Identifying and Treating Common Fungal Infections

Plant infections are common, widespread, and can be a real challenge to treat – especially fungal infections, which are responsible for 85% of plant diseases.

Fungal diseases can wreak havoc on crops, causing damage ranging from deformation and defoliation to twig, limb, and plant death. From leaf spot to root rot, infections cause a 10-20% loss in crops each year – equivalent to $100-$200 billion in the U.S. alone. The keys to preventing fungal pests are proactive treatment and prevention.

To effectively treat a fungus, it is crucial to first recognize the signs and symptoms of an infestation. But with more than 19,000 kinds of fungi causing disease in crops worldwide, identifying which type is causing damage can be overwhelming.

In this blog, you’ll learn about some common fungal infections found in both plants and crops, how abiotic and biotic factors play a role in fungal infections, how to diagnose and manage fungal disease, and tips for preventing infections.

Common Fungal Infections

The sheer number of fungal pathogens makes it near impossible for the average grower to isolate what type of fungi is damaging or destroying a plant, but there are common fungus infections that you are most likely to encounter.

Following are some of the most common fungal infections and visible signs and symptoms they may be infecting your foliage:

Blight affects fruits, vegetables, and flowers, as well as trees and shrubs. It impacts the leaves, stems, fruit, and tubers, and limits yield. It can live within the soil and has the potential to cause a total crop loss.

There are multiple varieties of blight. “Early blight” is caused by Alternaria tomatophila and Alternaria solani and appears in moderate to warm temperatures – 50 to 86 degrees. It thrives in humid conditions, including wet weather and heavy dew. Early blight symptoms first appear as brown spots on leaves and stems at the base of affected plants. These roughly circular spots may have a yellow halo, and as they enlarge, concentric rings appear, giving the areas a target-like appearance. Multiple spots on a single leaf will eventually combine, leading to extensive destruction of leaf tissue. Early blight can lead to total defoliation of lower leaves and kill an infected plant.

Despite its name, “late blight” may occur at the same time as early blight but may not be visible until later in the season. Leaves of plants infected with late blight exhibit pale-green or olive-green areas that quickly enlarge, becoming brown-black, water-soaked, and oily-looking. Stems may also exhibit dark-brown to black areas.

This fungus can impact multiple plants, but tomatoes and potatoes are especially susceptible, and it is responsible for $6.7B in potato crop losses annually. (Late blight was responsible for the 1840’s Irish Potato Famine that killed 1 million people.) Potato tubers may develop sunken areas with a reddish-brown skin discoloration. Tomato fruits develop large, often sunken, golden- to chocolate-brown, firm spots with distinct rings. Affected leaf, stem, fruit or tuber tissue often eventually develops a white-gray, fuzzy look as the late blight organism begins to reproduce. Cool, wet weather conditions can lead to plant death from late blight in just seven to 10 days.
Anthracnose is another type of blight. It involves a group of related fungal diseases that can produce dark lesions that may be black, brown, or tan, depending on the host plant affected. Sometimes called bud, leaf, shoot, or twig blight, this fungus causes the most damage when prolonged spring rains take place soon after new growth develops.

This pest may appear as irregular, dead areas on leaf margins, between and across and/or along veins, often moving onto the shoots and small twigs; sometimes whole leaves are engulfed. Large areas of infected leaves turn brown and fully infected leaves will drop. Anthracnose can cause gnarls or growth distortion, but doesn’t threaten the host plant unless defoliation, dieback, or cankering happen yearly.
Black spot, caused by the fungus Diplocarpen rosae, targets roses. Infected areas range from brown to black, are roughly circular shaped, and may be up to half-inch in diameter, often with feathery margins. Black spot initially appears during periods of wet weather, starting on lower leaves before spreading to the entire plant. Severe black spot infections cause yellowing and leaf defoliation. It is not considered fatal but can weaken infected rose bushes, making them more susceptible to other pests.
Leaf spot appears as a dead area on a leaf surrounded by healthy tissue. These spots are usually seen on the lower, inner branches, where humidity is high and leaves are shaded, and may occur on the upper, lower, or both surfaces of the leaf. Spots may range in color from yellow to orange to red, tan, brown, or black and appear in every size and shape, although they may start as random spots that appear after the pathogen lands there through wind or water splashes.

