9.2 Fungal diseases

Fungal diseases pose significant challenges to plant health and crop production. These pathogens infect various plant parts, causing symptoms like spots, blights, and wilts. Understanding their types, symptoms, and transmission methods is crucial for effective management.

Diagnosing and managing fungal diseases involves visual inspection, microscopy, culturing, and molecular techniques. Control strategies include cultural practices, chemical applications, and biological agents. Developing resistant cultivars and implementing integrated disease management are key to minimizing economic losses and ensuring sustainable crop production.

Types of fungal diseases

• Fungal diseases are caused by various pathogenic fungi that infect different parts of plants, leading to a wide range of symptoms and damage
• These diseases can affect various plant organs, including leaves, stems, roots, and fruits, resulting in reduced plant health, yield, and quality

Foliar diseases

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  Septoria leaf spot and canker (Septoria musiva) View original

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  Alternaria leaf blight (Alternaria cucumerina) View original

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  Septoria leaf spot (Septoria chrysanthemi) View original

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  Septoria leaf spot and canker (Septoria musiva) View original

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  Alternaria leaf blight (Alternaria cucumerina) View original

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Top images from around the web for Foliar diseases

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  Septoria leaf spot and canker (Septoria musiva) View original

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  Alternaria leaf blight (Alternaria cucumerina) View original

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  Septoria leaf spot (Septoria chrysanthemi) View original

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  Septoria leaf spot and canker (Septoria musiva) View original

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  Alternaria leaf blight (Alternaria cucumerina) View original

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• Affect the leaves and cause symptoms such as spots, blights, and discoloration
• Examples include leaf spot diseases caused by fungi like Alternaria and Septoria
• Can reduce photosynthetic capacity and lead to premature leaf drop

Stem and branch diseases

• Infect the stems and branches, causing cankers, dieback, and galls
• Examples include apple scab caused by Venturia inaequalis and Dutch elm disease caused by Ophiostoma novo-ulmi
• Can disrupt water and nutrient transport, weaken the plant structure, and lead to branch or tree death

Root and crown diseases

• Affect the roots and crown region of the plant, causing rot, decay, and stunting
• Examples include Phytophthora root rot and Armillaria root rot
• Can impair water and nutrient uptake, leading to wilting, yellowing, and plant decline

Vascular wilt diseases

• Invade the vascular system of the plant, causing wilting, yellowing, and vascular discoloration
• Examples include Fusarium wilt and Verticillium wilt
• Can block the xylem vessels, disrupting water transport and leading to plant death

Fruit and seed diseases

• Infect the fruits and seeds, causing rots, molds, and quality deterioration
• Examples include gray mold (Botrytis cinerea) on strawberries and aflatoxin contamination in corn caused by Aspergillus flavus
• Can reduce yield, marketability, and pose food safety risks

Symptoms of fungal diseases

• Fungal diseases can manifest through a variety of visible symptoms on different plant parts, aiding in their identification and diagnosis
• Recognizing these symptoms is crucial for timely management and prevention of further spread

Leaf spots and blights

• Appear as circular or irregular spots on the leaves, often with a distinct margin or halo
• Examples include black spot on roses caused by Diplocarpon rosae and early blight on tomatoes caused by Alternaria solani
• Can coalesce to form larger blighted areas, leading to leaf yellowing and premature defoliation

Cankers and dieback

• Develop as sunken or raised lesions on the stems and branches, often accompanied by bark cracking and discoloration
• Examples include citrus canker caused by Xanthomonas citri and fire blight on apples and pears caused by Erwinia amylovora
• Can girdle the affected area, causing dieback of shoots and branches

Root rot and decay

• Manifest as soft, brown, or black rot of the roots, often accompanied by a foul odor
• Examples include Rhizoctonia root rot and Pythium root rot
• Can lead to stunted growth, wilting, and eventual plant death

