| Alternaria dauci | |
|---|---|
Scientific classification  | |
| Domain: | Eukaryota |
| Kingdom: | Fungi |
| Division: | Ascomycota |
| Class: | Dothideomycetes |
| Order: | Pleosporales |
| Family: | Pleosporaceae |
| Genus: | Alternaria |
| Species: | A. dauci |
| Binomial name | |
| Alternaria dauci (J.G. Kühn) J.W. Groves & Skolko, (1944) | |
| Synonyms | |
Alternaria brassicae var. dauci(J.G. Kühn) P.C. Bolle, (1924) Contents | |
Alternaria dauci is a plant pathogen. The English name of the disease it incites is "carrot leaf blight".
Alternaria Leaf Blight is a foliar disease of carrots caused by the fungus Alternaria dauci. Alternaria dauci is included in the porri species group of Alternaria, which is classified for having large conidium and a long, slender filiform beak. [1] Because many of the members of this group have similar morphology, Alternaria dauci has also been classified as formae specialis of carrots, or A. porri f. sp. dauci. [1] It has been well established that the host range of this disease is on cultivated and wild carrot, but it has also been claimed that Alternaria dauci has the ability to infect wild parsnip, celery, and parsley. [1] A study in 2011 by Boedo et al. evaluated the host range of Alternaria dauci in a controlled environment and concluded that several non-carrot species could constitute alternate hosts, such as Ridolfia segetum (corn parsley) and Caucalis tenet (hedge parsley). [2] Despite their findings, reports of A. dauci colonization on non-carrot hosts continues to be debated because the use of Koch's Postulates on recovered isolates of A. dauci is challenging and is rarely reported; in addition, few reports are often made of such infections in field settings. [1]
Symptoms of A. dauci appear first as greenish-brown, then water-soaked, and finally necrotic lesions 8–10 days following an infection event. [1] These lesions will appear on carrot leaflets and petioles, and have a characteristic chlorotic, yellow halo. [3] The lesions can be irregularly shaped, and will often appear on older leaves first. [4] Older leaves are the most susceptible to infection; when approximately 40% of the leaf surface area has become infected by Alternaria dauci, the leaf will completely yellow, collapse, and die. [5] It is during extended conditions of warm, moist weather that lesions can coalesce and cause entire tops of carrot plants to die off, a phenomenon that is sometimes mistaken for frost damage. [6] The symptoms of this disease are also commonly confused with Cercospora Leaf Blight of carrots as well as bacterial blight, and microscopic analysis is frequently needed to accurately diagnose the pathogen. [7] A. dauci produces characteristically dark to olive-brown hyphae and elongated conidiophores, with conidia typically borne singly. [1] Petiole infection can also occur without any lesion development on leaflets, and A. dauci can additionally result in damping-off of seedlings, seed stalk blight, and inflorescence infections. [6] These symptoms can significantly reduce yield due to lost photosynthetic activity, prevention of mechanical harvest, and infection of commercial carrot seeds. [3]
Sexual reproduction of Alternaria dauci is not known to occur, [1] and the disease is most active during spring, summer, and autumn cropping cycles. [8] The disease cycle begins when fungus overwinters on or in host seed and in soil-borne debris from carrot. [6] A. dauci may also be spread into fields via contaminated carrot seeds during cultivation. [9] Once introduced, the pathogen can persist in carrot debris or contaminated seeds in the soil for up to two years. [9] Seedling infection near the hypocotyl-root junction (just below the soil line) then occurs in the early spring following overwintering of Alternaria dauci mycelium or conidia. [1] This infected region will become necrotic and lead to the production of more asexual conidia on conidiophores, which will serve as secondary inoculum. [1] Wind and rain cause conidia to disperse to neighboring host species, and multiple germination tubes will be produced from each conidium that successfully colonizes a new host. [1] As penetration occurs, Alternaria dauci will produce a chemical known as phytotoxin zinniol, which degrades cell membranes and chloroplasts, ultimately leading to the chlorotic symptoms characteristic of the disease. [1] These germination tubes will pierce host cell walls to initiate infection, or if wounds are present the pathogen may enter in that manner. [1] The process of germination, penetration, and symptom development generally occurs in a timespan of 8 to 16 days, but the presence of wounds shortens the amount of time needed to carry out the process. [6]
Following these events, conidia are repeatedly produced from leaf and stem lesions throughout the summer months, allowing the pathogen to be dispersed to its surrounding environment. [9] Inflorescence that is infected by A. dauci early in the summer will produce nonviable seeds, but plants infected later in the summer or early fall may still carry viable seeds; this fungus remains in the pericarp and does not penetrate the embryo or endosperm (non-systemic). [1] Following harvest in the fall, Alternaria dauci will persist in remaining carrot debris in the soil or be concentrated in infected seedlings, and the disease cycle will be repeated. [1]
Production and transmission of Alternaria dauci is heightened during moderate to warm temperatures and extended periods of leaf wetness due to rainfall, dew, or sprinkler irrigation. [1] [5] [10] Infection can occur between temperatures of 57 - 95 degrees Fahrenheit, with 82 degrees Fahrenheit being optimal. [6] Mycelium and spores are spread through splashing rain, tools for cultivation or contaminated soils. [11] Alternaria diseases, in general, tend to infect older, senescing tissues, and on plants developing under stress. [12] A study conducted by Vital et al. in 1999 assessed the influence of the rate of soil fertilization on the severity of Alternaria Leaf Blight in carrots and found low levels of nitrogen and potassium increased the severity of the pathogen, while high levels of the nutrients reduced disease severity. [8] Though it is not well understood why this occurs, it is postulated that higher nitrogen levels may extend a plant's vigor and delay maturation, which is important because A. dauci is more likely to infect senescing tissue. [8]