This type of fungus weakens trees and shrubs by interrupting photosynthesis. It may affect only a small portion of the host plant but, if it causes moderate to severe damage during consecutive seasons, it may result in a weakened host that is more susceptible to other pests and diseases.
Powdery mildew is one of the most easily diagnosable fungal infections, identified by its resemblance to baby powder being shaken onto a leaf. Caused by multiple genus of fungi and most often seen on the surface and sometimes the stems of leaves, powdery mildew does not require moisture to infect a host and can survive dry seasons as a result, unlike most other fungi. The extent of damage from powdery mildew ranges from purely cosmetic to instances of leaf drop and distortion, and the death of young shoots.
Downy mildew can be difficult to diagnose because there are many species of the disease, sometimes only targeting one type of plant, and each producing lesions of different sizes and colors. Caused by the fungus Pseudoperonospora cubensis, downy mildew affects a number of edible plants, including cauliflower, broccoli, lettuce, cucumbers, grapes, as well as ornamental varieties ranging from trees and shrubs to annual and perennial flowers.

Downy mildew symptoms begin as small, water-soaked spots, appearing first as slightly yellow-green and progressing to a bright yellow on the upper leaf surface. Lesions become angular, brown and distorted as they progress, and plants may defoliate. More harmful than powdery mildew, untreated downy mildew can lead to damaged fruit and crop losses.
Oak leaf blister is caused by infections of the fungus Taphrina caerulescens. Leaf blisters usually appear several weeks after an infection has occurred, often as leaf buds swell and open during wet, spring days. These blisters may appear as light green bulges on the top of the leaf, with the underside appearing to have a sunken, or depressed, area. Given time, the blisters may become dry, brown spots. Affected leaves may bend or curl but the fungus does not endanger the host plant.
Rust is a common fungal disease in the order Uredinales that is often identified by the red, orange, brown and yellow spots it produces on leaves. Raised pustules of powdery spores may form on the underside of the leaf surface, but may also appear on any green part of the plant, including the stem and the flower calyx. Severely infected leaves may turn brown or yellow, and may fall off. Plants infected with rust disease may experience stunted growth or easily wilt.
Wilt is caused by fungus Fusarium and Verticillium, two different species that cause similar symptoms in broad leaf plants. Individual branches or even single leaves may be affected at first, with infected foliage displaying a wilted appearance before developing a yellow color, often in V-shaped sectors between the major veins. Leaves eventually die and fall. Wilt may cause discoloration or brown streaking in vascular tissues. Diseased plants may die soon after first symptoms or they may sprout at the base after the top dies.
Root rot may result from a number of soil-borne fungi, including Pythium, Phytophthora, Rhizoctonia solani, and Fusarium, which are the most common varieties. Because they have wide host ranges, these four fungi types can cause root rots on a wide variety of plants. Infected plants may exhibit wilt, even when the soil is moist, and have leaves with a yellow or red color consistent with a nutritional deficiency. Roots often have a bad odor and may be soft and brown. Root rot can lead to plant death.

Several of the common fungal diseases above affect both edible and ornamental plants, trees and shrubs, but there are a few crop-specific fungal diseases that are also frequent pests for farmers.

Septoria leaf spot is a fungal infection that targets tomatoes and is caused by the fungus Septoria lycopersici. Symptoms typically begin as plant canopies start to close and usually first appear at the base of affected plants, where small, whitish spots with a dark border spots appear on leaves and stems. Eventually multiple spots on a single leaf will merge, leading to extensive destruction of leaf tissue. Septoria leaf spot can lead to total defoliation of lower leaves and the death of an infected plant.
Clubroot commonly occurs in brassicas roots, affecting crops like cabbage, cauliflower, turnips, and brussels sprouts. From the Plasmodiophora brassicae family, clubroot causes roots to swell and become distorted, sometimes appearing spindle shaped, and hinders the roots’ ability to absorb water and nutrients. Above-ground foliage may be stunted and appear purplish, with wilting leaves. The disease may kill young plants; older plants may fail to produce yield.
Damping off impacts seedlings. Commonly caused by the fungus Pythium, it may also be the result of pathogens from the Rhizoctonia, Fusarium, and Phytophthora species. Seeds infected before germination may fail to grow, and those seedlings that do emerge may become soft and mushy, turn brown, shrivel, and decay. Seedlings attacked after they emerge will show decayed and weakened stem tissue near the soil line, usually causing plants to topple and die. If only the roots are decayed, seedlings may continue to grow slowly or may eventually die.
Black leg disease impacts canola crops, including cabbage, cauliflower, turnips, and occasionally mustard, and is caused by two species of the Leptosphaeria fungus. Signs of the infection include circular, grey spots on leaves, with black pepper-like specks appearing in the center of the spots. When symptoms are severe, numerous leaf spots develop in the lower canopy, resulting in lower leaf yellowing and death. The spots become tan as they age. With time, lesions or cankers develop on the stems, and may extend into the stem, resulting in premature ripening and incomplete pod fill.
Fusarium is a common fungal disease in agriculture. Fusarium oxysporum is a complex fungus that has many host-specific strains often infecting tomatoes, cucumber, watermelon, eggplant, and pepper crops. Symptoms may begin on lower leaves before moving up to impact the entire plants. Temporary wilting – during the heat of the day and with overnight recovery – eventually becomes permanent and yellowing follows. The discoloration may only develop on one side of a shoot or branch. Brown leaf tips may occur, and will give way to full leaf death and the loss of the host plant.
Scleratina blight, which may also be called white mold, timber rot, drop, or head rot, is caused by Scleratina minor, and impacts leafy greens and cole crops, legumes, and root vegetables. The first symptoms of infection appear as a fluffy or cottony, white moldy growth around diseased areas of lower stems. On affected stems, tan to pale white lesions will develop and appear sunken and elongated. Soon entire limbs wilt and turn yellow. Affected branches die and turn dark brown in color and eventually whole plants die. The fungus rots stem tissue, leading to stem shredding, which means pods are unable to develop.

