9.2 Fungal diseases

Types of fungal diseases

Fungal diseases are caused by pathogenic fungi that can infect virtually every part of a plant. The type of disease is often categorized by which organ or tissue the fungus attacks, and knowing the category helps you narrow down the pathogen and choose the right management approach.

Foliar diseases

• Target the leaves, producing spots, blights, and discoloration
• Common culprits include Alternaria (causing leaf spots on many crops) and Septoria (a major problem in wheat)
• Because leaves are where photosynthesis happens, heavy foliar infection reduces the plant's energy production and can trigger premature leaf drop

Stem and branch diseases

• Infect stems and branches, causing cankers (sunken, dead areas of bark), dieback, and sometimes galls
• Apple scab (Venturia inaequalis) and Dutch elm disease (Ophiostoma novo-ulmi) are well-known examples
• These infections can cut off water and nutrient transport, weaken structural integrity, and kill entire branches or trees

Root and crown diseases

• Attack the roots and crown (the junction between root and stem), causing rot, decay, and stunting
• Phytophthora root rot and Armillaria root rot are two of the most damaging examples
• Since roots are underground, these diseases often go unnoticed until above-ground symptoms like wilting and yellowing appear

Vascular wilt diseases

• The fungus colonizes the xylem (water-conducting vessels), physically blocking water transport
• Fusarium wilt and Verticillium wilt are the classic examples
• Symptoms include progressive wilting, leaf yellowing, and brown streaking visible when you cut the stem lengthwise

Fruit and seed diseases

• Infect fruits and seeds, causing rots, molds, and quality loss
• Gray mold (Botrytis cinerea) on strawberries and aflatoxin contamination of corn by Aspergillus flavus are major concerns
• Beyond reducing yield and marketability, some fruit and seed fungi produce mycotoxins that pose serious food safety risks

Symptoms of fungal diseases

Recognizing symptoms early is the first step toward diagnosis and timely management. Different fungal groups tend to produce characteristic symptoms, so learning these patterns gives you a head start in identifying the problem.

Leaf spots and blights

• Appear as circular or irregular spots, often with a distinct margin or halo of discolored tissue
• Black spot on roses (Diplocarpon rosae) and early blight on tomatoes (Alternaria solani) are textbook examples
• Individual spots can merge into large blighted areas, causing yellowing and premature defoliation

Cankers and dieback

• Show up as sunken or raised lesions on stems and branches, often with cracked or discolored bark
• Note: citrus canker is actually caused by a bacterium (Xanthomonas citri), and fire blight is caused by Erwinia amylovora (also a bacterium). True fungal cankers include Nectria canker on hardwoods and Cytospora canker on stone fruits
• When a canker girdles (encircles) a branch, everything above it dies back

Root rot and decay

• Roots become soft, brown, or black and may have a foul odor
• Rhizoctonia and Pythium are common causes (though Pythium is technically an oomycete, not a true fungus, it's traditionally studied alongside fungal pathogens)
• Stunted growth, wilting, and plant death follow as the root system loses its ability to absorb water and nutrients

Vascular discoloration

• Visible as brown or dark streaks in the vascular tissue when you cut the stem lengthwise
• Fusarium wilt of tomatoes and Verticillium wilt of cotton both produce this telltale symptom
• This discoloration confirms that the pathogen is inside the vascular system, not just on the surface

Fruit rots and molds

• Start as soft, water-soaked lesions that quickly become covered with visible fungal growth
• Brown rot of stone fruits (Monilinia fructicola) and blue mold of citrus (Penicillium italicum) are common examples
• These can cause losses both before and after harvest, making post-harvest handling and storage conditions critical

Fungal disease transmission

Fungi have evolved multiple ways to spread from plant to plant. Understanding these transmission routes is essential for designing management strategies that actually interrupt the disease cycle.

Spore dispersal mechanisms

• Wind: Rust fungi produce enormous numbers of lightweight spores that can travel hundreds of kilometers on air currents
• Water splash: Pathogens like Phytophthora infestans (potato late blight) spread when rain or irrigation splashes spore-laden water onto nearby plants
• Insect vectors: Some fungi hitch a ride on insects that move between plants
• These mechanisms allow fungi to spread across both short and long distances

Environmental factors influencing spread

• Temperature, humidity, and moisture are the big three factors controlling fungal growth and spore germination
• Powdery mildew thrives under high humidity (but often doesn't need free water on the leaf surface), while many other fungi require a film of water on plant tissue for spores to germinate
• Tracking weather conditions helps predict outbreaks and time control measures effectively

