A leaf spot is a limited, discoloured, diseased area of a leaf that is caused by

fungal A fungus (: fungi , , , or ; or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as one of the tradit ...

bacteria Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...

l or viral plant diseases, or by injuries from

nematode The nematodes ( or ; ; ), roundworms or eelworms constitute the phylum Nematoda. Species in the phylum inhabit a broad range of environments. Most species are free-living, feeding on microorganisms, but many are parasitic. Parasitic worms (h ...

s, insects, environmental factors,

toxicity Toxicity is the degree to which a chemical substance or a particular mixture of substances can damage an organism. Toxicity can refer to the effect on a whole organism, such as an animal, bacteria, bacterium, or plant, as well as the effect o ...

herbicide Herbicides (, ), also commonly known as weed killers, are substances used to control undesired plants, also known as weeds.EPA. February 201Pesticides Industry. Sales and Usage 2006 and 2007: Market Estimates. Summary in press releasMain page f ...

s. These discoloured spots or lesions often have a centre of

necrosis Necrosis () is a form of cell injury which results in the premature death of cells in living tissue by autolysis. The term "necrosis" came about in the mid-19th century and is commonly attributed to German pathologist Rudolf Virchow, who i ...

(cell death).Horst, R. (2008). Westcott’s Plant Disease Handbook (Seventh Edition.). Springer Netherlands. 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 Irrigation (also referred to as watering of plants) is the practice of applying controlled amounts of water to land to help grow crops, landscape plants, and lawns. Irrigation has been a key aspect of agriculture for over 5,000 years and has bee ...

that carry the disease to other leaves.Lucas, G., & Campbell, L. (1992). Introduction to Plant Diseases Identification and Management (2nd ed. 1992.). Springer US. https://doi.org/10.1007/978-1-4615-7294-7

Description

Leaf spots are a type of

plant disease Plant diseases are diseases in plants caused by pathogens (infectious organisms) and environmental conditions (physiological factors). Organisms that cause infectious disease include fungi, oomycetes, bacteria, viruses, viroids, virus-like or ...

that are usually caused by pathogens and sometimes other cases such as herbicide injuries.Gunsolus, J. L. (1999). Herbicide Mode of Action and Injury Symptoms. ''North Central Regional Extension Publications'', ''377'', 1–24. Retrieved from https://appliedweeds.cfans.umn.edu/sites/appliedweeds.cfans.umn.edu/files/herbicide_mode_of_action_and_injury_symptoms.pdf 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 Photosynthesis ( ) is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their metabo ...

. Other forms of leaf spot diseases include leaf rust,

downy mildew Downy mildew refers to any of several types of oomycete microbes that are obligate parasites of plants. Downy mildews exclusively belong to the Peronosporaceae family. In commercial agriculture, they are a particular problem for growers of c ...

and blights.Grabowsk, M. & University of Minnesota Extension. (2018). Leaf spot diseases of trees and shrubs. Retrieved 9 September 2020, from https://extension.umn.edu/plant-diseases/leaf-spot-diseases-trees-and-shrubs#leaf-rusts-1156511 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.Isleib, J. & Michigan State University Extension. (2012). Signs and symptoms of plant disease: Is it fungal, viral or bacterial? Retrieved 10 October 2020, from https://www.canr.msu.edu/news/signs_and_symptoms_of_plant_disease_is_it_fungal_viral_or_bacterial 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.University of Maryland Extension. (2020). Foliar Nematodes - Flowers , University of Maryland Extension. Retrieved 10 October 2020, from https://extension.umd.edu/hgic/topics/foliar-nematodes-flowers ''

Aphelenchoides ''Aphelenchoides'' is a genus of mycetophagous nematodes. Some species are plant pathogenic foliar nematodes. Taxonomy In 1961 Sanwal listed 33 species and provided a key. The most important species of these are '' Aphelenchoides ritzemabosi' ...

'' are common foliar nematodes which produce angular leaf spots. The ''

Aphelenchoides ritzemabosi ''Aphelenchoides ritzemabosi'' (black currant nematode, chrysanthemum foliar nematode, chrysanthemum leaf nematode, chrysanthemum nematode, chrysanthemum foliar eelworm) is a plant pathogenic nematode. It was first scientifically described in 18 ...