The above is by no means a comprehensive list of all fungal infections but it comprises many of the diseases growers regularly face.

How Do Fungal Infections Develop and Spread?

Interestingly, a plant disease is considered a malfunction in the plant in response to continuous irritation by a causal agent.

Development. The causal agents of plant disease are biotic (living) and are called pathogens. However, not every pathogen can cause an infection. A disease can only develop with three criteria in place simultaneously: A pathogen comes in contact with a susceptible host plant during favorable environmental conditions that enable disease development.

Fungi have several different lifecycle patterns based on the genus. Regardless of the species, growth and feeding take place during the vegetative stage, which is when microscopic threads called hyphae develop and absorb nutrients from the environment. Hyphae growth can distribute the pathogens that spread disease. Hyphae may form a visible network of thread-like structures called mycelium, which release spores to spread the fungus to other host plants.

For a disease to spread, a pathogen requires inoculum, which is any part of the pathogen that can cause infection, including spores, particles, cells, or nematodes. In the case of fungal diseases, fungal spores or mycelium make up the inoculum. These inoculum can be found in soil, seeds, weeds, and other crops or crop residue.

The fungal disease cycle follows this pattern:

The pathogen is spread.
The inoculum penetrates the host plant.
The infection develops within the host plant.
Secondary cycles begin, producing new pathogens.
The inoculum survives between growing seasons.

Dispersal. Fungal diseases may reside in seed, soil, crop residue, other crops, and weeds, and are easily dispersed by any number of factors. Weather elements (wind and rain), contaminated items like gardening equipment, gloves, or clothing, or a vector—a living organism that transports a diseased spore between plants, like an insect or a pollinator.

Many fungi survive the winter in infected plant parts, including bulbs, roots, stems, and bud scales, emerging during the growing season and spreading as mentioned above when the conditions permit. Some, like powdery mildew, may spread for hundreds of miles carried by a gust of wind, making these pests challenging to contain and manage.

Diagnosing and Confirming the Disease

Plant diseases may be biotic or abiotic, or both. We covered several biotic, or living, diseases above. Abiotic plant disorders are not associated with a living organism like a fungus, but are attributed to physical, environmental, or chemical factors.

Physical. Types of physical factors that may impact plant growth include poor planting methods – not providing for deep enough root development, for example – along with improper pruning and soil management, and take their toll on gardens and landscapes.
Environmental. Climate conditions play a significant role in plant disease. Soil that is consistently too dry or too wet weakens a plant’s immunity, as do temperature extremes and too much or too little shade. Drought stress and cold injury lead to conditions like leaf scorch, leaf drop, and branch dieback, all of which lead to weakened immunity against fungal infections.
Chemical. Abiotic plant disorders result from chemicals like fertilizers and pesticides used to fight plant disease, but they can also result from factors unrelated to agriculture, like air pollution, spilled motor oil or drain cleaners, and ice-melt mixtures that make their way into soil and impact vegetation. These chemicals cause phototoxicity in impacted plants, which can result in leaf cupping, distortion, wilting, browning, stunted growth and plant death.

