2 Pathogen infection, How do they attack?

mycelium01 copy.jpgPathogens are organisms that have the ability to cause disease and, when we think of turfgrass disease, we automatically think of fungi as the causal pathogens. Fungi are certainly the cause of a number of well-known turfgrass diseases and new fungal diseases that continue to be identified on amenity turf. However, over recent years, we have become increasingly aware of the ability of other pathogens, most notably plant parasitic nematodes, to cause disease on turfgrasses. They have long been regarded as pathogens of warm-season turfgrasses and also of horticultural and agricultural crops, but the number of different types found to be associated with disease on cool-season turfgrasses is now increasing.

One characteristic that is shared by these pathogenic fungi and plant parasitic nematodes is that they are microscopic. They cannot generally be seen without magnification even if some of the symptoms that they cause are clearly visible on the plant. Some fungi, like Laetisaria that causes red thread disease, produce accumulations of their mycelium (the characteristic red needles) that do become visible on the sward. However, the individual strands of mycelium from which these needles are composed are not themselves visible without magnification. So, the question is often asked, how can these microscopic organisms have such a damaging effect on the quality of the sward?

In order to answer the question and appreciate how pathogens attack the turf and cause disease, it is first necessary to understand the healthy plant, how it grows and what functions are essential for its day-to-day existence.

The grass plant is somewhat different to most other plants in the way that the leaf tissues develop. All leaf tissues develop from a growing point, the so-called meristem and, for most plants, these growing points are located near the end of the plant shoots. In grasses, the meristem is located at the base of the leaf tissues in the crown and, in turfgrasses, the crown area is below the height of cut. In this position, new leaf tissues develop furthest from the cut tip and new leaves emerge from the protection of the previous leaf growth. The roots of turfgrasses develop, as do all other plant roots, with their meristem region towards the root tip and new root tissues developing from the leading edge of the root system.

As the roots grow, they facilitate the uptake of water and nutrients from the rootzone. The water and nutrients (in the form of elements) are used within the plant cells for growth and development. Water is an essential component of many processes within the healthy plant and one of these is its maintenance of so-called turgor pressure. When plant cells contain sufficient water, a pressure builds on the plant cell wall and this pressure maintains the plant in its upright state. If the water content of the cells falls, turgor pressure is reduced, the plant cells lose their 'strength' and the plant wilts. Water is moved up through the plant in the xylem, not only to maintain turgor pressure and for use in many biochemical processes within the plant, but also to help maintain the temperature of the plant at an acceptable level. As water moves out through the stomata on the leaf, it exerts a cooling effect on the leaf as it evaporates.

Water is one of the key elements of photosynthesis, the process whereby plants produce their own food. Using the energy of sunlight trapped by the chlorophyll molecules in the leaf, plants convert water and carbon dioxide in to sugars and oxygen. These sugars are moved or translocated away from the leaf where they are formed, to the areas that require them or to areas where they can be stored for later use. They are moved away from the leaf in the phloem tubes that, along with the xylem, make up the vascular system of plants.

If plants cannot photosynthesise for some reason, they cannot produce the food that they need for their growth and development. Incidentally, we don't help them too much by reducing the height of cut that dramatically reduces the leaf available for photosynthesis.

Assuming photosynthesis is progressing normally, the sugars produced during this process are used in respiration, a process that gives the plant the energy it needs to grow. The rate of respiration will vary with the growing conditions but, as long as the amount of energy required by the plant can be sustained by the amount of food it produces via photosynthesis, the plant will remain healthy.

We have now touched on the main processes that maintain the healthy plant; photosynthesis, respiration, translocation and transpiration. Each of these processes is regulated by a large number of biochemical reactions within the plant cell and enzymes regulate the speed of these reactions. There are a huge number of different enzymes produced by the plant that are specific to certain biochemical reactions and some of these are only produced when they are required. In addition to enzyme-mediated reactions, cells are also directly affected by the activity of plant growth regulators or plant hormones. Some of the more commonly known hormones are auxins, gibberellins and cytokinins, although there are others. Plants rely on the accurate release of these growth regulators and specific ones are produced as and when required and at the right concentration within the individual cells. Since they are required in such small amounts, any deviation from their normal levels can have a dramatic effect on the growth and development of the plant.

Therefore, normal, healthy, turfgrass growth occurs as a result of sufficient uptake of available nutrients and water from the rootzone via the roots and their translocation upwards through the plant. It is reliant upon the correct functioning of photosynthesis to produce sugars and the use of these sugars through respiration to release energy that the cells need to fuel the cellular processes. So, how do pathogens cause disease?

Pathogens attack the turfgrass plant because they utilise the substances manufactured by the plant for their own nutrition. In order to gain access to that nutrition, both fungi and nematodes need to get inside the plant cells and they have various methods of doing this. Once inside the plant, the pathogen ensures the continued supply of nutrition by overcoming the plants natural defences or causing death of the plant cells.

The main way in which they do this is by affecting the biochemical processes within the plant cells, disrupting photosynthesis, respiration and translocation, by secreting substances including enzymes, toxins and growth regulators that either directly, or indirectly, affect the normal biochemical reactions in the plant. Fungi and nematodes can produce enzymes and growth regulators and the fungi, or at least some of them, can produce toxins as well.

