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 (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, 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. 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).
!Ascomycetes
!Cause and Host
|-
|Cochliobolus
|Leaf spot on most grasses, and cereals.
|-
|Blumeriella (Higginsia)
|Leaf spot on cherries and plums.
|-
|Magnaporthe, M. grisea
|Grey leaf spot of cereals and turf grasses.
|-
|Elytroderma deformans
|Leaf spot of pines.
|-
|Mycosphaerella fragariae
|Leaf spot of strawberry.
|-
|Pseudopeziza
|Leaf spot of alfafa and clovers.
|-
|Pyrenophora
|Leaf spot on many cereals and grasses.
|-
|Cercospora
|Reddish lesions at first then enlarges and becomes white to light brown in the centre. Circular and varying in size.
Can appear concentric with red margins.
Leaf spot on most cereals and grasses, field crops, vegetables, ornamentals, and trees.
|-
|Myrothecium roridum
|Target-like spots with light brown centres and dark circumference on Gardenia augusta,
New Guinea Impatiens, Begonia species, and Gloxinia, and pansy. Distinctive dark green
and black spore producing bodies edged by white hyphae occurs in lesions.
|}
{| class="wikitable"
|+Common Deuteromycetes that cause leaf spot disease:
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
thumb|Bacterial leaf spot caused by Pseudomonas cichorii on a hibiscus leaf
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. 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.
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.