30. Top 10 Fungal Diseases of Wheat

 Fungal Diseases of Wheat

Here short detail of every disease is discussed.

1. Loose Smut of Wheat:

    Loose smut of wheat is a fungal disease caused by Ustilago tritici, which infects the plant at the flowering stage and produces a seed containing smut spores. 

• The disease can spread easily through air and rain splashes and has been reported worldwide wherever wheat is grown. 
• Loose smut was first identified by Romans and later characterized by Fabricius in 1774. 
• The disease was introduced to countries like Australia, America, and South Africa by European settlers who brought infected wheat with them. 
• While resistant genotypes have been developed, they have not been effective in stopping the spread of the disease. 
• Loose smut can cause yield reductions of up to 20% and renders seeds unfit for sowing. 
• The disease is seed-borne, with teliospores present in the kernel of infected plants. 
• The teliospores enter the ovary during anthesis and can survive inside the embryo without causing external symptoms. 
• The external symptom is the presence of black spores on a mature spike, which are released as soon as the ear emerges.

2. Wheat leaf Rust or brown rust:

    Wheat leaf rust, caused by Puccinia triticina, is a highly adaptable disease that affects wheat worldwide. 

• The fungus has a five-stage spore cycle and requires two hosts to complete its life cycle.
• The disease reduces the size and weight of wheat kernels and is spread by wind. 
• The fungus produces asexual and sexual spores, with the latter requiring an alternate host to complete its life cycle. 
• The disease cycle repeats every 8-14 days under optimal temperature and moisture conditions. 
• External symptoms include small, circular pustules filled with orange to brown urediniospores on the upper side of the leaf surface. 
• Teliospores produced at the end of the season help the fungus overwinter and become a source of inoculum for the next growing season.

3. Wheat stem rust or black rust:

    Stem rust is a devastating disease of wheat caused by Puccinia graminis f. sp. tritici, which can cause yield losses of up to 50% and even up to 90% in severe cases. 

• The pathogen is an obligate parasite with a biotrophic mode of nutrition and requires living hosts to complete its life cycle. 
• Stem rust is heteroecious, requiring both wheat (primary host) and the barberry plant (alternate host) to complete its life cycle, although it can also complete its life cycle without an alternate host. 
• Stem rust produces urediniospores, teliospores, basidiospores, pycnidiospores, and aeciospores to complete its life cycle. 
• The disease can cause annual losses of 8-54 billion dollars globally, and its optimum temperature for infection is 30°C with 2 hours of leaf wetness.

4. Stripe Rust or yellow Rust:

    Stripe rust is a destructive disease caused by the fungus Puccinia striiformis f. sp. tritici, which can infect a wide range of host plants including wheat, rye, and barley. 

• It can cause yield losses of up to 100% if conditions are favorable and the variety is susceptible and has caused significant losses in Asia. 
• The disease infection cycle is similar to other rust pathogens, with infection caused by urediniospores and the movement of spores facilitated by the wind. 
• The disease is spreading rapidly and changing strains, making it difficult to produce resistant genotypes. 
• Environmental factors, the vulnerability of host plants, and alternate hosts all contribute to the disease's severity.

5. Powdery mildew:

    Powdery mildew, caused by Blumeria graminis f. sp. tritici, is a highly contagious disease in plants and is considered the 6th most threatening fungal yield disease and the 8th major disease in the world.

• It is an endemic disease that attacks wheat at the vegetative stage, mostly affecting high-yielding and high-nitrogen fertilizer varieties. 
• Infection reduces photosynthesis and leaf assimilation index, and increases respiration, affecting grain quality and ultimately leading to plant death. 
• Yield losses range from 5-35% and can be as high as 40% if the flag leaf is infected.
• Powdery mildew on wheat appears as a grayish powdery colony on the upper and lower surfaces of the leaf and stem. 
• It produces white cottony pustules early in the season, and asexual spores known as conidia are spread by wind. 
• Sexual spores are also produced in cleistothecia. 
• The disease progresses to cover the entire plant and turns yellow, with black fruiting bodies and a gray powdery mass appearing on the leaves. 
• The disease is favored by temperatures between 10 and 21°C, and high humidity and temperatures between 15 and 20°C are most suitable for disease spread. 
• The fungus grows inside the tissues during winter and spreads from plant to plant and field to field by wind. 
• The life cycle repeats in 7-10 days, and new strains may develop.

6. Karnal bunt:

    Karnal bunt of wheat is caused by Tilletia indica, a disease that affects the quality of seeds and makes them unfit for eating and sowing. 