Effective management for Alternaria dauci involves preventing the introduction and development of the disease. One of the best practices to avoid infection is to plant pathogen-free seed or seed treated with hot water at 50 degrees Celsius for twenty minutes. [5] In addition to seed treated with fungicide or hot water, once harvest is complete it is imperative to turn the carrot residue under the soil. The pathogen only survives on infected plant debris, allowing this practice to hasten decomposition of the debris. [10] [13] [14] Crop rotation will allow the debris enough time to decompose. Recommendations vary depending on location, but 2 years is the minimum allowance for rotation. [10] [13] [14] Planting carrots continuously in the same field will result in increased infection. New fields should not be located near previously infected fields in order to prevent contamination through dispersal. Dispersal can occur through multiple avenues such as rain splash, farm equipment, workers, and insects.
Cultural practices can also promote reduction of Alternaria dauci. They include practices that will lower the duration of leaf wetness and soil moisture. Planting on raised beds with wider row spacing has been shown to reduce soil moisture, thereby limiting the spread of the disease. [13] Symptoms tend to be more severe on carrots that are stressed or poorly fertilized. In order to avoid more severe symptoms, keep the plants free of injury, watered, and adequately fertilized. Although resistant varieties are not available, the susceptibility of the carrot differs by variety. The varieties least susceptible vary by state, and a list of varieties appropriate to a specific area can be found through the state's extension program. In the Midwest, the University of Wisconsin and the University of Michigan have bred varieties including Atlantis, Beta III, and Chancellor that exhibit resistance. [15] [16]
In the absence of treated seed, there are multiple chemical sprays available to treat Alternaria dauci. Azoxystrobin, chlorothalonil, iprodione, pyraclostrobin and bacillus are a few common fungicides to consider for foliar application. A few brand names to look for in the Midwest include RR Endura 70WG, Rovral, and Switch. [17] Gibberrillic acid has shown to be equally effective as the aforementioned fungicides. However, if sprayed in excess giberrillic acid can defer nutrients from the roots to foliage, resulting in undeveloped carrots. [18] [19] If chemically treating plants, scouting the crop is of utmost importance. Initial threshold recommendations vary depending on location, time of year, and moisture level. Different recommendations include spraying upon first evidence of symptoms or spraying once disease has reached 25% of the foliage [13] [14]
Alternaria dauci is one of two leading pathogens affecting carrots around the world. Most often found in temperate climates, the disease has been found in North America, the Netherlands, the Middle East, and even parts of Southern Asia and India. [20] Carrot leaf blight is especially damaging in that its leaf lesions not only reduce photosynthetic area, but also weaken the leaves and petioles structurally. This makes mechanical harvesting of the carrot crop less efficient, and yields are even worse when blighted leaves have been exposed to heavy frosts. [21]
Alternaria dauci can spread rapidly if not controlled. Between February and November 2003, when the disease first spread to Turkey, 73-85% of surveyed fields were shown to be infected. Of those fields, disease rates among individual plants ranged from 65-90% total infection within the field. The highest levels of occurrence were always in moist fields with low levels of drainage. [22]
Alternaria dauci is most well known for its characteristic dark lesions on the leaves of carrots. These lesions are most often found on mature leaves, where full necrosis often follows. Younger leaves remain, for the most part, relatively unharmed. Immediately after the lesions form on the leaves, chlorosis begins to occur. One phytotoxin in particular, Al toxin, has been shown to both reduce chlorophyll production in leaves as well as cause stunting. [20]
Alternaria dauci first infects its hosts using germ tubes to penetrate the host cell's epidermis. This is done using the phytotoxin zinniol, which is the first toxin produced when the Alternaria conidia germinate in water. [23] Zinniol is highly hydrophobic, and has been detected inside cell walls after infection occurs, so it is assumed that the toxin degrades the cellulose past the epidermal layer. [23] Afterwards, disintegration of neighboring cell walls is seen to occur, as well as breakdown of cytoplasmic structure and complete destruction of chloroplasts. This is followed by a sharp decrease in the availability of polyphenol micronutrients. Polyphenols are important growth and photosynthetic regulators. After only six days of infection, there is almost total loss of photosynthesis on inoculated leaves. [20]
Alternaria is a genus of Deuteromycetes fungi. All species are known as major plant pathogens. They are also common allergens in humans, growing indoors and causing hay fever or hypersensitivity reactions that sometimes lead to asthma. They are present in the human mycobiome and readily cause opportunistic infections in immunocompromised people such as AIDS patients.
