SECTION 1:   Disease Cycle is the chain of events that leads to the development and perpetuation of disease or it is that part of the life cycle of a pathogen spent in conjunction with the host.  Following is the major steps in the disease cycle:
 Inoculation
 Penetration
 Infection
 Invasion
 Growth and reproduction of pathogens
 Dissemination of the pathogen
 Overwintering/oversummering of pathogens
We will now look briefly at each of the stages in the disease cycle.
Inoculation:  Go to the glossary of your book and find the definitions of inoculum, primary inoculum, and secondary inoculum.  We will now consider sources and delivery of inoculum to the host plant.

 Sources of inoculum may involve seed, transplants, plant debris (from previous crop), resevoir weed hosts, etc.  Inoculum may reach the host plant by wind blown inoculum, rain or aerosol droplets, irrigation water, insect vectors and other biotic vectors.

Assignment 1:  Read the section in your textbook beginning on page 46 on pre-penetration phenomena and continue through page 48.

Penetration:  Generally, there are three main avenues of entrance into the host plant-direct penetration, wounds and natural openings.  The mechanism varies with the pathogen-some employing only one mechanism, while others may employ more than one mechanism.  Some fungi have the ability to penetrate the host by direct penetration or through wounds and natural openings.  We will look further into direct penetration in the succeeding paragraph.  Bacteria and viruses are passive and generally require wound, natural openings or vectors to accomplish entrance.  Nematodes enter by direct penetration and occasionally through natural openings.  As I mentioned in class on Tuesday, penetration does not necessarily lead to infection.  Studies have shown that pathogens may penetrate non-hosts, but be unable to establish a food relationship.
 Direct penetration through intact plant surfaces is a common mechanism employed by many fungi.  There are two ways that fungi penetrate directly.  One is by a fine hyphae produced by a spore or mycelium or an infection peg produced from an appressorium (refer to figure 2-4 on page 49 and figure 2-8 on page 51)  The fine haphae and appressorium forms when the mycelium or germ tube from a germinating spore comes into contact with the plant surface.  The fine hyphae penetrates the cuticle and the enzymatically softened cell wall by mechanical means. The more common feature with fungi is to produce an appressorium from which emerges an infection peg that penetrates the cuticle and cell wall.
 Many organisms utilized wounds for entrance into the hosts. As I mentioned previously, bacteria and viruses often gain entrance through wounds.  Wounds may be created by feeding action of insects, nematodes, pruning, cultivation equipment, etc.
 
 Many fungi and bacteria utilize natural openings to gain access.  Stomata, hydathodes (water pores), lenticels and rifts in  the root cortex  as a result emergence of secondary roots are examples of natural openings.  The black rot bacterium, Xanthomonas campestris p.v. campestris,  is the most important worldwide disease of crucifers.  Its usual method of spread is through infected seed.  This bacterium enters through hydathodes at the leaf margin and progresses inter and intracellular and eventually reaches the vascular system becoming systemic.  Symptoms begin at the point of entrance and spread producing characteristic v-shaped chorotic lesions with blackened veins within the chlorotic area.  This organism does not use stomata for invasion.  A closely-related organism, Xanthomonas campestris p.v. armoraceae,  invades stomata and not hydathodes resulting in a different characteristic symptomotology "leaf spots", hence the disease is referred to as Xanthomonas leaf spot.
Infection:  This is the next step in the disease cycle and is the process where the pathogen has not only penetrated, but has established a food relationship with the host and is able to extract nutrients from the host.  There is usually a lag phase between successful infection and the development of symptoms that may only be measurable biochemically.  Some pathogens may produce no symptoms, in which case we refer to that as latent.  An example illustrating latency can be found in strawberries in Louisiana.
It was determined by indexing that most of the commercial strawberries in Louisiana were viral infected, but with latent symptoms. When known virus-free plants were compared to those with latent virus symptoms, a marked increase in yield was demonstrated in virus free treatments.  Sometimes the time of year determines symptoms or latency.  An example illustrating this may be seen with TMV (tobacco mosaic virus) in tomatoes.  In the spring crop symptoms may be very mild or absent completely, while the same seed lot used in the fall may result in a crop with severe virus symptoms.
 During infection some pathogens cause severe damage and often death to the host, while others extract nutrients without killing the cells, at least for long periods of time.  Pathogens utilize several weapons such as enzymes, toxins, growth hormones, etc.
At the same time, the host responds with an array of defense mechanisms that may be structural or biochemical.
 Another important concept to remember is that pathogens usually are composed of races that differ in virulence, thus impacting particular varieties differently.

 Invasion:  Following infection pathogens invade hosts in different ways.  Some grow along the surface and send haustoria as in the powdery mildews or in the case of the scab diseases grow only in the area between the cuticle and the epidermis.  In most fungi, the mycelium grows intercellularly and intracellularly as the organism invades.
    Bacteria mainly invade intercellularly, but as the cell walls degrade may move intracellularly.  Nematodes invade intercellularly and sometimes intracellularly, while some are ectoparasitic and feed by piercing epidermal cells with their dagger-like stylets.
    Viruses, viroids, fastidious vascular bacteria invade by cell to cell intracellular movement.  When studying invasion by these organisms, one finds that viruses and viroids invade all types of plant cells and tissues, while mycoplasma and spiroplasmas invade phloem sieve tubes and the fastidious vascular bacteria invade xylem vessels and on some occasions, phloem sieve tubes.
    Invasion may be restricted by the host giving local lesions (viruses) or leafspots or lesions (roots).  Invasion in many hosts may be systemic where spread is from an initial foci and then spread or invades most or all of the tissues of the plant.
Growth and Reproduction:  As the pathogen invades tissues of the plant, generally growth is occurring whether by fungal vegetative hyphae branching and growing from hyphal tips, or viruses and viroids replicating producing progeny particles, or bacteria increasing in numbers rapidly by fission or nematodes producing eggs from which the infective juveniles are produced.  Plant pathogens are varied in their reproduction strategies (See Figure 1-3 on page 7, Agrios text).  Fungi reproduce by a variety of spores both sexual and asexual and as previously mentioned, viruses produce progeny virions by replicating in the cell, nematodes produce eggs.

Dissemination of the Pathogen:  This step in the disease cycle ensures the movement or secondary spread of the pathogen.  Previously we differentiated between primary inoculum and secondary inoculum.  The extent of spread may be as few as a few millimeters with nematodes in soil to long distance across an entire continent with others.
Following are the major ways that plant pathogens are disseminated:
    Dissemination by air
    Dissemination by water
    Dissemination by insects, mites, nematodes or other vectors
    Dissemination by seed, transplants, budwood and nursery stock
    Dissemination by man
    Dissemination by air:  This is the primary mechanism of dissemination with fungi.  The extent of spread of fungi depends on how fragile or hardy are the spores.  Fungi with fragile spores are limited in their spread to a few hundred or thousand meters.  In the cereal rusts, this is not the case, spores are quite hardy and may be windblown over large areas, including entire continents.  Air dissemination is not frequent with other pathogens, but occasionally bacteria, nematodes, viruses carried by their vectors may be moved by wind currents.

Other modes of dissemination will be covered in class on Tuesday, September 11.

Assignment 2: Go to apsnet.org and click on Education Center. Next click on Teaching Resources sidebar and go to Introductory Plant Pathology Resources and scroll to Selected ASPNet feature articles. Here scroll to “The Future of World Food Situation and the Role of Plant Diseases” or use the Url ==== www.apsnet.org/education/feature/Food Security/Top.html

Check Lesson 2 through Friday September 7.