While these abiotic physical, environmental, and chemical factors may result in plant disease in and of themselves, they also create ideal conditions for biotic fungal infections to occur should a pathogen be introduced.

Given the information above, growers may be able to diagnose some fungal infections by doing a bit of research without expert input. Following these steps can help you identify the cause of a plant disease and ultimately take steps to manage an infection:

Identify the host plant and how a healthy specimen should look. Multiple species of the host will have different appearances and it is key to ensure the correct variety.
Examine the plant for symptoms and signs. Carefully inspect each aspect of the diseased plant, from the leaves to the stem and roots, taking note of color and texture variations that appear unusual.
Research common problems for the plant. Understanding frequently seen issues may help dismiss disease concerns.
Consider which causal factors might be biotic and abiotic. Plants that were placed in unfavorable growing locations may be failing due to location versus infection.

If you’re unsure whether your plant is infected by a fungal disease, reference materials also serve as excellent resources. Experts at local garden nurseries or college extension offices, which are staffed by people with knowledge of plants native to that location, are also able to help diagnose plant diseases. Many states also have a plant diagnostic lab that can assist in identifying infections.

Treating, Managing, and Preventing Fungal Diseases
Integrated pest management, known as IPM, is a science-based approach that employs multiple methods to manage plant health by managing the ecosystem.

Cultural disease management strategies are based on good sanitation and husbandry. Controlling plant diseases with cultural practices is based on improving the growing conditions for maximum plant health while preventing the environment that enables pathogen growth.

Some examples of IPM practices include yearly crop rotation, which can disrupt year-to-year pest cycles; interplantings, or alternating different plants within rows or patches to minimize infestation; weed management to diminish habitats for pests develop; watering in the morning to enable plants to dry thoroughly before cooler, humid evenings foster fungal growth; and pruning to promote air circulation around plants to discourage infection.

IPM also involves the premise of biol o gic control: using nature against itself. For instance, to control insect populations, introduce their natural enemies. Biologic practices promote long-term sustainability by preserving natural predator-prey relationships and reducing the risk of pesticide resistance.

Chemical solutions are considered part o f IPM, but only those with natural ingredients should be introduced. Unfortunately, growers relied on toxic chemicals to manage plant diseases for decades with disastrous results. The damage from prolonged use of pesticides and fungicides is well documented on people and our planet. Pesticide use is responsible for causing everything from physical issues including chronic illnesses and human death to environmental impacts including polluted waterways and soil and endangering beneficial insects and wildlife. What’s more, after years of exposure, many diseases have developed an immunity to pesticides, rendering their use ineffective against pests like fungal infections.

As part of a safe IPM practice, growers have a safe, organic option when it comes to treating, managing, and preventing fungal infections: GrowSafe. An all-natural formulation made of a proprietary blend of food-grade ingredients, GrowSafe kills fungal disease on contact and provides a physical coating that acts as a defensive barrier against figure fungal attacks, promoting higher yields and healthy, disease-free plant growth.

Better and safer than chemical-based pesticides and fungacides, as well as neem and mineral-based oils, GrowSafe’s all-natural ingredients won’t harm the environment if it gets into the soil. The product is safe for people, pets, pollinators, and beneficial insects that come into contact with treated plants, and treatment will not change the smell or taste of herbs and botanicals, fruits, or vegetables.

Regular monitoring and a proactive approach are key to managing fungal infections. The conditions will always provide an environment in which fungi can attack foliage, but taking steps to recognize the early signs and symptoms of a fungal infection, and utilizing a combination of IPM cultural, biological, and chemical controls like GrowSafe as a preventative and treatment tool, creates a strong pest management strategy against this destructive disease.

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Blog Outline: Identifying and Treating Common Fungal Infections

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Introduction

Plant infections are common, widespread, and can be challenging to treat
Fungal infections aren’t the same as viral or bacterial types and require different disease management
Know what to look for and how to treat various fungal infections

In this blog, you’ll learn about common fungal infections found in both plants and crops, how abiotic and biotic factors play a role in fungal infections, how to diagnose and manage fungal disease, and tips for preventing infections.

Source: https://www.canr.msu.edu/news/signs_and_symptoms_of_plant_disease_is_it_fungal_viral_or_bacterial

Section 1: Fungal infections

Common types of fungal infections, with signs and symptoms of each
- Blackspot
- Leafspot
- Powdery mildew
- Downy mildew
- Blight
Plant versus crop fungal infections: Differences and similarities