'' affects

chrysanthemum Chrysanthemums ( ), sometimes called mums or chrysanths, are flowering plants in the Asteraceae family. They are native to East Asia and northeastern Europe. Most species originate from East Asia, and the center of diversity is in China. Co ...

and other plants such as dry beans and bird's nest fern, and the ''

Aphelenchoides fragariae ''Aphelenchoides'' is a genus of mycetophagous nematodes. Some species are plant pathogenic foliar nematodes. Taxonomy In 1961 Sanwal listed 33 species and provided a key. The most important species of these are '' Aphelenchoides ritzemabosi ...

'' affects strawberry and other ornamentals ferns.Agrios, G. N. (2005). ''Plant Pathology'' (5th ed.). https://doi.org/10.1016/C2009-0-02037-6

Fungi

Foliar diseases such as leaf spots are commonly caused by

ascomycetes Ascomycota is a phylum of the kingdom Fungi that, together with the Basidiomycota, forms the subkingdom Dikarya. Its members are commonly known as the sac fungi or ascomycetes. It is the largest phylum of Fungi, with over 64,000 species. The de ...

and so-called

deuteromycetes The fungi imperfecti or imperfect fungi are fungi which do not fit into the commonly established taxonomic classifications of fungi that are based on biological species concepts or morphological characteristics of sexual structures because their ...

(mitosporic fungi).

Bacteria

The most common cause of bacterial leaf spots are by bacteria in the genera ''

Pseudomonas ''Pseudomonas'' is a genus of Gram-negative bacteria belonging to the family Pseudomonadaceae in the class Gammaproteobacteria. The 348 members of the genus demonstrate a great deal of metabolic diversity and consequently are able to colonize a ...

'' and ''

Xanthomonas ''Xanthomonas'' (from greek: ''xanthos'' – "yellow"; ''monas'' – "entity") is a genus of bacteria, many of which cause plant pathology, plant diseases. There are at least 27 plant associated ''Xanthomonas spp.'', that all together infect at l ...

.'' For example, ''

Pseudomonas syringae ''Pseudomonas syringae'' is a rod-shaped, Gram-negative bacterium with polar flagella. As a plant pathology, plant pathogen, it can infect a wide range of species, and exists as over 50 different pathovars, all of which are available to research ...

pv. tabaci'' is known to cause angular leaf spots of cucumber, ''Pseudomonas syringae pv. phaseolicola'' to cause bean 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

enzyme An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...

toxin A toxin is a naturally occurring poison produced by metabolic activities of living cells or organisms. They occur especially as proteins, often conjugated. The term was first used by organic chemist Ludwig Brieger (1849–1919), derived ...

s and

spore In biology, a spore is a unit of sexual reproduction, sexual (in fungi) or asexual reproduction that may be adapted for biological dispersal, dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores fo ...

s, virus infections affect the host by means of synthesising new

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...

s that are biologically active substances such as enzymes which may sabotage the

metabolism Metabolism (, from ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cellular processes; the co ...

of the host. Viruses can inhibit

chlorophyll Chlorophyll is any of several related green pigments found in cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Greek words (, "pale green") and (, "leaf"). Chlorophyll allows plants to absorb energy ...

development in leaves and the lack of photosynthetic activity can cause yellowing and

chlorosis In botany, chlorosis is a condition in which leaves produce insufficient chlorophyll. As chlorophyll is responsible for the green color of leaves, chlorotic leaves are pale, yellow, or yellow-white. The affected plant has little or no ability to ...

. 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.

Herbicide

Leaf spots may also be from injuries made by herbicides coming in contact with the plant. A low rate of contact from nitrile and pyridazine herbicides, can result in spotting or speckling of the plant's foliage. Diphenylether herbicides can result in reddish-colour spots shortly after application. Accurate identification of leaf spot disease is needed as to distinguish signs of illness from damage done by herbicides.

