Introductory Plant Pathology

Class 32

Environmental Effects on the Pathodeme and Plant Disease

Objectives for Today's Class    
Reference: Agrios Chapter 6    

Concept:

Pathodeme refers to the observation, anecdotal and scientific, that diseased tissue exhibits metabolic activity that is more that additive of host and parasite contributions. Thus when considering "disease" one should be constantly cognizant of the complexities involved. If one ponders the concept of the disease triangle with greater than a superficial perusal, one is drawn to the inevitable conclusion that the Environment is an equal player in disease. There is nothing in the disease triangle concept that precludes the environment from, in and of itself, being causal in the disease. With only the most rudimentary observation of plant culture it becomes obvious that an adverse environment may be solely responsible for a plants demise.

In consideration of the usual interpretation of the disease triangle, one must consider the effects of environment on both the host and the parasite as well as on the pathodeme. When the environment has been considered as effecting/affecting the host and parasite, various characteristics have been elucidated which have helped to classify diseases as "high temperature", "water Mould", etc. The environment then becomes recognized for it's post-infectional role as a modulator of the host-parasite interaction by placing constraint son each of the interactants individually, as well as on the pathodeme.

While this discussion uses temperature as the primary environmental example, it is well documented that drought, anoxia, heavy metals, light quality and quantity, pesticides and many other external environmental stimuli can, and do, activate the stress response mechanisms of all organisms. The point here is that the environmental stress is recognized by, and responded to, both host and parasite; and that the threshold level(s) for induction may be the same, or different, for each.

Effect of temperature

Now that there is an understanding of response of all living entities to Hyperthermal stress through the expression of the "heat shock response" and knowing the plants and their resident parasites will undergo diurnal temperature (and other) fluctuation; what/how will this affect/effect disease/resistance parasitism/pathogenesis?







The Parasitic Balance that occurs that occurs between hosts and parasites is dependent on active metabolism by each partner. If one of the partners is inhibited by temperature, either high or low, then the other partner will have the advantage. There is a large body of literature accumulating which indicates that organisms respond to temperature stress by turning on stress response genes to protect themselves from the deleterious effects of untoward temperatures. During the time that the heat stress response genes are active other metabolic processes are subjugated to the heat stress response. This may be a pivotal event in nature for parasitic systems. The temperature at which organisms respond are unique to the organism, thus it is reasonable to expect that hosts and parasites may have very different temperature thresholds.



Therefore, there is every likelihood to expect that temperature stress on a host conditions susceptibility, even if the host possesses genes for resistance. Conversely, temperature stress may condition resistance as is often observed in the field when "cool temperature" diseases are not observed when temperatures rise.







Under these conditions both the host and parasite have their respective Temperature Stress Responses turned on and subjugate their "normal" cellular processes to repair from the heat stress. Disease will be expressed primarily due to fact that the recovery time of micro-organisms is short (2 minutes to and hour) while the host recovery period may extend to 2 days. Consequently, even a relatively avirulent pathogen may cause the interaction to "shift" towards the disease side of the equation while the plant recovers. This type of "asymptomatic" disease may be far more common in nature than has been either appreciated or demonstrated,






Effect of moisture

Moisture in the forms of; free water, relative humidity, soil moisture, mists, fogs, rains, floods, etc plays an essential role in dissemination, infection, and disease processes. Free water (rain, floods, running water, wind driven rain) is essential for dissemination of soil organisms and for "water soaking" (forcing water into sub-stomatal cavities) of leaves which provides an avenue (or infection court) for bacterial infection. Standing water becomes depleted in dissolved oxygen creating an oxygen deficit (anoxia) for roots and submerged plant parts. In such condition, plants may be more susceptible to infection. This presupposes, of course, that the potential infecting organism is not inhibited, or killed, under these water conditions. Soil bourne "water moulds" are dependent on water for sporulation and dissemination.

After infection, if the plant does not receive sufficient water, is prevented from taking it into its root, or can not transport water in its vascular tissue; then water stress and wilting will occur. Water stress under drought conditions or poor watering habits places the plant in a similar physiological and metabolic condition as temperature stress.

"Petunia Blight" is a common term applied to over watering of plants, usually in house plants or home landscapes. People become so conscientious for the "health" of their plants that they continue to water, even in the face of wilt. Thus anoxia occurs, soil inhabiting organisms infect and become pathogenic followed by death due to collapse of the crown root tissue. In the home landscape "Petunia Blight" way be surpassed only by "Lawn Mower Blight" (mechanical girdling) as the most common management error.

Effect of wind

Wind while it is usually considered for its role in dissemination is also a factor in micro-environment modification at the plant surface.

Effect of light

Restricted light quantity will lead to etiolation. Etiolated tissue is less cutinized and thus easier to penetrate. For some not understood reason, viruses are easier to mechanically infect under experimental conditions if the host plant is restricted from light for a couple days prior to infection

Effect of Soil pH

What is pH?

The effects of pH are usually on the parasite prior to infection. Thus low pHs favor fungi while high pHs favor bacteria (as a general rule - but not always).

Effect of Host-Plant nutrition

Effect of Herbicides

Because herbicides affect enzyme and/or hormone systems of the host, would one be surprised if herbicides exacerbated disease?

Questions, Comments, Complaints and Complements?

This page is authored and maintained by:

Dr. J.E. Partridge, Department of Plant Pathology, University of Nebraska-Lincoln

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