Vectors and alternate hosts

• Insects and mites can physically carry spores from infected to healthy plants. Elm bark beetles spread Dutch elm disease, and nitidulid beetles transmit oak wilt
• Alternate hosts are other plant species (including weeds) that harbor the pathogen between crop seasons, serving as a source of inoculum
• Removing alternate hosts near crop fields can reduce disease pressure

Fungal survival strategies

• Many fungi produce tough resting structures to survive between growing seasons:
  • Sclerotia: compact masses of hardened hyphae that can persist in soil for years
  • Chlamydospores: thick-walled spores that resist drying and temperature extremes
• Knowing how a pathogen survives helps you target management practices at reducing the inoculum that carries over from one season to the next

Diagnosis of fungal diseases

Accurate diagnosis matters because the wrong identification leads to the wrong treatment. Diagnosis typically moves from simple observation to increasingly precise laboratory methods.

Visual inspection techniques

• The first step: examine the plant for characteristic symptoms and signs (visible fungal structures like spore masses or mycelium)
• Some diseases have distinctive visual clues, like the "shot hole" appearance of cherry leaf spot (Blumeriella jaapii) or the white powdery coating of powdery mildew
• Visual inspection narrows down the possibilities but rarely confirms a specific pathogen on its own

Microscopic examination

• A light microscope lets you observe fungal structures like spores (conidia), spore-bearing structures (conidiophores), and hyphae
• For example, Botrytis cinerea has distinctive grape-like clusters of conidia, and Phytophthora species produce characteristic oospores
• Morphological features under the microscope can often identify the fungus to genus or species level

Culturing and isolation

• Infected plant tissue is placed on nutrient media (like potato dextrose agar) to grow the pathogen in pure culture
• Colony appearance, growth rate, and spore production on the media help confirm identification
• Culturing also enables further testing, such as pathogenicity assays and fungicide sensitivity screening

Molecular detection methods

• PCR-based techniques (including real-time PCR and DNA barcoding) detect pathogen DNA directly from plant or soil samples
• The ITS region (internal transcribed spacer) of ribosomal DNA is the most commonly used barcode for fungal identification
• Molecular methods offer high sensitivity and speed, making them especially valuable for early detection and for identifying quarantine pathogens that require rapid confirmation

Management of fungal diseases

No single tactic controls fungal diseases perfectly on its own. Effective management combines multiple approaches tailored to the specific pathogen, crop, and growing conditions.

Cultural control practices

Cultural practices modify the growing environment to make it less hospitable for fungi:

• Crop rotation breaks disease cycles by depriving soil-borne pathogens of their host
• Pruning improves air circulation, which reduces the leaf wetness that many fungi need
• Irrigation management (e.g., drip irrigation instead of overhead sprinklers) minimizes water on foliage

These are often the first line of defense and can significantly reduce the need for chemical inputs.

Chemical control options

• Protectant fungicides (e.g., chlorothalonil, mancozeb) coat plant surfaces and prevent spore germination. They must be applied before infection occurs
• Systemic fungicides (e.g., triazoles, strobilurins) are absorbed into plant tissue and can have both preventive and curative activity
• Proper timing, correct dosage, and rotation between fungicide classes are critical to avoid fungicide resistance, where pathogen populations evolve to tolerate the chemical

Biological control agents

• Beneficial microorganisms suppress pathogens through competition for resources, production of antimicrobial compounds (antibiosis), or direct parasitism of the pathogen
• Trichoderma species are widely used against soil-borne pathogens, and Bacillus subtilis is applied to control foliar diseases
• Biocontrol is more environmentally sustainable than chemical control, but efficacy can vary depending on environmental conditions and proper application

Integrated disease management strategies

Integrated disease management (IDM) combines cultural, chemical, and biological tactics into a coordinated program. For example:

• Managing apple scab might involve planting resistant cultivars, applying fungicides at key infection periods, and removing fallen leaves (sanitation) to reduce overwintering inoculum
• Controlling soybean rust could combine crop rotation, seed treatments, and foliar fungicides timed to weather-based disease forecasts

The goal of IDM is to reduce reliance on any single method, which slows resistance development and improves long-term sustainability.

Resistance to fungal diseases

Planting resistant cultivars is one of the most effective and sustainable ways to manage fungal diseases. Resistance reduces or eliminates the need for fungicide applications, saving money and reducing environmental impact.

Types of resistance

Two main categories:

• Qualitative (vertical) resistance: controlled by one or a few major genes, often provides complete resistance but only against specific races of the pathogen. Example: Rps genes in soybeans that confer resistance to specific races of Phytophthora sojae
• Quantitative (horizontal) resistance: controlled by many genes, each with a small effect. Provides partial resistance against a broad range of pathogen races. Example: slow-rusting genes in wheat against Puccinia triticina

Qualitative resistance can be overcome when the pathogen evolves a new race, while quantitative resistance tends to be more durable over time.