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. One distinct feature of fungal infections is that there may be visible spores in the centre of leaf spots. Fungal leaf spots often have a brown, black, tan or reddish centre with a darker margin and vary in size.Douglas, S. M. (2020). Fungal Leaf Spots of Trees and Ornamentals. Retrieved from https://portal.ct.gov/CAES/Fact-Sheets/Plant-Pathology/Fungal-Leaf-Spots-of-Trees-and-Ornamentals

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.Koike, S. T., Turini, T. A., & Davis, R. M. (2017). Bacterial Leaf Spot / Lettuce / Agriculture: Pest Management Guidelines / UC Statewide IPM Program (UC IPM). Retrieved 10 October 2020, from https://www2.ipm.ucanr.edu/agriculture/lettuce/Bacterial-leaf-spot/ 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 ''

'' show red-brown spots which can distort the infected leaves, whilst those caused by ''

'' 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 An electron microscope is a microscope that uses a beam of electrons as a source of illumination. It uses electron optics that are analogous to the glass lenses of an optical light microscope to control the electron beam, for instance focusing it ...

for detection.

Effect on transpiration

Transpiration Transpiration is the process of water movement through a plant and its evaporation from aerial parts, such as leaves, stems and flowers. It is a passive process that requires no energy expense by the plant. Transpiration also cools plants, c ...

increases in affected plants. This is because in leaf spots, the

plant cuticle A plant cuticle is a protecting film covering the outermost skin layer (epidermis) of leaves, young shoots and other aerial plant organs (aerial here meaning all plant parts not embedded in soil or other substrate) that have no '' periderm''. The ...

epidermis The epidermis is the outermost of the three layers that comprise the skin, the inner layers being the dermis and Subcutaneous tissue, hypodermis. The epidermal layer provides a barrier to infection from environmental pathogens and regulates the ...

, and cell tissues, including the

xylem Xylem is one of the two types of transport tissue (biology), tissue in vascular plants, the other being phloem; both of these are part of the vascular bundle. The basic function of the xylem is to transport water upward from the roots to parts o ...

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. 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. Fungal leaf spot pathogens follow the path of attaching to the plant surface, germinating via

s and entering into the host tissue. Colonisation of the host tissue follows and then the expression of symptoms. Usually fungi will

overwinter Overwintering is the process by which some organisms pass through or wait out the winter season, or pass through that period of the year when "winter" conditions (cold or sub-zero temperatures, ice, snow, limited food supplies) make normal activ ...

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 A pycnidium (plural pycnidia) is an asexual fruiting body produced by mitosporic fungi, for instance in the order Sphaeropsidales ( Deuteromycota, Coelomycetes) or order Pleosporales (Ascomycota, Dothideomycetes). It is often spherical or inve ...

, acervuli and

perithecia An ascocarp, or ascoma (: ascomata), is the fruiting body ( sporocarp) of an ascomycete phylum fungus. It consists of very tightly interwoven hyphae and millions of embedded asci, each of which typically contains four to eight ascospores. Ascoc ...

, within the affected leaf tissue. Viruses can survive in cells that have been infected by the viral agent called ''alternate hosts''. For infection to occur virus replication needs to happen, and in doing so uses the host cell's products, disrupting cell processes.Burchett, S., & Burchett, S. (2017). Plant pathology. Garland Science, Taylor & Francis Group. 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 Bordeaux mixture (also called ''Bordo Mix'') is a mixture of copper(II) sulphate (CuSO4) and quicklime ( Ca O) used as a fungicide. It is used in vineyards, fruit-farms, vegetable-farms and gardens to prevent infestations of downy mildew, powder ...

, the first fungicide to have been developed, which treats many fungal and bacterial leaf spots. Other fungicides such as

zineb Zineb is the chemical compound with the formula n. Structurally, it is classified as a coordination polymer and a dithiocarbamate complex. This pale yellow solid is used as fungicide. Production and applications It is produced by treating ethyle ...

chlorothalonil Chlorothalonil (2,4,5,6-tetrachloroisophthalonitrile) is an organic compound mainly used as a broad spectrum, nonsystemic fungicide, with other uses as a wood protectant, pesticide, acaricide, and to control Mold (fungus), mold, mildew, bacteria ...

and

Captan Captan is a general use pesticide (GUP) that belongs to the phthalimide class of fungicides. It is a white solid, although commercial samples appear yellow or brownish. Applications Although it can be applied on its own, Captan is often added as ...

, also treat leaf spot disease and

Benomyl Benomyl (also marketed as Benlate) is a fungicide introduced in 1968 by DuPont. It is a systemic benzimidazole fungicide that is selectively toxic to microorganisms and invertebrates (especially earthworms), but relatively nontoxic toward mammals. ...

specifically treats ''Cercospora'' leaf spots, cherry leaf spot and black spot of roses.

