How to Identify and Control Blight On Plants

The word “blight” does not refer to one specific plant disease but rather is a general term that describes conditions that severely hinder the healthy growth of plants. Blight on plants has had a significant impact on human populations, movements, and diets throughout history.

Some of the most notable blights include the potato blight that caused the Irish Potato Famine of 1846 to 1850, the bacterial leaf blight in rice that swept through Asia in the 1960s and caused the loss of up to 80% of crops, and the southern leaf corn blight epidemics of 1970 to 1971, which totaled one billion dollars in economic damages.

There are many different causes of blight on various plants and economically important vegetables. Many of these pathogens are specific to a few plant species and are not genetically related to each other. In this article, we will provide an overview of different plant diseases known as blights and offer a general description of each type and their control methods.

Bacterial Diseases

Bacteria are single-celled prokaryotic organisms with a wide range of genetic and metabolic diversity. Pathogenic bacteria typically occupy spaces between cells and tend to colonize xylem vessels. They may also produce plant toxins that lead to cell death or necrosis.

Bacterial Blight

Bacterial blight includes two types of pathogenic bacteria: Pseudomonas savastanoi, which affects soybeans, and Pseudomonas syringae pv. pisi, which impacts field peas. Both are early-season diseases most visible on young leaves. P. savastanoi overwinters on host debris and is transmitted by rain, wind, or surface contact with infected material. P. syringae pv. pisi, on the other hand, is predominantly a seed-borne pathogen that impacts peas after a frost or severe weather conditions.

After the bacteria enter a plant through the stomata or wounds, they produce a toxin that stops chlorophyll production. Brown spots may be visible on the margins of cotyledons, and young plants might appear stunted or even die. In later growth stages, the leaves will develop yellow spots that eventually turn reddish-brown and dry out as lesions. Seed pods might also develop lesions and shrivel, although seeds will not exhibit symptoms.

You can manage both types of bacterial blight by planting resistant varieties, waiting to plant after wet or severe weather, and practicing crop rotation. For P. syringae pv. pisi, growers can also source disease-free seeds from reputable suppliers.

Bacterial Leaf Blight Of Rice 

Bacterial leaf blight (BLB), or kresek disease, is caused by the pathogenic bacterium Xanthomonas oryzae. This is a serious disease for Oryza sativa (rice) and impacts rice growers worldwide in tropical and temperate regions. In addition to rice, it can also affect grass and wild rice species.

The bacteria favor warm, wet weather and nitrogen-rich environments. Monsoon and typhoon seasons are particularly damaging. This pathogen can travel by rain, wind, irrigation, plant-to-plant contact, and human handling during transplanting or while using contaminated tools. The bacterium enters the host through the natural uptake of water or via wounds. Since rice cultivation requires transplanting, this process might result in wounds at the roots or openings when the tip of the seedlings is clipped as part of operations. Once inside the plant, the pathogen moves systematically through the xylem.

The two symptoms of this disease are long yellow lesions along the blade and plant wilt (kresek disease). Bacterial ooze may be visible on the lesions. Total production will decrease, and the seeds of infected plants may also appear discolored. In plants suffering from kresek disease, the leaves will curl and turn a greenish-gray color. Plants often do not survive wilt. The most reliable methods of prevention are using resistant varieties, practicing field sanitation, and implementing proper spacing and fertilizer management.

Bacterial Seedling Blight Of Rice 

Burkholderia plantarii is a relatively new disease first recorded at a rice nursery in Japan in 1985. It has since been found across rice-growing regions in Asia. Once inside the host plant, this bacterium produces a phytotoxin called tropolone, which causes the blight symptoms. Infected seedlings show chlorosis and stunted growth of leaves and roots.

Common Blight

Common blight is a warm-weather disease caused by the bacterium Xanthomonas campestris pv. phaseoli that impacts runner beans, French beans, mung beans, garden lupin, and peas in the vegetable garden. It enters the plant through natural openings or wounds and can affect leaves, stems, pods, and seeds.

This disease first manifests as water-soaked, angular leaf spots. The spots expand over time and dry out as plant tissues die. The brown patches will be surrounded by a ring of yellow leaf tissue. On stems, the bacterium results in brown patches with no yellow ring. Infected stems may also wilt. Discoloration can also appear on pods and seeds, along with bacterial ooze under very humid conditions.

You can control common blight by using disease-free seeds, practicing crop rotation, removing volunteer plants and weed hosts, and limiting overhead watering to keep foliage dry. Good sanitation practices are also important, as the bacterium overwinters on host debris and on the soil surface. There are also cultivars with some disease tolerance or resistance.