Pseudomonas cichorii is a Gram-negative soil bacterium that is pathogenic to plants. It has a wide host range, and can have an important economical impact on lettuce, celery and chrysanthemum crops. P. cichorii was first isolated on endives, from which it derives its name. It produces 6-aminopenicillanic acid. Based on 16S rRNA analysis, P. cichorii has been placed in the P. syringae group.
Glomerella graminicola is an economically important crop parasite affecting both wheat and maize where it causes the plant disease Anthracnose Leaf Blight.
Alternaria alternata is a fungus causing leaf spots, rots, and blights on many plant parts, and other diseases. It is an opportunistic pathogen on over 380 host species of plant.
Alternaria japonica is a fungal plant pathogen. It is a cause of black spot disease in cruciferous plants. It is not a major source of crop loss, but is considered dangerous for plants during the seedling stage.
Alternaria triticina is a fungal plant pathogen that causes leaf blight on wheat. A. triticina is responsible for the largest leaf blight issue in wheat and also causes disease in other major cereal grain crops. It was first identified in India in 1962 and still causes significant yield loss to wheat crops on the Indian subcontinent. The disease is caused by a fungal pathogen and causes necrotic leaf lesions and in severe cases shriveling of the leaves.
Stemphylium solani is a plant pathogen fungus in the phylum Ascomycota. It is the causal pathogen for grey leaf spot in tomatoes and leaf blight in alliums and cotton, though a wide range of additional species can serve as hosts. Symptoms include white spots on leaves and stems that progress to sunken red or purple lesions and finally leaf necrosis. S. solani reproduces and spreads through the formation of conidia on conidiophores. The teleomorph name of Stemphyllium is Pleospora though there are no naturally known occurrences of sexual reproduction. Resistant varieties of tomato and cotton are common, though the pathogen remains an important disease in Chinese garlic cultivation.
Ascochyta is a genus of ascomycete fungi, containing several species that are pathogenic to plants, particularly cereal crops. The taxonomy of this genus is still incomplete. The genus was first described in 1830 by Marie-Anne Libert, who regarded the spores as minute asci and the cell contents as spherical spores. Numerous revisions to the members of the genus and its description were made for the next several years. Species that are plant pathogenic on cereals include, A. hordei, A. graminea, A. sorghi, A. tritici. Symptoms are usually elliptical spots that are initially chlorotic and later become a necrotic brown. Management includes fungicide applications and sanitation of diseased plant tissue debris.
Alternaria carthami is a necrotrophic plant pathogen of safflower. The fungus is in the order Pleosporales and family Pleosporaceae. It was first isolated in India, has spread globally and can have devastating effects on safflower yield, and resultant oilseed production. A. carthami is known to be seed-borne and appears as irregular brown lesions on safflower leaves and stems.
Alternaria solani is a fungal pathogen that produces a disease in tomato and potato plants called early blight. The pathogen produces distinctive "bullseye" patterned leaf spots and can also cause stem lesions and fruit rot on tomato and tuber blight on potato. Despite the name "early," foliar symptoms usually occur on older leaves. If uncontrolled, early blight can cause significant yield reductions. Primary methods of controlling this disease include preventing long periods of wetness on leaf surfaces and applying fungicides. Early blight can also be caused by Alternaria tomatophila, which is more virulent on stems and leaves of tomato plants than Alternaria solani.
