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Introduction
A Leaf Spot is a limited, discoloured, diseased area of a leaf that is caused by fungal, bacterial or viral plant diseases, or by injuries from nematodes, insects, environmental factors, toxicity or herbicides. These discoloured spots or lesions often have a centre of necrosis or cell death. Symptoms can overlap across causal agents, however differing signs and symptoms of certain pathogens can lead to the diagnosis of the type of leaf spot disease. Prolonged wet and humid conditions promote leaf spot disease and most pathogens are spread by wind, splashing rain or irrigation that carry the disease to other leaves.

Description
Leaf spots are a type of plant disease that are usually caused by pathogens and sometimes other cases such as herbicide injuries. Leaf spots can vary in size, shapes and colours depending on the age and type of the cause or pathogen. Plants, shrubs and trees are weakened by the spots on the leaves as they reduce available foliar space for photosynthesis. Other forms of leaf spot diseases include leaf rust, downy mildew and blights. Although leaf spot diseases can affect a small percentage of the host’s leaves, more severe consequences of leaf spot disease results in moderate to complete loss of leaves.

Causes
The causes of leaf spots are mainly from fungi, bacteria, and viruses. However leaf spots may also be caused by abiotic factors such as environmental conditions, toxicities and herbicide injuries. Foliar nematodes are another cause of leaf spots where the saliva injected into the cell walls during feeding result in the affected cells to discolour and become lesions. Aphelenchoides are common foliar nematodes which produce angular leaf spots. The Aphelenchoides ritzemabosi affects chrysanthemum and other plants such as dry beans and bird's nest fern, and the Aphelenchoides fragariae affects strawberry and other ornamentals ferns.

Fungi
Foliar diseases such as leaf spots are commonly caused by Ascomycetes and Deuteromycetes (mitosporic fungi).

Bacteria
The most common cause of bacterial leaf spots are by bacteria in the genera Pseudomonas and Xanthomonas. For example, ''Pseudomonas syringae pv. tabaci is known to cause angular leaf spot of cucumber, Pseudomonas syringae pv. phaseolicola to cause been leaf spot and Xanthomonas campestris pv. phaseoli,'' angular leaf spot of cotton.

Virus
Whilst other pathogenic causes such as fungi and bacteria induce leaf spot disease by way of enzymes, toxins and spores, virus infections affect the host by means of synthesising new proteins that are biologically active substances such as enzymes which may sabotage the metabolism of the host. Viruses can inhibit chlorophyll development in leaves and the lack of photosynthetic activity can cause yellowing and chlorosis. Viruses inducing low levels of carbohydrates in plant tissues can result in mosaic diseases. Viral leaf spot diseases include the Apple chlorotic leaf-spot virus from the genus Trichovirus, Tospoviruses, and Coconut cadang-cadang viroid.

Fungi
Leaf spots caused by fungi occur due to the necrosis of plant tissues. These necrotic lesions, localised in area and shape, consist of dead and collapsed cells of the host leaves. . One distinct feature of fungal infections is that there may be visible spores in the centre of leaf spots.

Bacteria
Bacterial leaf spots show as necrotic, circular or angular lesions and may have a yellowish outline or halo. Early symptoms of bacterial leaf spots show on older leaves and lesions appear water-soaked. Bacterial spots affecting dicytyledounous plants that have net-like leaf veins sometimes take a more angular shape as they are restricted by the large leaf veins. Bacterial spots on monocotyledonous plants with parallel leaf veins have a streak or striped appearance. The most obvious symptom of bacterial leaf spots are the blackening of the spots after infection. Eventually older lesions dry out and become papery in texture. Bacterial spots can also produce white, yellow, light cream or silver bacterial exudate depending on the type of bacteria, which may ooze from splitting lesions and/or from the underside of the spots.

Bacterial leaf spots caused by Pseudomonas show red-brown spots which can distort the infected leaves, whilst those caused by Xanthomonas are angular or circular in shape outlined with a yellow halo.

Virus
Leaf spots are visible symptoms of virus infections on plants, and are referred to as systemic symptoms. In systematic virus infections leaf spots caused by viruses show a loss of green colour in leaves, due to chlorosis which is a repression of chlorophyll development. Leaves may yellow and have a mottled green or yellow appearance, show mosaic (e.g. chlorotic spotting) and ringspots (chlorotic or necrotic rings). However there are no signs of the viral pathogen itself, as compared to visible spores of fungal pathogens and bacterial ooze or water-soaked lesions of bacterial spots as the viruses are difficult to see and requires an electron microscope for detection.

Effect on Transpiration
Transpiration increases in affected plants. This is because in leaf spots, the plant cuticle, epidermis, and cell tissues, including the xylem may be destroyed in the infected areas. The cuticle is what protects the leaf. The destruction of these cell tissues results in an uncontrollable loss of water from the affected areas. This can result in wilting of leaves.

Effect on plant growth
Leaf spots reduce the surface area available on leaves for photosynthesis and so can result in smaller growth and yield of plants. Weakened plants may produce lesser fruit. Virus caused leaf spots reduces chlorophyll in the leaves, resulting in less photosynthetic activity. This can lead to smaller leaves and blossoms, smaller growth and reduced yield.

