Alternaria solani
Encyclopedia
Alternaria solani is a fungal pathogen, producing a disease in tomato
and potato
plants called early blight. The pathogen produces distinctive "bullseye" patterned leaf spots and can also cause stem lesions and fruit rot on tomato and tuber blight on potato. Despite the name "early," foliar symptoms usually occur on older leaves. If uncontrolled, early blight can cause significant yield reductions. Primary methods of controlling this disease include preventing long periods of wetness on leaf surfaces and fungicide
use.
Geographically, A. solani is problematic in tomato production areas east of the Rocky Mountains
and is generally not an issue in the less humid Pacific or inter-mountain regions. A. solani is also present in most potato production regions every year but has a significant effect on yield only when frequent wetting of foliage favors symptom development.
(Solanum lycopersicum L.), potato
(S. tuberosum), eggplant (S. melongena L.), bell pepper
and hot pepper (Capsicum
spp.), and other plants of the Solanum family. Distinguishing symptoms of A. solani include leaf spot
and defoliation, which are most pronounced in the lower canopy. In some cases, A. solani may also cause damping off
.
rings - a distinguishing characteristic of the pathogen - and measure up to 1.3 cm (0.51 inches) in diameter. Both the area around the leaf spot as well as the entire leaf may become yellow or chlorotic. Under favorable conditions (e.g., warm weather with short or abundant dews), significant defoliation of lower leaves may occur, leading to sunscald of the fruit. As the disease progresses, symptoms may migrate to the plant stem and fruit. Stem lesions are dark, slightly sunken and concentric in shape. Basal girdling and death of seedlings may occur, a symptom known as collar rot. In fruit, A. solani invades at the point of attachment to the stem and through growth cracks and wounds made by insects, infecting large areas of the fruit Fruit spots are similar in appearance to those on leaves – brown with dark concentric circles. Mature lesions are typically covered by a black, velvety mass of fungal spores that may be visible under proper light conditions.
, moderate potassium
and low phosphorus
in the soil can reduce susceptibility of infection by the pathogen.
The life cycle starts with the fungus overwintering in plant material. This can be crop residues or wild members of the Solanaceae
family, such as black nightshade
. In the spring, conidia are produced. Conidia are splashed by water or by wind onto an uninfected plant. The conidia infect the plant by entering through small wounds, stomata, or direct penetration. Infections usually start on older leaves close to the ground. The fungus takes time to grow and eventually forms a lesion. From this lesion, more conidia are created and released. These conidia infect other plants within the same growing season. Every part of the plant can be infected and form lesions. This is especially important when fruit or tubers are infected.
In general, development of the pathogen can be aggravated by an increase in inoculum from alternative hosts such as weeds or other solanaceous species. Disease severity and prevalence are highest when plants are mature.
Free water is required for Alternaria spores to germinate; spores will be unable to infect a perfectly dry leaf.
Alternaria spores germinate within 2 hours over a wide range of temperatures but at 26.6-29.4ºC (80-85ºF) may only take 1/2 hour. Another 3 to 12 hours are required for the fungus to penetrate the plant depending on temperature. After penetration, lesions may form within 2-3 days or the infection can remain dormant awaiting proper conditions [15.5ºC (60ºF) and extended periods of wetness]. Alternaria sporulates best at about 26.6ºC (80ºF) when abundant moisture (as provided by rain, mist, fog, dew, irrigation) is present. Infections are most prevalent on poorly nourished or otherwise stressed plants.
Chemical Control
This disease can be prevented with some fungicides, including azoxystrobin
, potassium bicarbonate
, hydrogen dioxide as well as the biological control agent Bacillus subtilis
.
A. solani is also one of the most important foliar pathogens of potato worldwide. Primary damage is caused by premature defoliation of the potato plants, which results in decreased tuber quality and yield reduction. In the U.S., yield loss estimates attributed to foliar damage can reach 20-30%. In storage, A. solani can cause dry rot
of tubers and reduce storage length, which both of which diminish the quantity and quality of marketable tubers.
Because A. solani is one of numerous tomato/potato pathogens that are typically controlled with the same products, accurately estimating both the total economic loss and the total expenditure on fungicides for control of early blight is difficult. Best estimates suggest that total annual global expenditures on fungicide control of A. solani is approximately $77 million: $32 million for tomatoes and $45 million for potatoes.
Tomato
The word "tomato" may refer to the plant or the edible, typically red, fruit which it bears. Originating in South America, the tomato was spread around the world following the Spanish colonization of the Americas, and its many varieties are now widely grown, often in greenhouses in cooler...
and potato
Potato
The potato is a starchy, tuberous crop from the perennial Solanum tuberosum of the Solanaceae family . The word potato may refer to the plant itself as well as the edible tuber. In the region of the Andes, there are some other closely related cultivated potato species...
plants called early blight. The pathogen produces distinctive "bullseye" patterned leaf spots and can also cause stem lesions and fruit rot on tomato and tuber blight on potato. Despite the name "early," foliar symptoms usually occur on older leaves. If uncontrolled, early blight can cause significant yield reductions. Primary methods of controlling this disease include preventing long periods of wetness on leaf surfaces and fungicide
Fungicide
Fungicides are chemical compounds or biological organisms used to kill or inhibit fungi or fungal spores. Fungi can cause serious damage in agriculture, resulting in critical losses of yield, quality and profit. Fungicides are used both in agriculture and to fight fungal infections in animals...
use.