Vascular discoloration

• Visible as brown or black streaks in the vascular tissue when the stem is cut longitudinally
• Examples include Fusarium wilt of tomatoes and Verticillium wilt of cotton
• Indicates the presence of fungal infection in the vascular system, disrupting water and nutrient transport

Fruit rots and molds

• Appear as soft, water-soaked lesions on the fruits, often covered with fungal growth
• Examples include brown rot of stone fruits caused by Monilinia fructicola and blue mold of citrus caused by Penicillium italicum
• Can cause pre- and post-harvest losses and reduce fruit quality and marketability

Fungal disease transmission

• Understanding the various mechanisms of fungal disease transmission is essential for developing effective management strategies and preventing the spread of infections
• Fungal pathogens employ diverse means to disseminate their spores and infect new hosts

Spore dispersal mechanisms

• Fungal spores can be dispersed by wind, water splashes, and insect vectors
• Examples include the wind-borne spores of rust fungi and the water-splashed spores of Phytophthora infestans, the cause of potato late blight
• Spore dispersal enables fungi to spread over short and long distances, infecting new plants and fields

Environmental factors influencing spread

• Temperature, humidity, and moisture play a crucial role in fungal growth and spore germination
• Examples include the favoring of powdery mildew development under high humidity and the requirement of free water for spore germination in many fungal pathogens
• Understanding the environmental conditions that favor disease development helps in predicting outbreaks and implementing timely control measures

Vectors and alternate hosts

• Insects, mites, and other organisms can act as vectors, carrying fungal spores from infected to healthy plants
• Examples include the spread of Dutch elm disease by elm bark beetles and the transmission of oak wilt by nitidulid beetles
• Alternate hosts, such as weeds or other plant species, can harbor fungal pathogens and serve as inoculum sources for crop infections

Fungal survival strategies

• Fungi employ various strategies to survive unfavorable conditions and persist in the environment
• Examples include the formation of resistant structures like sclerotia and chlamydospores, which can remain dormant in the soil for extended periods
• Understanding fungal survival strategies helps in developing management practices that target inoculum sources and reduce disease carry-over between growing seasons

Diagnosis of fungal diseases

• Accurate diagnosis of fungal diseases is crucial for selecting appropriate management strategies and preventing unnecessary or ineffective treatments
• A combination of visual inspection, microscopic examination, culturing, and molecular techniques can be used for reliable diagnosis

Visual inspection techniques

• Involve examining plant symptoms and signs, such as leaf spots, cankers, and fungal growth, with the naked eye or a magnifying lens
• Examples include the characteristic "shot hole" appearance of cherry leaf spot caused by Blumeriella jaapii and the white, powdery growth of powdery mildew fungi
• Visual inspection is often the first step in disease diagnosis, providing clues for further investigation

Microscopic examination

• Utilizes light microscopy to observe fungal structures, such as spores and hyphae, from infected plant samples
• Examples include the examination of conidiophores and conidia of Botrytis cinerea, the cause of gray mold, and the observation of oospores of Phytophthora species
• Microscopic examination aids in the identification of fungal genera and species based on their morphological characteristics

Culturing and isolation

• Involves plating infected plant tissues on nutrient media to isolate and grow the causal fungal pathogen
• Examples include the isolation of Fusarium species from infected vascular tissues and the culturing of Colletotrichum species from fruit lesions
• Culturing allows for the identification of fungal species based on colony morphology and growth characteristics, and enables further studies on pathogenicity and fungicide sensitivity

Molecular detection methods

• Employ PCR-based techniques, such as real-time PCR and DNA barcoding, to detect and identify fungal pathogens from plant samples
• Examples include the detection of Verticillium dahliae in soil using PCR assays and the identification of Puccinia species using DNA barcoding of the ITS region
• Molecular methods offer high sensitivity, specificity, and rapid diagnosis, particularly for early detection and identification of quarantine pathogens

Management of fungal diseases

• Effective management of fungal diseases relies on a combination of cultural, chemical, and biological control strategies, often integrated into a comprehensive disease management program
• The choice of management tactics depends on the specific pathogen, crop, and environmental conditions

