empireorchard.htm | Apple orchard infested with Erwinia amylovora, the causative agent of fire blight http://www.canr.msu.edu/vanburen/fbpicts.htm |
Fungi 
Fungi belong to the higher Eukarya. They combine features typical for plants and animals, but represent a separate kingdom within the tree of life. Historically they were considered more closely related to plants, but modern taxonomy places them closer to animals. Fungi are most widely known as typical destructive agents, being responsible for the decomposition of organic matter. Apart from this 'saprophytic' life style (growth and propagation on already dead material), fungi have established a number of interactions with other organisms. These interactions range from mutualistic symbiosis (mycorrhiza, lichens), via co-habitation without symptoms (endophytes) to parasitic relationships (necrotrophic and biotrophic).
| The tree of life from Alexopoulos et al (1996) Introductory Mycology, 4 ed. John Wiley & Sons, Inc., New York |  |
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| Close up of brown rot on wood. The wood cracks and checks into cubicle pieces. Little to no integrity remains. http://forestpathology.coafes.umn.edu/microbes.htm | Cross section of an oak tree with white rot The fungus has decayed the sapwood and dark heartwood turning it white. http://forestpathology.coafes.umn.edu/microbes.htm |
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| Ectomycorrhiza http://www.biology.usu.edu/biol5410-kropp/mycorrhizae.htm | Arbuscle of a VA-Mycorrhiza http://cgee.hamline.edu/see/ questions/dp_interliving/dp_ inter_mycorrhiza.htm |
Lichens http://www.gregorymarderos. com/pages/lichens.htm |
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Endophytic Stagonospora
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Botrytis cinerea on grapes of Vitis vinifera http://www.wine4you2.com/ Lexikon/Suchbegriffe/ Botrytis.htm |
Puccinia recondita uredia on a wheat leaf http://www.arc.agric.za/institute /sgi/ main/intro/diseases.htm#leafrust |
Some fungal pathogens colonize a plant and subsequently kill their host quickly through the action of degrading enzymes and/or toxins. This lifestyle is called necrotrophic (e.g. Botrytis cinerea). Other fungi have to rely on a living host to complete their life cycle. These are called obligate biotrophic (e.g. Uromyces fabae). There is a smooth transition between the two extremes which is characterized by a number of pathogens classified as hemibiotrophes. Hemibiotrophic fungi, like Colletotrichum spp., are typified by a more or less extended biotrophic phase, before switching to necrotrophic growth and killing of their host.
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| Botrytis cinerea Conidiophore with conidia http://www.vscht.cz/obsah/fakulty /fpbt/ostatni/miniatlas/bot.htm |
Uromyces fabae Spore with appressorium ME nicht veröffentlicht |
Colletotrichum spp. Germinating conidia
http://www.botany.hawaii.edu/faculty /gardner/biocontrol/Miconia/Colletotrichum% 20germinating%20conidia.jpg |
Biotrophy
Some of the most serious fungal plant pathogens world wide are obligate biotrophic parasites. In order to mark off the true obligate biotrophic fungi from hemibiotrophs or necrotrophs we suggest the following criteria: a) highly differentiated infection structures; b) limited secretory activity; c) a narrow contact zones separating fungal and plant plasma membranes; d) long term suppression of host defense responses; e) the formation of haustoria (Mendgen, K. W., and Hahn, M. (2002) Trends Plant Sci. 7:352-356).
The application of these criteria narrows down the range of obligate biotrophic fungi to the rust – and the powdery mildew fungi. However, another group of organisms, evolutionary remote from the true fungi, also form haustoria, the downey mildews (Kingdom: Chromista, Phyllum: Oomycota). Given the broad phylogenetic spectrum of haustoria-forming organisms it seems more than likely that these structures have arisen more than once in the course of evolution. Extending this idea to the structurally similar arbuscules produced by arbuscular mycorrhizas such an apparatus seems to represent a particularly successful adaptation of these organisms to their interactions with their respective host plants.
Early infection structures of biotrophic fungi:
A germ tube (GT) emerges from an urediospore (S) attached to the host by an adhesion pad (P). After recognition of the guard cell lip, an appressorium (A) develops over the stomatal pore. The penetration hypha (PE) penetrates into the substomatal chamber and elongates into an infection hypha (IH). When the tip of the infection hypha contacts a host cell wall, a haustorial mother cell (HM) is formed from which the haustorium (H) invades the host cell. Unique features of the dikaryotic haustorium are the dark-staining neckband (NB) around the haustorial neck and the interfacial, extrahaustorial matrix (yellow) surrounded by the extrahaustorial membrane (EHM). After forming the first haustorium, the infection hypha branches and further intercellular hyphae, haustorial mother cells (HM) and haustoria are formed
hemibiotrophic Colletotrichum spp.
The spore (S) forms a short germ tube, which differentiates into an appressorium
(A). The penetration hypha (PE) develops, transforming internal pressure into
mechanical force to pierce the cuticle and the cell wall. The penetration
hypha swells to form a vesicle (V) and broad primary hyphae (PH), which are
surrounded by the invaginated plant plasma membrane. The host protoplast remains
alive during the biotrophic stage (1) and an interfacial matrix separates
the protoplasts of fungus and host (yellow). One or two days after penetration,
plant plasma membrane disintegration starts (2). As new host cells are colonized
by primary hyphae, the sequence of a transient biotrophic phase followed by
cell killing is repeated (3)
(from Mendgen, K. W., and Hahn, M. (2002) TiPS 7:352-356)
Haustoria are specifically differentiated hyphae breeching the plant cell wall and penetrating into the host cell. These structures have generated the interest of plant pathologists ever since their discovery by the Italian botanist Giovanni Antonio Maria Zanardini about 150 years ago. The name was introduced the german physician Heinrich Anton de Bary, and is derived from the Latin word haustor, which means the pail. This illustrates that the structure and location of these haustoria already suggested a function in nutrient uptake to researchers at that time.
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Transmission electron micrograph image of a haustorium (Szabo & Bushnell. 2001. PNAS USA 98:7654-5; left)and schematic representation of the ultrastructural details (right).
However, knowledge about this key element of the obligate biotrophic life style is still fairly scarce. The main reasons for this are the preserving lack of functional stable transformation systems for haustoria forming fungi and the fact that these organs are not formed in culture. This excludes them from the application of many molecular techniques successfully in use for other systems.
This is the point to switch to PROJECTS, since the majority of our research is centered around the elucidation of the role of haustoria.