Fire Blight 

Fire blight, a bacterial disease caused by Erwinia amylovora, attacks pome trees and mountain ash. It can also impact raspberry, hawthorn, serviceberry, and cotoneaster. It prefers warm weather conditions and surges after severe weather like storms, strong winds, or hail that create wounds in susceptible plants.

The fire blight bacterium overwinters on infected plants and in cankers formed during the previous season. It can multiply in leaves, flowers, fruits, bark, twigs, and shoots. The disease’s name comes from the telltale symptom that turns infected plant parts black or brown, resulting in a burned appearance. In shoots, this produces a very characteristic hook shape as they wilt and start to die back. Infected fruits will shrivel and mummify.

There are no plants in the pome family that are completely resistant to fire blight, although some have a better chance of survival. Act quickly and prune infected branches with sanitized tools to prevent disease progression before the bacteria eventually kills the entire plant.

Halo Blight 

Halo blight is a significant agricultural disease for the bean industry. It is a seed-borne disease caused by the bacterium Pseudomonas savastanoi pv. phaseolicola and is very hard to control once it has taken hold. This disease leads to halo-like leaf chlorosis and lesions that stunt plant growth and eventually kill infected plants. The bacterium also attacks pods, appearing as water-soaked brown spots with crusty bacterial ooze.

Fungal Diseases

Fungi are the leading cause of plant ailments, with over 8,000 pathogenic species. They can cause both preharvest and postharvest blemishes and rot on leaves, fruits, and tubers and produce toxins that impact humans and other animals.

Chestnut Blight

Chestnut blight has been one of the most devastating ailments for the chestnut tree population in North America. Chestnut trees were a pervasive native tree on the continent until the arrival of the fungus Cryphonectria parasitica with imported Japanese chestnut trees in the late 19th century.

While Japanese and Chinese chestnut trees can be infected, researchers found that they have some resistance and the disease is not fatal. However, American chestnuts do not have any resistance; by the 1940s, over 3.5 billion chestnut trees had been felled by this disease.

Chestnut blight first manifests as cankers on the above-ground parts of the tree. Fungal spores of C. parasitica germinate and enter the tree via the bark. Plant tissue dies surrounding the cankers, causing the tree to become water-stressed and vulnerable to other infections. Over time, cankers can girdle the entire tree and all above-ground parts will die. The disease can persist on plant debris for up to two years after the death of the tree. This fungus can also exhibit orange spore tendrils that protrude from the cankers.

Researchers are currently exploring ways to create hybrid chestnut varieties with better resistance to C. parasitica.

Coryneum Blight

Coryneum blight, or shothole blight, is a disease that impacts almonds and stone-fruit trees. It is caused by the fungus Wilsonmyces carpophilus, which infects leaves, twigs, branches, buds, flowers, and fruits. This fungus prefers cold, wet conditions in spring and is commonly dispersed by rain during this period.

As the name suggests, plants suffering from shothole blight have holes in their leaves that resemble shot holes. Raised purple spots or lesions can be found on leaves and fruits.

One of the most recommended and effective control methods is to prune infected branches when the trees are dormant. Commercial growers may also apply fungicide treatments as a preventative method during wet periods and throughout the growing season.

This plant disease is significant for plants in the Solanaceae family, including potatoes and tomatoes, and is often called tomato blight. Early tomato blight first appears as brown lesions on leaves surrounded by a yellow halo. Over time, the lesions may develop concentric rings or a “bullseye” pattern, which is a characteristic sign of this disease. Lesions can girdle the stem and lead to “collar rot.”

Early blight can impact fruits and tubers, manifesting as brown patches that may also have concentric rings. Tomato fruits may drop prematurely. Infected tubers have leathery patches, often with a raised purple border.

The fungus Alternaria solani causes early blight and prefers humid, warm conditions. It may persist from season to season on volunteer potato or tomato plants and on weed hosts from the nightshade family.

Controlling weeds and volunteers and removing infected plant material are key cultural practices. Limiting overhead watering can help keep leaf moisture low during the growing season, as can drip irrigation. Plant tomatoes in a way that provides good airflow to reduce risk. This disease is also associated with nitrogen deficiency in the soil. Minimizing plant stress and setting a routine fertilization schedule later in the season can aid in preventing blight.

Leaf Blight (Botrytis Blight)

Botrytis blight, or gray mold, is a fungal disease that impacts a range of plants including ornamentals such as roses. It is caused by the fungus Botrytis cinerea, which thrives in the cool, humid temperatures of spring and fall. B. cinerea produces grape-like clusters of fungal spores that spread by wind or water. Spores can enter hosts through wounds, and the fungus can also overwinter on plant debris.

