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Reinette Gouws1, Johan Habig2

1Tshwane University of Technology, Department of Crop Sciences, Private Bag X680, Pretoria, 0001. Email address: GouwsR2@tut.ac.za

2Agricultural Research Council-Plant Protection Research Institute, Private Bag X134, Queenswood, 0121. Email address: HabigJ@arc.agric.za

Since the earliest times of man’s existence, he was confronted with plant diseases. The Greeks were the first civilization to study and write about plant diseases, and even in the Old Testament of the Bible, blasting and mildew of crops were listed amongst the scourges of mankind during those early times. Little scientific knowledge was however added until around 1800, and it was only with the discovery of fungi and other micro-organisms as well as the development of the compound microscope, that more scientific knowledge of plant diseases was achieved. During the 1840's and 50's the classic development of microbiology triggered a rapid increase of scientific knowledge of the cause of various plant diseases, and it was only from this time that the applied science of plant pathology found its feet among the other sciences.

In the first article we published: “The positive side of our soils.” [October 2016], we elucidated on the various micro-organisms that contribute to the overall ‘health’ and balance of the soil and its ecology. However, like with most things in life, there is a flip side to this coin and it’s not pretty. It usually leaves us with a diseased crop, economic loss and various unanswered questions.

Question 1: How do I know my plant is diseased?

Plant disease symptoms are defined as the internal or external reactions or changes which indicate that a plant is diseased. Symptoms can be caused by various factors which are not easily identifiable to the untrained eye. Some symptoms are caused by disease-causing micro-organisms [pathogens] such as fungi, bacteria, viruses and nematodes and are relatively distinct and easy to identify on specific host plants. Symptoms can also be caused by factors that are not related to micro-organisms which complicate the matter. Great care must thus be taken not to identify a disease merely according to symptoms observed on the host plant.

Plants exhibit a variety of morphological and physiological symptoms. In order to make correct diagnoses it is necessary to conduct further tests or to examine the diseased plant further for signs of the possible cause, or the presence of a pathogen. When the symptoms are not uniquely characteristic, it is necessary to do a microscopic examination. An initial examination with a microscope is sometimes sufficient to identify the causal organism. In other cases, thin sections or scrapings of the infected parts will have to be mounted on a microscope slide and examined under a microscope in order to observe the presence of specific fungi or bacteria, after which the disease can usually be identified by reference to textbooks or scientific publications. It might also be necessary to isolate the causal agent and to conduct a full molecular identification of the pathogen to confirm the diagnosis and recommend appropriate management strategies.

Question 2: What causes plant disease?

The cause [etiology] of plant diseases constitutes an important body of knowledge of the science of Plant Pathology. If the real cause of a disease is known, we are able to suggest general control measures which will be able to control the disease acceptably, even if the detailed knowledge of the disease is not yet fully known. On the basis of cause, plant diseases can be grouped into two categories; namely biotic [living factors] (Fig. 1) and abiotic [non-living factors] (Fig. 3) diseases.

A. Biotic / Transmissible / Parasitic diseases

These diseases are caused by a transmissible entity or pathogen and can be transferred from a healthy to a diseased plant (Fig. 1). Diseases can be caused by micro-organisms such as fungi, bacteria, viruses, nematodes, mycoplasmas (a kind of bacteria lacking a cell wall around their cell membrane) and protozoa (a complex, single-celled organism).

Diseases caused by fungi and bacteria

When fungi or bacteria are observed, but no reference of a disease with similar symptoms on the same crop can be found in the literature, the following steps must be followed to ascertain if the particular organism is the cause of the disease. These steps are called Koch's Postulates, and were already stated during the previous century by the pioneer of microbiology, Robert Koch. Koch’s Postulates for facultative parasites (i.e. do not rely on a host to complete its life-cycle) are listed as:

1) The pathogen under suspicion must consistently be found to be in association with the disease.

2) The pathogen under suspicion must be isolated in a pure culture and be grown in the laboratory in order to ascertain its characteristics.

3) The pathogen under suspicion in pure culture must be transferred to a healthy plant of the same cultivar as on which the disease was found, and must cause the same symptoms as were initially found.

4) The pathogen under suspicion must be re-isolated from the artificially infected plant and be grown in pure culture. Its characteristics must be exactly the same as the pathogen isolated in step two.

If all these steps have been successfully completed, one can then state with certainty that the pathogen under suspicion is indeed the cause of the disease.

Diseases Caused by Nematodes

The presence of a species of parasitic nematodes on or in the plant or in its rhizosphere (the soil in the area of the living root) indicates that the nematode is probably involved in the production of the symptoms observed (Fig. 2a, 2b, 2c). If the nematode can be identified as to belong to a species or genus known to cause such symptoms, the diagnosis of the problem can be made with a degree of certainty.

Diseases Caused by Viruses, and Mycoplasmas.

Many of these entities cause distinct and characteristic symptoms on their hosts and can be identified by the symptoms alone. In the many other cases where this is not possible, the diseases are diagnosed and the entities identified by:

1) virus transmission tests to specific host plants by sap inoculation, grafting or by certain insect, nematode or fungus vectors (i.e. carriers);

2) using sero-diagnostic tests for viruses, for which specific antiserum are available;

3) using electron microscopy;

4) Electrophoretic tests for the detection of the nucleic acids of viruses etc.