Cultural control practices

• Involve the manipulation of the growing environment and cultural practices to create conditions less favorable for disease development
• Examples include crop rotation to reduce inoculum build-up, pruning to improve air circulation, and irrigation management to minimize leaf wetness duration
• Cultural practices are often the first line of defense against fungal diseases and can help reduce the need for chemical interventions

Chemical control options

• Involve the use of fungicides to prevent or cure fungal infections
• Examples include the application of protectant fungicides like chlorothalonil and mancozeb, and the use of systemic fungicides like triazoles and strobilurins
• Chemical control can be highly effective but requires proper timing, dosage, and application methods to minimize the risk of fungicide resistance development and environmental impacts

Biological control agents

• Utilize beneficial microorganisms, such as bacteria and fungi, to suppress fungal pathogens through various mechanisms, including competition, antibiosis, and parasitism
• Examples include the use of Trichoderma species as biocontrol agents against soil-borne pathogens and the application of Bacillus subtilis to control foliar diseases
• Biological control offers a more sustainable and environmentally friendly approach to disease management but may have variable efficacy and require specific environmental conditions for optimal performance

Integrated disease management strategies

• Combine multiple control tactics, such as cultural practices, chemical control, and biological control, into a comprehensive program to manage fungal diseases effectively
• Examples include the integration of resistant cultivars, fungicide applications, and sanitation practices to manage apple scab, and the use of crop rotation, seed treatment, and foliar fungicides to control soybean rust
• Integrated disease management aims to optimize disease control while minimizing the reliance on any single control method, reducing the risk of resistance development and enhancing the sustainability of the cropping system

Resistance to fungal diseases

• Developing and deploying resistant cultivars is a key strategy for managing fungal diseases in a sustainable and environmentally friendly manner
• Resistance can be achieved through various mechanisms and can be influenced by both genetic and environmental factors

Types of resistance

• Include qualitative (vertical) resistance, which is often race-specific and controlled by single genes, and quantitative (horizontal) resistance, which is typically polygenic and provides partial resistance against multiple pathogen races
• Examples of qualitative resistance include the Rps genes in soybeans conferring resistance to Phytophthora sojae, while quantitative resistance is exemplified by the slow-rusting genes in wheat against Puccinia triticina
• Understanding the types of resistance helps in designing appropriate breeding strategies and deploying resistance genes effectively

Genetic basis of resistance

• Involves the identification and characterization of genes and quantitative trait loci (QTLs) that confer resistance to fungal pathogens
• Examples include the cloning of the Cf genes in tomato conferring resistance to Cladosporium fulvum and the mapping of QTLs for resistance to Fusarium head blight in wheat
• Elucidating the genetic basis of resistance enables marker-assisted selection and facilitates the introgression of resistance genes into elite cultivars

Breeding for disease resistance

• Involves the development of resistant cultivars through traditional breeding methods, such as cross-breeding and recurrent selection, or modern techniques like genetic engineering and genome editing
• Examples include the development of potato cultivars with stacked resistance genes against late blight and the creation of transgenic bananas resistant to Fusarium wilt
• Breeding for disease resistance is a continuous process that requires the identification of new resistance sources, the incorporation of resistance genes into adapted cultivars, and the monitoring of pathogen populations for the emergence of new races

Induced resistance mechanisms

• Involve the activation of the plant's innate defense responses through the application of biotic or abiotic elicitors, leading to enhanced resistance against fungal pathogens
• Examples include the use of salicylic acid and its analogs to induce systemic acquired resistance (SAR) and the application of beneficial microbes to trigger induced systemic resistance (ISR)
• Induced resistance offers a promising approach to disease management, as it can provide broad-spectrum protection and reduce the reliance on chemical fungicides

Economic impact of fungal diseases

• Fungal diseases cause significant economic losses in agriculture worldwide, affecting crop yield, quality, and marketability
• The economic impact of fungal diseases extends beyond direct yield losses, encompassing management costs, trade restrictions, and broader societal implications