Genetic basis of resistance

• Researchers identify and characterize specific resistance genes and quantitative trait loci (QTLs) that contribute to disease resistance
• The Cf genes in tomato (conferring resistance to Cladosporium fulvum) and QTLs for Fusarium head blight resistance in wheat are well-studied examples
• Understanding the genetic basis enables marker-assisted selection, where breeders use DNA markers to track resistance genes during crosses, speeding up the breeding process

Breeding for disease resistance

• Traditional methods include cross-breeding resistant and susceptible parents, then selecting offspring that combine resistance with desirable agronomic traits
• Modern approaches include genetic engineering and genome editing (e.g., CRISPR) to introduce or modify resistance genes more precisely
• Breeding is a continuous effort because pathogen populations keep evolving. New resistance sources must be identified, and pathogen populations must be monitored for emerging races

Induced resistance mechanisms

• Plants have innate defense systems that can be "primed" or activated by certain triggers:
  • Systemic acquired resistance (SAR): triggered by chemicals like salicylic acid or its synthetic analogs (e.g., acibenzolar-S-methyl). Provides broad-spectrum protection throughout the plant
  • Induced systemic resistance (ISR): triggered by beneficial root-colonizing microbes
• Induced resistance doesn't replace other management tools, but it can complement them by boosting the plant's own defenses

Economic impact of fungal diseases

Fungal diseases cost global agriculture billions of dollars annually. The economic damage goes well beyond the crops lost in the field.

Yield losses and crop damage

• Yield reductions can be severe. Soybean rust (Phakopsora pachyrhizi) can destroy up to 80% of the yield in susceptible cultivars. Wheat stem rust (Puccinia graminis f. sp. tritici) has caused famines historically
• These losses directly reduce farmer income and can threaten food security in regions that depend heavily on affected crops

Quality reduction and marketability

• Fungal infections cause visual defects (spots, discoloration), off-flavors, and mycotoxin contamination
• Apple scab makes fruit unsellable for fresh markets, and aflatoxin contamination of maize (from Aspergillus flavus) poses direct health risks to consumers
• Lower quality means lower prices, rejected shipments, and potential regulatory action

Management and control costs

• Fungicide applications, resistant seed, scouting labor, and cultural practice modifications all add up
• For example, managing grape powdery mildew can require multiple fungicide sprays per season, each with material and application costs
• These expenses hit smallholder farmers with limited resources especially hard

Trade restrictions and quarantines

• Certain fungal diseases trigger import bans and quarantine regulations. Citrus black spot (Phyllosticta citricarpa) and oak wilt (Bretziella fagacearum) are examples of diseases that restrict international trade
• These measures protect importing countries but can devastate exporting regions by cutting off market access and disrupting supply chains

Notable fungal diseases

Some fungal diseases stand out for their historical impact, global reach, or ongoing threat to major crops.

Rusts and smuts

• Rust fungi (order Pucciniales) and smut fungi (order Ustilaginales) are obligate parasites, meaning they can only grow on living host tissue
• Wheat stem rust has caused devastating epidemics throughout history. Corn smut (Ustilago maydis) can destroy entire ears
• Both groups are known for complex life cycles (rusts often require two different host species to complete theirs) and the ability to rapidly evolve new races that overcome host resistance

Powdery and downy mildews

• Powdery mildew fungi (order Erysiphales) grow on the leaf surface and produce a characteristic white, powdery coating. Downy mildew fungi (family Peronosporaceae) grow inside leaf tissue and produce sporulation on the underside of leaves
• Grapevine powdery mildew (Erysiphe necator) and cucurbit downy mildew (Pseudoperonospora cubensis) are economically important examples
• Note: downy mildew organisms are oomycetes, not true fungi, but they're traditionally covered alongside fungal diseases in plant pathology

Anthracnose and scab

• Anthracnose (Colletotrichum spp.) and scab (Venturia spp.) cause distinctive lesions on leaves and fruits
• Apple scab is one of the most important diseases in apple production worldwide. Sorghum anthracnose (Colletotrichum sublineolum) causes severe losses in tropical and subtropical regions
• Management typically requires combining resistant cultivars, well-timed fungicide applications, and cultural practices like sanitation

Fusarium and Verticillium wilts

• Both are soil-borne fungi that invade through the roots and colonize the vascular system
• Fusarium wilt of banana (Fusarium oxysporum f. sp. cubense, especially Tropical Race 4) is currently one of the biggest threats to global banana production. Verticillium dahliae attacks cotton, strawberries, and hundreds of other crops
• These pathogens are extremely difficult to manage because they persist in soil for years (sometimes decades), making crop rotation alone insufficient. Long-term strategies combining resistant cultivars, soil management, and sanitation are essential