Ascochyta pisi is a fungal plant pathogen that causes ascochyta blight on pea, causing lesions of stems, leaves, and pods. These same symptoms can also be caused by Ascochyta pinodes, and the two fungi are not easily distinguishable.
Didymella bryoniae, syn. Mycosphaerella melonis, is an ascomycete fungal plant pathogen that causes gummy stem blight on the family Cucurbitaceae, which includes cantaloupe, cucumber, muskmelon and watermelon plants. The anamorph/asexual stage for this fungus is called Phoma cucurbitacearum. When this pathogen infects the fruit of cucurbits it is called black rot.
Colletotrichum coccodes is a plant pathogen, which causes anthracnose on tomato and black dot disease of potato. Fungi survive on crop debris and disease emergence is favored by warm temperatures and wet weather.
Alternaria helianthi is a fungal plant pathogen causing a disease in sunflowers known as Alternaria blight of sunflower.
Ascochyta blights occur throughout the world and can be of significant economic importance. Three fungi contribute to the ascochyta blight disease complex of pea. Ascochyta pinodes causes Mycosphaerella blight. Ascochyta pinodella causes Ascochyta foot rot, and Ascochyta pisi causes Ascochyta blight and pod spot. Of the three fungi, Ascochyta pinodes is of the most importance. These diseases are conducive under wet and humid conditions and can cause a yield loss of up to fifty percent if left uncontrolled. The best method to control ascochyta blights of pea is to reduce the amount of primary inoculum through sanitation, crop-rotation, and altering the sowing date. Other methods—chemical control, biological control, and development of resistant varieties—may also be used to effectively control ascochyta diseases.
Southern corn leaf blight (SCLB) is a fungal disease of maize caused by the plant pathogen Bipolaris maydis.
Gummy stem blight is a cucurbit-rot disease caused by the fungal plant pathogen Didymella bryoniae. Gummy stem blight can affect a host at any stage of growth in its development and affects all parts of the host including leaves, stems and fruits. Symptoms generally consist of circular dark tan lesions that blight the leaf, water soaked leaves, stem cankers, and gummy brown ooze that exudes from cankers, giving it the name gummy stem blight. Gummy stem blight reduces yields of edible cucurbits by devastating the vines and leaves and rotting the fruits. There are various methods to control gummy stem blight, including use of treated seed, crop rotation, using preventative fungicides, eradication of diseased material, and deep plowing previous debris.
Alternaria black spot of canola or grey leaf spot is an ascomycete fungal disease caused by a group of pathogens including: Alternaria brassicae, A. alternata and A. raphani. This pathogen is characterized by dark, sunken lesions of various size on all parts of the plant, including the leaves, stem, and pods. Its primary economic host is canola. In its early stages it only affects the plants slightly by reducing photosynthesis, however as the plant matures it can cause damage to the seeds and more, reducing oil yield as well.
Alternaria brassicicola is a fungal necrotrophic plant pathogen that causes black spot disease on a wide range of hosts, particularly in the genus of Brassica, including a number of economically important crops such as cabbage, Chinese cabbage, cauliflower, oilseeds, broccoli and canola. Although mainly known as a significant plant pathogen, it also contributes to various respiratory allergic conditions such as asthma and rhinoconjunctivitis. Despite the presence of mating genes, no sexual reproductive stage has been reported for this fungus. In terms of geography, it is most likely to be found in tropical and sub-tropical regions, but also in places with high rain and humidity such as Poland. It has also been found in Taiwan and Israel. Its main mode of propagation is vegetative. The resulting conidia reside in the soil, air and water. These spores are extremely resilient and can overwinter on crop debris and overwintering herbaceous plants.
Alternaria leaf spot or Alternaria leaf blight are a group of fungal diseases in plants, that have a variety of hosts. The diseases infects common garden plants, such as cabbage, and are caused by several closely related species of fungi. Some of these fungal species target specific plants, while others have been known to target plant families. One commercially relevant plant genus that can be affected by Alternaria Leaf Spot is Brassica, as the cosmetic issues caused by symptomatic lesions can lead to rejection of crops by distributors and buyers. When certain crops such as cauliflower and broccoli are infected, the heads deteriorate and there is a complete loss of marketability. Secondary soft-rotting organisms can infect stored cabbage that has been affected by Alternaria Leaf Spot by entering through symptomatic lesions. Alternaria Leaf Spot diseases that affect Brassica species are caused by the pathogens Alternaria brassicae and Alternaria brassicicola.