Northern corn leaf blight (NCLB) or Turcicum leaf blight (TLB) is a foliar disease of corn (maize) caused by Exserohilum turcicum, the anamorph of the ascomycete Setosphaeria turcica . With its characteristic cigar-shaped lesions, this disease can cause significant yield loss in susceptible corn hybrids. [1]
Lesions can eventually expand to a more oblong or “cigar” shape. They may also coalesce to form large areas of necrotic tissue.
There are several host-specific forms of E. turcicum. The most economically important host is corn, but other forms may infect sorghum, Johnson grass, or sudangrass. [2] The most common diagnostic symptom of the disease on corn is cigar-shaped or elliptical necrotic gray-green lesions on the leaves that range from one to seven inches long. [3] These lesions may first appear as narrow, tan streaks that run parallel to the leaf veins. Fully developed lesions typically have a sooty appearance during humid weather, as a result of spore (conidia) formation. As the disease progresses, the lesions grow together and create large areas of dead leaf tissue. The lesions found in Northern corn leaf blight are more acute if the leaves above the ear are infected during or soon after flowering of the plant. [4] In susceptible corn hybrids, lesions are also found on the husk of ears or leaf sheaths. In partially resistant hybrids, these lesions tend to be smaller due to reduced spore formation. In highly resistant hybrids, the only visible disease symptoms may be minute yellow spots. [5]
On severely infected plants, lesions can become so numerous that the leaves are eventually destroyed. Late in the season, plants may look like they have been killed by an early frost. Lesions on products containing resistance genes may appear as long, chlorotic, streaks, which can be mistaken for Stewart’s wilt or Goss's wilt. [6]
In nature, E. turcicum lives and reproduces in an asexual phase with a relatively simple life cycle. In temperate regions, the fungus overwinters mycelia, conidia, and chlamydospores in the infected corn debris. [2] When conditions become favorable the following season, conidia are produced from the debris and dispersed by rain or wind to infect new, healthy corn plants. [5] Once on a leaf, conidia will germinate and directly infect the plant. The damage to the plant is relatively localized, although diseased corn plants are more susceptible to stalk rot than are healthy plants. [2] In conditions with high humidity, the fungus will produce new spores at the leaf surface, which are spread by rain or wind through the crop and create cycles of secondary infection. [5] One complete cycle on susceptible plants takes approximately 10 to 14 days, whereas it takes about 20 days on plants with resistance. [2] At the end of the season, E. turcicum goes into a state of dormancy in crop residue.
The ideal environment for NCLB occurs during relatively cool, wet seasons. [5] Periods of wetness that last more than six hours at temperatures between 18 and 27 °C (64 and 81 °F) are most conducive to disease development. [2] Infection is inhibited by high light intensity and warm temperatures. Leaving large amounts of infected residue exposed in the field and continuing to plant corn in those fields will promote disease progress by providing large amounts of inoculum early in the season. [5] Also, the number of conidia produced in an infected field increases significantly after rain due to the increase in moisture. [7]
Sporulation requires a 14-hour dew period between 20 and 25 °C (77 °F). When there is not a sufficiently long period of continuous humidity, the fungus will stop making spores and resume conidia production only when humidity level rises again. For this reason, sporulation often occurs during nighttime and is halted when humidity falls during the day. [7]
In the United States, NCLB is a problem during the spring in southern and central Florida and during the summer months in the Midwestern states. [8] On a global scale, NCLB is a problem in corn-growing areas in the mid-altitude tropics, which have the wet, cool environment that is favorable for disease development. These susceptible areas include parts of Africa, Latin America, China, and India. [1]
Preventative management strategies can reduce economic losses from NCLB. Preventative management is especially important for fields at high risk for disease development. In-season disease management options, such as fungicides, are also available.