Disease Cycle
Leaf spot disease occurs when the following factors are all present: favourable environmental conditions, a pathogenic agent, and susceptible host. Different types of pathogens, including fungal, bacterial and viral agents have unique ways to suppress and attack the host plant's immune system, thereby resulting in the progression of leaf spot disease. Knowing the disease cycle of each microbial agent also helps in managing leaf spot disease.

Pathogens can survive in infected debris, seeds or soil throughout winter, and

Fungal leaf spots develop in distinct stages, which involve i) the attachment of the fungal pathogen to the plant surface, ii) the germination of spores ii) penetration or entry into the host tissue, i) colonisation of the host tissue v) and the expression of symptoms. (naraysansmy)

i)

Fungal Pathogens Plant pathogens exhibit a wide array of strategies during the infection process. Direct entry may occur via wounds caused by herbivores, wind damage, and/or human activity, such as pruning. Fungi can also enter plants via their natural openings, such as the stomata and lenticels. However, in addition, fungi exhibit an array of complex biological interactions between host and pathogen. A number of infection strategies employed by pathogens require cell-wall-degrading enzymes to facilitate entry of the pathogen into plant tissues via degradation of the cuticle. Other strategies require the establishment of a pathogen–host interface via haustoria and the differentiation and function of other infection structures; many of these are related to the nutritional strategy (see Chapter 1) of the pathogen, with biotrophic and necrotrophic pathogens being two important and contrasting nutritional states. (Burchett)

The process whereby a pathogen comes in contact with the host is called inoculation. Mineral salts, sugars, and amino acids exuded from seeds, roots, and leaves during normal plant growth may stimulate germination of pathogens in the spermosphere (seeds), rhizosphere (roots), and phyllosphere (leaves). For disease to develop, the pathogen must penetrate the host and invade tissue, in a process called infection. After infection, the pathogen begins to grow and establish an intimate relationship with the host. This process is called colonization of the host. (Lucas)

Pathogens penetrate host tissues in one of three ways: by direct penetration of the outer layers of plant tissue; by entry into natural openings; or by entry through wounds.

Nematodes pierce plant cells with their stylets and suck out the cell contents.

Spores of many fungi germinate and get inside a plant by growing through natural plant openings such as stomates and hydathodes; some fungal produce infection pegs that penetrate the leaf surface directly; some fungi produce substances called enzymes that dissolve plant tissue and allow growth of the mycelium into the plant.

Most bacteria gain entry into the plant through natural openings or through wounds. Many viruses are injected into plants by insect vectors while feeding; others penetrate through the microscopic injuries resulting from the wounding or rubbing of plants.

As the pathogen continues to grow in the plant, symptoms will begin to appear at a certain time after penetration, depending on the particular pathogen and host plant involved. Visible symptoms are the responses of the plant to disease processes that have been occurring over a period of time. The time between inoculation and the appearance of symptoms is known as the incubation period

Many biochemical and physiological changes occur in the plant before and during the development of disease symptoms. These changes may be due to the production of toxins by the pathogen or may result from the disruption of normal metabolic processes in the plant by the growth and reproduction of the pathogen. In fact, most of the major physiological processes in a plant can be disturbed by the invasion of a pathogen. Photosynthesis can be reduced dramatically if pathogens grow into and disrupt or kill green leaf and stem tissue. The respiration rate of plant tissues invaded by a pathogen may increase as a result of a stimulation of some physiological processes by the pathogen. As disease continues to develop and more and more tissue becomes chlorotic (yellow due to lack of active chlorophyll) or necrotic (dead), the respiration rate will decline.

Other diseases, particularly ones that occur in aerial parts of the host, develop secondary, or repeating, disease cycles during a growing season. Recurrent crops of spores, maturing every week or 10 days, provide inoculum to infect nearby healthy plants. These are called polycyclic diseases. The disease cycle is completed when the pathogen reaches the survival stage. Many pathogens produce specialized structures, such as sclerotia and resistant spores, that enable the pathogen to resist freezing, drying, or other adverse conditions.

Management
Strains of plant pathogenic bacteria becoming resistant to chemicals contributes to the difficulty of managing bacterial leaf spot disease. An example is Xanthomanos vesicatoria, which causes bacterial spot of tomato and pepper, that is now resistant to streptomycin. Apart from chemicals, alternative management methods include using bacteriophages, bacteriocins, and heat therapy.

The host-specific characteristic of many leaf spot pathogens makes diversity in plant species a way to reduce and regulate leaf spot pathogens infection levels.

Practicing Citations
This fifth edition of the textbook in plant pathology provides comprehensive information about how to identify and manage plant diseases and offers recent techniques in molecular and biological control in plant diseases.

This website offers factual information about leaf spot diseases, including causes, identification, life cycle and management.

This journal article provides information about the effects of herbicides on plants and how they can cause injury such as leaf spots.

This journal article gives insightful information through investigation and experimental analysis, about the effects of plant diversity ultimately decreasing the pathogen infection levels of plants.

This plant disease handbook in its seventh edition, revised and edited by Kenneth Horst, offers detailed information about leaf spot pathogens and hosts in its own separate chapter.

Answers to Module 7 Questions

 * Description of media: A photograph of a leaf spot, showing grey and dried necrosis.


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