Thiabendazole Tiabendazole (International Nonproprietary Name, INN, British Approved Name, BAN), also known as thiabendazole (Australian Approved Name, AAN, United States Adopted Name, USAN) or TBZ and the trade names Mintezol, Tresaderm, and Arbotect, is a pr ...

is used to treat leaf spot diseases of turf and ornamentals. Both fenarimol and nuarimol are

pyrimidine Pyrimidine (; ) is an aromatic, heterocyclic, organic compound similar to pyridine (). One of the three diazines (six-membered heterocyclics with two nitrogen atoms in the ring), it has nitrogen atoms at positions 1 and 3 in the ring. The oth ...

s that also treats leaf spot disease. More chemicals include

Triazole A triazole is a heterocyclic compound featuring a five-membered ring of two carbon atoms and three nitrogen atoms with molecular formula C2H3N3. Triazoles exhibit substantial Isomer, isomerism, depending on the positioning of the nitrogen atoms w ...

s an organosphosphate fungicide, Imazalil, Procholora and Fentin hydroxide. Registered fungicides in use are thiophanate methyl, chlorothalonil, ferban and

mancozeb Mancozeb is a dithiocarbamate non-systemic agricultural fungicide with multi-site, protective action on contact. It is a combination of two other dithiocarbamates: maneb and zineb. The mixture controls many fungal diseases in a wide range of fi ...

. These chemicals can be applied as foliar sprays, seed and soil treatments or as post-harvest treatment. 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 Streptomycin is an antibiotic medication used to treat a number of bacterial infections, including tuberculosis, Mycobacterium avium complex, ''Mycobacterium avium'' complex, endocarditis, brucellosis, Burkholderia infection, ''Burkholderia'' i ...

. Apart from chemicals, alternative management methods include using

bacteriophage A bacteriophage (), also known informally as a phage (), is a virus that infects and replicates within bacteria. The term is derived . Bacteriophages are composed of proteins that Capsid, encapsulate a DNA or RNA genome, and may have structu ...

bacteriocin Bacteriocins are proteinaceous or peptide, peptidic toxins produced by bacteria to inhibit the growth of similar or closely related bacterial strain(s). They are similar to yeast and paramecium killing factors, and are structurally, functionally ...

s, and heat therapy. 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 Sanitation refers to public health conditions related to clean drinking water and treatment and disposal of human excreta and sewage. Preventing human contact with feces is part of sanitation, as is hand washing with soap. Sanitation systems ...

is key as well as the avoidance of handling plants when wet, planting pathogen-free and resistant

cultivar A cultivar is a kind of Horticulture, cultivated plant that people have selected for desired phenotypic trait, traits and which retains those traits when Plant propagation, propagated. Methods used to propagate cultivars include division, root a ...

s and moving out infected plants.Agriculture Victoria. (2020a, November 24). Bacterial leaf spot of ornamentals and vegetables - Agriculture. Retrieved from https://agriculture.vic.gov.au/biosecurity/plant-diseases/vegetable-diseases/bacterial-leaf-spot-of-ornamentals-and-vegetables 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. Disinfection of tools and washing of hands is important when handling infected plants. Susceptibility to leaf spot disease can occur due to insufficient or excessive fertilising of plants. Checking plants periodically for any signs and symptoms of disease is also good practice. Avoiding overhead watering and increasing air circulation by pruning plants should be done to prevent humid conditions. Collecting and removing fallen leaves is important in reducing the amount of pathogenic agents on the ground to prevent dispersal of disease. Fungicides should be used only when necessary, and if applied, early before the leaf spot disease progresses.Nix, S. (2017, November 29). How to Prevent and Control Leaf Spot Tree Disease. Retrieved from https://www.thoughtco.com/leaf-spot-tree-disease-prevention-control-1342869 Chemical control is necessary for severe leaf spotting and defoliation occurring over several years.

References

External links

{{Commons category, Leaf spot
Leaf Spot and Lawn Care
Fungal plant pathogens and diseases Bacterial plant pathogens and diseases Leaf diseases

Description

Causes

Fungi

Bacteria

Virus

Herbicide

Symptoms

Fungi

Bacteria

Virus

Effect on transpiration

Effect on plant growth

Disease cycle

Dispersion

Management

See also

References

External links