Management of NCLB can be achieved primarily by using hybrids with resistance, but because resistance may not be complete or may fail, it is advantageous to utilize an integrated approach with different cropping practices and fungicides. Scouting fields and monitoring local conditions is vital to control this disease. [5]
Major (vertical) resistance of corn hybrids comes from the race specific Ht1, Ht2, Ht3, and HtN genes, with the Ht1 gene being most prevalent. Plants with Ht1 Ht2, or Ht3 genes have smaller, chlorotic lesions and reduced sporulation. [2] The HtN gene delays symptoms until after the pollen shed. Individually, each Ht gene has limited effectiveness because there are races of E. turcicum that are virulent in the presence of one or the other. For example, widespread use of the Ht1 gene has reduced the prevalence of the Race 0 to which it has resistance against, but has increased Race 1. Breeders are now focusing on incorporating several resistance genes into corn hybrids. Incorporating both the Ht1 and Ht2 provide resistance against both Races 0 and 1. Thus far, this multigenic approach has proven to be effective. However resistant plants still show some symptoms, and the threat of new races showing up lends to the need for other management practices, especially in areas where the disease is present. [9]
Ways to change cropping practices to control the disease include reducing the amount of infected residue left in a field, managing weeds to improve airflow and reduce humidity, and encouraging residue decomposition with tillage. The tillage will assist in breaking down crop debris and reducing existing inoculum. In a system with normal tillage, a one-year rotation out of corn can be effective, but a two-year rotation may be required for a reduced-tillage system. If possible, planting in low areas that receive heavy dew and fog should be avoided. [5] A combination of crop rotation for one to two years followed by tillage is recommended to prevent NCLB disease development. [10]
The use of foliar fungicides for corn have also been shown to control NCLB. [5] Research suggests that using fungicides to keep the upper 75% of the leaf canopy disease-free for three quarters of the grain-filling period will eliminate yield loss [11] To ensure that newly emerging leaf tissue is protected from infection, before the plants are in tassel, fungicides should be applied on the same day that significant conidial dispersal is expected to occur. After tasseling and silking, timing becomes less important since plant expansion will have slowed down. [12] The disease pressure in the field and weather conditions should be monitored and evaluated beforehand to determine if fungicides are needed or not. [5]
NCLB can cause significant yield loss in corn. If severe disease is present two to three weeks after silking in field corn, grain yields may be reduced by 40 to 70 percent. In the U.S. Corn Belt and Ontario, NCLB has recently become a significant disease, [5] causing estimated yield losses of an alarming 74.5 million bushels of grain in 2012 and 132.3 million bushels of grain in 2013. [13]
In susceptible varieties of sweet corn, yields can be reduced by up to 20 percent. In fresh market sweet corn, not only is yield lost, but market value will decrease if the ear husks become infected. The lesions cause the ears to appear old and poor quality even if they are fresh. [7]
Researchers in Hokkaido, Japan have also discovered that NCLB reduces the quality of corn silage as animal feed. Their study showed that the digestibility of dry matter, organic matter, and gross energy was significantly lower in the inoculated silage compared to the control. Total digestible nutrients and digestible energy were reduced by 10.5 and 10.6 percent, respectively [14]
Spores of the fungus that causes this disease can be transported by wind long distances from infected fields. Spread within and between fields locally also relies on wind blown spores.
E. turcicum causes disease and reduces yield in corn primarily by creating the necrotic lesions and reducing available leaf area for photosynthesis. [5] Following conidia germination, the fungus forms an appressorium, which penetrates the corn leaf cell directly using an infection hypha. Once below the cuticle, the infection hypha produces infection pegs to penetrate the epidermal cell wall. After penetration through the cell wall, the fungus produces intracellular vesicle to obtain nutrients from the cell. After approximately 48 hours after infection, necrotic spots begin to form as the epidermal cells collapse. [15]
Fungal toxins also play an important role in disease development. Researchers have found that a small peptide called the E.t. toxin allows a non-pathogenic isolate of E. turcicum to infect corn when suspensions of conidia and the toxin were in contact with the leaves. This toxin has also been shown to inhibit root elongation in seedlings and in chlorophyll synthesis. Another toxin produced by E. turcicum, called monocerin, is a lipophilic toxin known to cause necrosis of leaf tissue. [16]
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 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.
Gibberella zeae, also known by the name of its anamorph Fusarium graminearum, is a fungal plant pathogen which causes fusarium head blight (FHB), a devastating disease on wheat and barley. The pathogen is responsible for billions of dollars in economic losses worldwide each year. Infection causes shifts in the amino acid composition of wheat, resulting in shriveled kernels and contaminating the remaining grain with mycotoxins, mainly deoxynivalenol (DON), which inhibits protein biosynthesis; and zearalenone, an estrogenic mycotoxin. These toxins cause vomiting, liver damage, and reproductive defects in livestock, and are harmful to humans through contaminated food. Despite great efforts to find resistance genes against F. graminearum, no completely resistant variety is currently available. Research on the biology of F. graminearum is directed towards gaining insight into more details about the infection process and reveal weak spots in the life cycle of this pathogen to develop fungicides that can protect wheat from scab infection.