Yield losses and crop damage

• Fungal diseases can cause substantial yield reductions by affecting various plant parts and growth stages
• Examples include the yield losses caused by soybean rust (Phakopsora pachyrhizi), which can reach up to 80% in susceptible cultivars, and the impact of wheat stem rust (Puccinia graminis f. sp. tritici) on grain production
• Yield losses directly affect farmers' income and can threaten food security in regions heavily dependent on affected crops

Quality reduction and marketability

• Fungal infections can reduce crop quality by causing visual defects, off-flavors, and contamination with mycotoxins
• Examples include the impact of apple scab on fruit appearance and marketability and the contamination of maize with aflatoxins produced by Aspergillus flavus
• Quality reduction can lead to lower prices, rejection of produce by buyers, and potential health risks for consumers

Management and control costs

• Implementing disease management strategies, such as fungicide applications, cultural practices, and resistant cultivars, incurs additional costs for farmers
• Examples include the expenses associated with fungicide sprays to control grape powdery mildew and the costs of developing and deploying resistant cultivars
• These management costs can significantly impact the profitability of crop production, particularly for smallholder farmers with limited resources

Trade restrictions and quarantines

• The presence of certain fungal diseases can result in trade restrictions and quarantine measures imposed by importing countries
• Examples include the ban on the import of citrus fruits from areas affected by citrus black spot (Phyllosticta citricarpa) and the quarantine regulations for the movement of oak wood to prevent the spread of oak wilt (Bretziella fagacearum)
• Trade restrictions and quarantines can limit market access, disrupt supply chains, and cause economic losses for exporting countries and industries

Notable fungal diseases

• Several fungal diseases have gained notoriety due to their widespread occurrence, economic impact, and historical significance
• These diseases continue to pose significant challenges to crop production and require ongoing research and management efforts

Rusts and smuts

• Rust fungi (order Pucciniales) and smut fungi (order Ustilaginales) are obligate parasites that infect a wide range of crops, causing significant yield losses
• Examples include wheat stem rust (Puccinia graminis f. sp. tritici), which has caused devastating epidemics throughout history, and corn smut (Ustilago maydis), which can lead to complete ear destruction
• Rusts and smuts are known for their complex life cycles, often involving alternate hosts, and their ability to rapidly evolve and overcome host resistance

Powdery and downy mildews

• Powdery mildew fungi (order Erysiphales) and downy mildew fungi (family Peronosporaceae) are widespread pathogens that infect a variety of crops, causing leaf damage and yield losses
• Examples include grapevine powdery mildew (Erysiphe necator) and cucurbit downy mildew (Pseudoperonospora cubensis)
• These diseases are favored by specific environmental conditions, such as high humidity and moderate temperatures, and can spread rapidly under conducive conditions

Anthracnose and scab

• Anthracnose fungi (genus Colletotrichum) and scab fungi (genus Venturia) cause fruit and foliar diseases in a range of crops, affecting yield and quality
• Examples include apple scab (Venturia inaequalis), which is a major constraint in apple production worldwide, and sorghum anthracnose (Colletotrichum sublineolum), which can cause severe yield losses
• Anthracnose and scab diseases are often characterized by distinctive lesions on leaves and fruits, and their management relies on a combination of cultural practices, fungicide applications, and resistant cultivars

Fusarium and Verticillium wilts

• Fusarium and Verticillium are soil-borne fungi that cause vascular wilt diseases in a wide range of crops, leading to plant wilting, yellowing, and death
• Examples include Fusarium wilt of banana (Fusarium oxysporum f. sp. cubense), which threatens global banana production, and Verticillium wilt of cotton (Verticillium dahliae), which can cause significant yield losses
• These diseases are particularly challenging to manage due to the persistence of the pathogens in the soil and their ability to infect plants through the root system, requiring long-term management strategies and the development of resistant cultivars