The symptoms of this disease appear on leaves, flowers, buds, and stems. Brown, water-soaked spots first appear at the onset of the disease. Over time, the brown spots develop fuzzy mold and petals may mat together. On stems, lesions may form and cause dieback and wilt.

The best control method for botrytis blight is proper sanitation and watering. Remove infected plant debris and do not add them to the compost pile along with other materials. Increasing airflow around plants also helps prevent this blight.

Leaf Blight of Grasses/Grains 

Ascochyta leaf blight is caused by the fungal pathogen Ascochyta spp. and affects Kentucky bluegrass, tall fescue, and perennial ryegrass in the summertime. Turf grasses typically recover quickly after being infected by Ascochyta spp.

Little is known about the environmental factors of this pathogen, although the disease is most prevalent during a hot, dry period followed by a cool and rainy period. The blades of the grasses will rapidly change to a straw color and form large, irregular patches very similar to drought. However, this disease progresses very quickly, and healthy leaves can be found interspersed within the patches. Leaves may also show bleached tips and die back.

Leaf Blight of Wheat

Alternaria triticina is a fungus that causes leaf blight of wheat and other cereals. The fungus transmits through infected seeds, contaminated soil and plant residue, and splashback from rain. Plants become more susceptible with age, and A. triticina does not impact seedlings less than four weeks old. Wet leaves in high-humidity environments are particularly at risk for this fungal disease.

Leaf blight on wheat starts as small lesions on lower leaves and can spread to upper leaves, sheaths, and seed heads. These lesions are caused by a plant toxin secreted by the fungus. Sporulation and secondary transmission by wind can also occur. Heavy infestations will cause a field of wheat to take on a bronze, burned appearance.

This pathogen can survive on plant residue for several months, so removing any infected material is recommended. Pre-soaking seeds in hot water, 126 to 129°F (52 to 54°C) for 10 minutes, can help reduce the fungal presence while preserving seed viability.

Southern Corn Leaf Blight 

Southern corn leaf blight is caused by the fungus Cochliobolus heterostrophus. Early symptoms include elongated lesions on lower leaves, which spread to upper leaves over time.

This fungal disease proliferates during rainy weather and under frequent dewy conditions. It is more prevalent in corn planted in fields that previously grew corn. Commercial growers can spray fungicides during flowering or early grain development periods.

Oomycete Disease

Oomycetes, or “water molds,” are some of the most devastating plant pathogens. Although oomycetes share some similarities with fungi, such as their filamentous growth habit and reproduction via spores, they are actually a distinct group of a few hundred organisms more closely related to algae.

Late Blight (Phytophthora infestans)

The infamous late blight is the cause of the Irish Potato Famine and the most notorious of all potato diseases. The study of late blight in the 1800s kick-started the field of plant pathology after the scientific community identified Phytophthora infestans as the cause, rather than the wrath of God.

Late blight on potato plants starts as water-soaked or chlorotic spots on leaves that rapidly expand to black and brown lesions on infected leaves and stems. In humid conditions, P. infestans creates white spores that can be seen on leaves. An entire plant can die from late blight within a few days of the first lesions appearing. Potato tubers can also be infected and will rot due to secondary infections.

Tomatoes are also susceptible to late blight. Tomato leaves, stems, and fruits can all be infected. Like potatoes, tomato plants can perish in a few days, and farmers can lose their entire crop. There was an epidemic of late blight in tomatoes in 2009 in the United States from infected transplants.

No potato cultivar is completely resistant to this disease, but some are more resistant than others. The top predictor of late blight spread is weather conditions. Humid environments with high condensation create the optimal conditions for this pathogen. Good drainage, air circulation, and regular foliar sprays are some control methods. Tubers are another way of harboring dormant inoculants. Remove all potato tubers from the ground and destroy any volunteer plants in subsequent seasons.

Citrus Blight

Citrus blight is a prevalent citrus disease found in many citrus-growing regions of the world, including Florida and Brazil. It is not as common in citrus-growing regions with a generally arid climate and winter rainfall, like California and the Mediterranean. Even though this disease has been observed for over 100 years, scientists today are still unable to pinpoint its cause.

This disease is considered a decline disease that impacts trees as young as four years of age. The pathogen causes blockage to the xylem and prevents the effective transportation of water and nutrients. During the onset of the disease, infected trees show some wilt and leaf drop along with a gray cast to the canopy.

As the disease progresses, trees develop more permanent wilt, increased leaf drop, and dieback. Infected trees do not typically die but will quickly become unproductive due to loss of canopy and nutritional deficiencies. The unknown characterization of the disease makes it difficult for researchers to recommend control methods.

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