Leaf spot

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, shape, and color 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 results 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 so-called deuteromycetes (mitosporic fungi). == Symptoms ==

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. Bacteria Bacterial leaf spots show as necrotic, circular or angular lesions and may have a yellowish outline or halo 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. 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 protects the leaf and 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. Usually fungi will overwinter on fallen leaves, or buds, branches and fruits, then in the warmer early spring to summer months produce spores during the germination process, on the exterior of leaves, as well as exist as pycnidia, acervuli and perithecia, within the affected leaf tissue. Viruses can survive in cells that have been infected by the viral agent called alternate hosts. Horizontal transmission of viral pathogens include dispersal through touching of nearby infected leaves and through root systems or through vectors for more distant hosts. Vertical transmission occurs by inheriting the virus from the parent host plant. Bacterial pathogens survive in infected plants, plant debris, seed and soil. Infection occurs when the bacteria enter into wounds, or by natural entry (cell adhesion), under favourable warm and moist conditions. Dispersion Pathogens can be dispersed by the wind that can lift nematode eggs, insects, and many tiny fungal spores as well as bacterial cells by air currents. Animal and insect vectors are another way in which fungal, bacterial and viral leaf spot diseases are spread. Rainwater spreads pathogens by transporting infested soil into areas that are disease-free. Infested water can also be spread by way of irrigation or transplanting. Blowing rain can also spread fungi and bacteria. Splashing water can also spread pathogens from the soil to leaf and amongst leaves. Plant material can also be the cause of leaf spot disease. These include infected seeds, transplants and discarded culls and leaves. Tools used by humans and worker's hands during transplants, watering, and market practices can contribute to the dispersal of leaf spot pathogens. == Management ==

Certain chemicals are used to treat leaf spot disease, such as the Bordeaux mixture, the first fungicide to have been developed, which treats many fungal and bacterial leaf spots. Diversity in plant species has also been found to reduce the prevalence of leaf spot disease. The host-specific characteristic of many leaf spot pathogens makes diversity in plant species a way to reduce and regulate leaf spot pathogen infection levels within plant populations. Prevention of leaf spot disease includes variety selection, crop rotations, plant hygiene and fungicide use for seeds and foliage. To stop the spread of pathogens good sanitation is key as well as the avoidance of handling plants when wet, planting pathogen-free and resistant cultivars and moving out infected plants. Reducing the humidity around plants and in greenhouses by good plant spacing for air circulation, and watering early in the day can also help prevent leaf spot disease. Chemical control is necessary for severe leaf spotting and defoliation occurring over several years. ==See also==