Diaporthe helianthi is a fungal pathogen that causes Phomopsis stem canker of sunflowers. In sunflowers, Phomopsis helianthi is the causative agent behind stem canker. Its primary symptom is the production of large canker lesions on the stems of sunflower plants. These lesions can eventually lead to lodging and plant death. This disease has been shown to be particularly devastating in southern and eastern regions of Europe, although it can also be found in the United States and Australia. While cultural control practices are the primary method of controlling for Stem Canker, there have been a few resistant cultivars developed in regions of Europe where the disease is most severe.
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.
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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.
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Peronosclerospora sorghi is a plant pathogen. It is the causal agent of sorghum downy mildew. The pathogen is a fungal-like protist in the oomycota, or water mold, class. Peronosclerospora sorghi infects susceptible plants though sexual oospores, which survive in the soil, and asexual sporangia which are disseminated by wind. Symptoms of sorghum downy mildew include chlorosis, shredding of leaves, and death. Peronosclerospora sorghi infects maize and sorghum around the world, but causes the most severe yield reductions in Africa. The disease is controlled mainly through genetic resistance, chemical control, crop rotation, and strategic timing of planting.
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Cercospora melongenae is a fungal plant pathogen that causes leaf spot on eggplant. It is a deuteromycete fungus that is primarily confined to eggplant species. Some other host species are Solanum aethiopicum and Solanum incanum. This plant pathogen only attacks leaves of eggplants and not the fruit. It is fairly common among the fungi that infect community gardens and home gardens of eggplant. Generally speaking, Cercospora melongenae attacks all local varieties of eggplants, but is most severe on the Philippine eggplant and less parasitic on a Siamese variety.
This article summarizes different crops, what common fungal problems they have, and how fungicide should be used in order to mitigate damage and crop loss. This page also covers how specific fungal infections affect crops present in the United States.
Grey leaf spot (GLS) is a foliar fungal disease that affects maize, also known as corn. GLS is considered one of the most significant yield-limiting diseases of corn worldwide. There are two fungal pathogens that cause GLS: Cercospora zeae-maydis and Cercospora zeina. Symptoms seen on corn include leaf lesions, discoloration (chlorosis), and foliar blight. Distinct symptoms of GLS are rectangular, brown to gray necrotic lesions that run parallel to the leaf, spanning the spaces between the secondary leaf veins. The fungus survives in the debris of topsoil and infects healthy crops via asexual spores called conidia. Environmental conditions that best suit infection and growth include moist, humid, and warm climates. Poor airflow, low sunlight, overcrowding, improper soil nutrient and irrigation management, and poor soil drainage can all contribute to the propagation of the disease. Management techniques include crop resistance, crop rotation, residue management, use of fungicides, and weed control. The purpose of disease management is to prevent the amount of secondary disease cycles as well as to protect leaf area from damage prior to grain formation. Corn grey leaf spot is an important disease of corn production in the United States, economically significant throughout the Midwest and Mid-Atlantic regions. However, it is also prevalent in Africa, Central America, China, Europe, India, Mexico, the Philippines, northern South America, and Southeast Asia. The teleomorph of Cercospora zeae-maydis is assumed to be Mycosphaerella sp.
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.
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Cherry leaf spot is a fungal disease which infects cherries and plums. Sweet, sour, and ornamental cherries are susceptible to the disease, being most prevalent in sour cherries. The variety of sour cherries that is the most susceptible are the English morello cherries. This is considered a serious disease in the Midwest, New England states, and Canada. It has also been estimated to infect 80 percent of orchards in the Eastern states. It must be controlled yearly to avoid a significant loss of the crop. If not controlled properly, the disease can dramatically reduce yields by nearly 100 percent. The disease is also known as yellow leaf or shothole disease to cherry growers due to the characteristic yellowing leaves and shot holes present in the leaves upon severe infection.
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.