• It is an air, seed, and soilborne disease that spreads easily to far-off places and produces a specific smell even if 1% contamination occurs. 
• It can cause yield losses of up to 40% in severely infected fields and is common in many countries, leading to bans on importing wheat. 
• The disease epidemics depend on suitable weather conditions, and teliospores require a temperature of 20-25°C, pH 6-9.5, and 80% moisture rate or free water for germination.
• The symptoms of the bunt disease in wheat include swollen and fallen grain in the spike, reduced spike length and number, and oval-shaped sori containing black to brown powdery spores. 
• The bunt fungus infects wheat after diazotization and spreads from cell to cell. Teliospores drop on soil and become a source of primary inoculum, and secondary sporidia play an important role in the disease cycle. 
• The disease produces a characteristic fishy smell due to the release of triethylamine. 
• In unfavorable conditions or if the wheat plant does not reach the vulnerable stage, it leads to "suicidal germination."
• Karnal bunt spores can persist in soil for up to 3 years and can easily spread from one farm to another through contaminated farm machinery. 
• These spores can withstand very low temperatures and remain viable. Airborne spores can be dispersed up to a distance of 3000 meters. 
• Teliospores, which are the reproductive structures of the fungus, exhibit resistance to various chemical treatments including chloropicrin, hydrogen peroxide, methyl bromide, ozone, and propionic acid.
• Isolates of the pathogen may differ from one another in physical and morphological features, chromosomal count, infection severity, and ability to overcome host defenses.
• Tilletia indica, the causative agent of Karnal bunt, is considered the most potent and highly pathogenic smut fungus.

7. Flag Smut:

    Flag smut, caused by Urocystis agropyri, is another significant disease that requires a living host to complete its life cycle, similar to other smuts and rusts. 

• Infected seedlings exhibit bending and twisting, along with the appearance of white patches and blistered coleoptiles. 
• Leaves of mature plants also develop white patches, which turn gray to black. 
• Infected plants exhibit stunted growth, distorted and twisted tillers, and may not produce spikes and grains. 
• Poor root development and reduced germination are also observed in infected grains. 
• The extent of losses due to flag smut ranges from 5 to 20% depending on environmental conditions.

8. Fusarium Head Blight:

    Fusarium head blight, also known as ear blight or head scab, is caused by several fungal pathogens, including F. culmorum, F. graminerarum, and F. avenaceum, with F. graminearum being the most common. 

• The pathogen is found in America, Europe, and Asia, and it can survive in crop residues as saprobes. 
• Inoculum consists of conidia and ascospores, with ascospores being sexual spores produced in perithecia fruiting bodies and released by the wind. 
• FHB infects wheat plants from the anthesis stage until kernel production, with external symptoms appearing on the head, peduncle, spikes, and grains. 
• Infected spikes turn yellow and contain pinkish and orange-colored spore colonies. 
• F. graminearum produces several mycotoxins, which can cause food poisoning in humans and animals.
• Moderate rainfall and a temperature range of 24-29°C for 2-3 days are the optimum conditions for disease infection. 
• Fungal spores can survive in crop residues, while high humidity and rainfall can increase inoculum and aid in dispersal.

9. Tan Spot:

    Pyrenophora tritici repentis is the causal agent of tan spot, also known as yellow spot or blotch, which is a prevalent disease in several European countries including the UK, Sweden, Germany, France, and Denmark, leading to significant yield loss. 

• This disease affects various plant species, including perennial crops, which serve as hosts for overwintering pathogens and contribute to the increased inoculum levels during epidemics. 
• Tan spot symptoms include small dark brown flecks that develop into black spots on the basal leaves. 
• The spots combine to form large tan, irregular-shaped lesions with yellow rings surrounding the lesion. 
• Under humid conditions, lesions produce dark spores, and the tissue surrounding the lesion dies. Infected seeds have pink to red spores, black spots, and poor germination rates.
• The fungus overwinters in the form of dormant mycelium on crop residues. 
• The pseudothecium is the fruiting body, and ascospores are produced inside, which can be spread by wind to distant locations. Infected seeds can also contribute to the spread of the disease. 
• Under warm and wet conditions, asexual conidia germinate and spread with rain, infecting the ear, glume, and developing grain. 
• Disease symptoms typically appear 7 to 14 days after infection, with the optimum temperature range being between 20 and 28°C.

10. Septoria disease:

    Septoria is caused by three different pathogens, including P. avenaria triticae, Mycosphaerella graminicola (anamorph; Zymoseptoria tritici), and Phaeosphaeria nodorum. 

• The disease caused by Mycosphaerella graminicola is called Septoria tritici blotch (STB), which reduces both the size and quality of the grain, causing 30-50% yield losses during epidemics. 
• It is prevalent in cooler, wet, and humid regions and is common in North and South America, North and South Africa, northern Europe, and Turkey. 
• Symptoms include chlorotic lesions on leaves in fall and spring that darken and produce fruiting bodies as the disease progresses. 
• Fruiting bodies include both sexual pseudothecia and asexual pycnidia.
• Septoria tritici blotch (STB) is caused by the fungal pathogen Mycosphaerella graminicola, and it can cause significant yield losses in wheat crops. 
• The pathogen overwinters in crop residues and infects plants through airborne ascospores.
• The spores attach to stomata and produce appressoria, which allows them to enter the plant and grow mycelium inside it. 
• The pathogen can grow in both biotrophic and necrotrophic modes, and lesions on leaves develop as the disease progresses. 
• Conidia are produced on necrotic sites and can spread to healthy plants through rainwater.
• Pycnidia produce conidia within 15-40 days after infection, depending on environmental conditions. 
• Spores can also undergo a dormant state until favorable conditions occur.

Note:

The link of every disease is given for detailed study. 

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