Mycoplasm diagnosis is based on recognition of specific symptoms, graft transmissibility, transmission by certain insect vectors, electron microscopy, sensitivity to tetracycline antibiotics but not penicillin, and sensitivity to moderately high temperatures (32 - 35 oC).

B. Abiotic disease / Non-Transmissible / Physiological diseases

If no pathogen can be found, cultured from, or transmitted from a diseased plant, it must be assumed that the disease is caused by an environmental (physiological) factor (Fig. 3).. The specific factor might be determined by observing a change of the environment, i.e. flooding, unseasonable frost, etc. Some environmental factors cause specific symptoms, while others cause general symptoms. Unless the history of the environmental conditions is known, these diseases are often difficult to diagnose.

Non-infectious (non-transmissible, physiological) diseases are usually caused by a surplus or deficiency of some or other factor which supports the life of the plant. These diseases are non-transmissible and no pathogens are present. The symptoms caused by these diseases differ in degree of severity, depending on the degree of deviation from the normal range of the factor.

Extremes of Temperature

For each kind of plant there exists a minimum-, optimum- and maximum temperature for growth. When the prevailing temperature falls outside this range, the plant will be damaged.

  • Low temperature:

    Cold and frost damage will depend on the time and degree of deviation from the minimum temperature for growth. Cold damage is caused by:

    1) the ice crystals formed by the freezing of the water vapour in the air spaces between plant cells, physically damaging the cells;

    2) ice crystals form in the cell sap and cytoplasm, thus disrupting the membranes of the cell;

    3) the freezing of water vapour in the spaces between plant cells causes more water vapour to diffuse out of the cells, leading to increased concentration of solutes in the cell which may damage the cell.

  • High temperature:

    Plant parts, especially leaves, become singed (burned) mostly on hot windless days. Tree branches are sometimes scorched by the sun leading to the formation of cankers.

    Extremes of Moisture

  • Too little soil moisture causes water stress in the above ground parts of the plants leading to temporary wilting which may become permanent if the moisture content of the soil decreases further.

  • Too much soil moisture leads to decreased aeration of the soil which may cause anaerobic (i.e. oxygen deprived) soil conditions under which root respiration is impeded, causing decreased water uptake by the roots.

    Extremes of Light

    Too much light on shade-loving plants may cause the loss of the normal green coloration (chlorosis) of the leaves, while too little light on sun-loving plants give rise to a sub-normal green colour, development of long internodes and lack of mechanical tissue in the stems called etiolation.

    Oxygen deficiency

    Lack of oxygen in roots growing in poorly aerated soil result in reduced water uptake and wilting. When potato tubers are stored in big piles under condition of high temperature, the tubers in the centre of the pile are subject to a state of oxygen deficiency because of the high respiration rate of the tubers at elevated temperatures. The characteristic ‘black heart’ symptom then develops.

    Air Pollution

    Crops grown in the proximity of industries sometimes develop chlorosis and/or necrosis of the leaves due to the presence of gases such as chlorine, sulphur dioxide, nitrogen dioxide, hydrogen fluoride and ozone.

    Toxic minerals

    Soils may contain excessive amounts of macro- as well as micro elements. Both may harm plants if they occur in high concentrations. Excessive macro elements are less harmful than excessive micro elements. Too much sodium induces a calcium deficiency. Excessive sodium salts increase the pH of the soil leading to alkali injury of plants. A low soil pH causes increased solubility of especially metal elements. This may lead to toxicities or interference with the uptake of other elements.

    Soil pH

    Soil pH has an important effect on the solubility and thus the availability of plant nutrient elements. Nutrient elements may be changed to insoluble forms when the soil pH falls outside the range of about 4.5 to 8. Very acidic or alkaline soils may lead to toxicities as mentioned.

    Toxic agricultural remedies

    The extensive use of herbicides leads to a lot of damage to crops. Most herbicides are safe to use for the purpose they were manufactured for, especially when it is used as indicated. Other agricultural remedies may also be harmful when used at too high concentrations or under the wrong conditions. It is mandatory to follow the instructions on the label closely.

    Concluding remarks

    Probably the single greatest management challenge to farmers the world over, is that represented by pests, diseases and weeds (biotic stresses). About one third of crop production is lost to biotic stresses and on average their control represents about one fifth of farming costs. Due to the fact that the serious diseases occur regularly and are rapidly disseminated from plant to plant, as well as the difficulty to cure a disease once it has established in or on a plant, almost all control measures are aimed at preventing plants from becoming diseased, rather than trying to cure them.

    The use of crop rotation systems and establishment of cover crops can assist in managing plant diseases by interrupting the infection chain between subsequent crops. However, the most important concept to follow when devising a management plan is to make use of Integrated Pest Management (IPM) technologies. IPM is defined as the sustainable control of pests and diseases, by combining alternative methods of control in a way that minimizes the use of chemical pesticides to reduce economic, health and environmental risks. The aims of IPM are to prevent pests and diseases, to keep pest and pathogen populations low and ultimately to promote vigorous, but balanced, plant growth. We will elaborate on this concept and its application in conservation agriculture in the next edition of this series.