Enzymes are specific to an individual chemical reaction and generally the first enzymes that are used by pathogens in attacking the plant, are those that will help to break down or degrade the plant cell walls and protective waxy layer and allow entry of the pathogen in to the cells. Even the plant parasitic nematodes may use cell wall degrading enzymes to facilitate their movement through plant roots. Not only are enzymes used by the pathogens to gain access to, and move between, adjacent plant cells but, once inside a given plant cell, enzymes are used to degrade larger nutrient sources that are contained within each cell. Thus, pathogens can degrade the structural integrity of the plant and enable the breakdown of the cellular contents by the production and secretion of different enzymes. Not all pathogens will be able to produce the same range of enzymes and this will, in part, explain why different pathogens attack different parts of the plant.

Growth regulators are naturally occurring compounds that play a vital role in regulating the growth of the plant. They are required in very small amounts and generally produced by the plant cells as and when they are required during the plants development. These growth regulators act by affecting enzyme production and therefore the rate of biochemical reactions in the plant. The result of these biochemical reactions ultimately affects the normal functioning of the plant. Plant pathogens may produce the same growth regulators as those produced by the plant. If this happens, they can stimulate the plant to develop abnormally or to initiate cell reactions that should not be activated at a given point in the cells development.

Pathogens also have the potential to produce new and different growth regulators to those that are naturally produced by the plant, or they produce substances that directly stimulate or slow down the plants natural production of its own growth regulators. It may all seem quite confusing but the important message is that healthy plant growth is maintained by the plants natural ability to regulate itself through production of growth regulators. Pathogens can affect this normal regulation process in many ways ultimately weakening the plant or adversely affecting its normal growth. The pathogen takes advantage of the weakened plant and disease can develop.

Finally, toxins. These are chemicals produced by some pathogens that act directly on the living cell content and severely damage or kill the plant cells. Not all pathogens produce toxins and of those that do, not all of them produce the same range of toxins. These chemicals are extremely poisonous to the plant in very low concentrations and are an effective tool for the pathogen to ensure its supply of nutrient from the damaged plant cells.

Through enzymes, growth regulators and toxins, the plant pathogens can ensure the disruption of plant growth at the cellular level, ensuring an adequate supply of nutrient for their own growth and development. The pathogens also affect the physiology of the plant in four main ways, as mentioned earlier, i.e. photosynthesis, respiration, translocation and transpiration.

Diseases like leaf spot and anthracnose foliar blight cause direct damage to the quality of the leaf tissues and, therefore, directly reduce photosynthesis through the reduction of functioning leaf area. Raising the height of cut on areas affected by these diseases will generally lessen disease severity due to an increase in the leaf surface area that allows the plant to increase its level of photosynthesis.

Photosynthesis can also be reduced if the chloroplast, the organelle within the leaf that contains the chlorophyll, is damaged and the chlorophyll content of the leaf is reduced (the leaf becomes chlorotic or pale in colour). In some fungal diseases, the toxins that are produce by the pathogen directly affect the function of certain enzymes that are necessary for photosynthesis. Regardless of the exact way in which photosynthesis is reduced, any negative effect on this fundamental plant process will reduce the plants ability to function in a healthy manner.

The nutrients that are produced via the process of photosynthesis are moved away from the leaf to the areas of the plant where they are required or stored for future use. In addition, water and soluble nutrients are taken up via the roots and moved around the plant to maintain the healthy functioning of the plants physiology. When any pathogen interferes with the movement, or translocation, of these materials, symptoms of disease will develop and disease is likely to spread around the plant as the effects of this reduced translocation become compounded.

For example, if water movement to the leaves is inhibited, photosynthesis will be reduced and few nutrients will be translocated to the roots that, therefore, become starved, diseased and may die. Some pathogens, both fungal and nematode, affect the ability of the root to take up water. Fungi may grow within or damage the root cells, e.g. Gaeumannomyces spp. and prevent water and nutrient movement around the plant. Most plant parasitic nematodes target the root tissues and cause stunting, deformity, galling or general decline in the quality of root development. In high populations, their detrimental effect on root function can be dramatic.

In many diseases that affect the leaf tissues, the rate of transpiration is often increased, resulting in the uncontrolled loss of water from the plant. If the amount of water taken up via the roots cannot keep up with the amount lost through the damaged leaf, plants will wilt and possibly die.

Finally, pathogens can adversely affect the rate of turfgrass respiration, increasing the speed at which the infected tissues use up their reserves of carbohydrate. In plants with greater resistance to a given disease, their rate of respiration rises rapidly after infection by the pathogen, but also falls quickly once the infection has been halted. In susceptible plants, the rate of respiration will also increase but by a lesser amount. However, the initial increased rate of respiration is sustained or may even rise during infection by the pathogen, depleting the plants carbohydrate reserves in the process.

The ways in which pathogens can adversely affect turf growth are varied and, more importantly, geared to the long-term need of the pathogen. If the pathogen needs to break down the plant cell walls in order to gain access to the required nutrition, it can. If the pathogen needs to adversely affect the functioning of the plant in order to gain continued access to the plan cell contents, it will. If we have the ability to understand the potential stresses that pathogens can put on the turf in order for us to help the sward through periods of high risk of infection, we should.

Additional reading: Agrios, GN 1997. Plant Pathology (4th Ed.), Academic Press.

Dr Kate Entwistle MBPR

The Turf Disease Centre

www.theturfdiseasecentre.co.uk

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