Geographically, A. solani is problematic in tomato production areas east of the Rocky Mountains
Rocky Mountains
The Rocky Mountains are a major mountain range in western North America. The Rocky Mountains stretch more than from the northernmost part of British Columbia, in western Canada, to New Mexico, in the southwestern United States...
and is generally not an issue in the less humid Pacific or inter-mountain regions. A. solani is also present in most potato production regions every year but has a significant effect on yield only when frequent wetting of foliage favors symptom development.
Hosts and Symptoms
Alternaria solani infects stems, leaves and fruits of tomatoTomato
The word "tomato" may refer to the plant or the edible, typically red, fruit which it bears. Originating in South America, the tomato was spread around the world following the Spanish colonization of the Americas, and its many varieties are now widely grown, often in greenhouses in cooler...
(Solanum lycopersicum L.), potato
Potato
The potato is a starchy, tuberous crop from the perennial Solanum tuberosum of the Solanaceae family . The word potato may refer to the plant itself as well as the edible tuber. In the region of the Andes, there are some other closely related cultivated potato species...
(S. tuberosum), eggplant (S. melongena L.), bell pepper
Bell pepper
Bell pepper, also known as sweet pepper or a pepper and capsicum , is a cultivar group of the species Capsicum annuum . Cultivars of the plant produce fruits in different colors, including red, yellow, orange and green. Bell peppers are sometimes grouped with less pungent pepper varieties as...
and hot pepper (Capsicum
Capsicum
Capsicum is a genus of flowering plants in the nightshade family, Solanaceae. Its species are native to the Americas where they have been cultivated for thousands of years, but they are now also cultivated worldwide, used as spices, vegetables, and medicines - and have become are a key element in...
spp.), and other plants of the Solanum family. Distinguishing symptoms of A. solani include leaf spot
Leaf spot
Leaf spots are round blemishes found on the leaves of many species of plants, mostly caused by parasitic fungi or bacteria.A typical spot is "zonal", meaning it has a definite edge and often has a darker border. When lots of spots are present, they can grow together and become a blight or a blotch...
and defoliation, which are most pronounced in the lower canopy. In some cases, A. solani may also cause damping off
Damping off
Damping off is the term used for a number of different fungus-caused ailments that can kill seeds or seedlings before or after they germinate....
.
A. solani on tomatoes
On tomato, foliar symptoms of A. solani generally occur on the oldest leaves and start as small, brown to black lesions. These leaf spots resemble concentricConcentric
Concentric objects share the same center, axis or origin with one inside the other. Circles, tubes, cylindrical shafts, disks, and spheres may be concentric to one another...
rings - a distinguishing characteristic of the pathogen - and measure up to 1.3 cm (0.51 inches) in diameter. Both the area around the leaf spot as well as the entire leaf may become yellow or chlorotic. Under favorable conditions (e.g., warm weather with short or abundant dews), significant defoliation of lower leaves may occur, leading to sunscald of the fruit. As the disease progresses, symptoms may migrate to the plant stem and fruit. Stem lesions are dark, slightly sunken and concentric in shape. Basal girdling and death of seedlings may occur, a symptom known as collar rot. In fruit, A. solani invades at the point of attachment to the stem and through growth cracks and wounds made by insects, infecting large areas of the fruit Fruit spots are similar in appearance to those on leaves – brown with dark concentric circles. Mature lesions are typically covered by a black, velvety mass of fungal spores that may be visible under proper light conditions.
A. solani on potatoes
In potato, primary damage by A. solani is attributed to premature defoliation of potato plants, which results in tuber yield reduction. Initial infection occurs on older leaves, with concentric dark brown spots developing mainly in the leaf center. The disease progresses during the period of potato vegetation, and infected leaves turn yellow and either dry out or fall off the stem. On stems, spots are gaunt with no clear contours (as compared to leaf spots). Tuber lesions are dry, dark and pressed into the tuber surface, with the underlying flesh turning dry, leathery and brown. During storage, tuber lesions may enlarge and tubers may become shriveled. Disease severity due to A. solani is highest when potato plants are injured, under stress or lack proper nutrition. High levels of nitrogenNitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
, moderate potassium
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
and low phosphorus
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
in the soil can reduce susceptibility of infection by the pathogen.
Disease Cycle
Alternaria solani is a deuteromycete with a polycyclic life cycle. Alternaria solani reproduces aesexually by means of conidia.The life cycle starts with the fungus overwintering in plant material. This can be crop residues or wild members of the Solanaceae
Solanaceae
Solanaceae are a family of flowering plants that include a number of important agricultural crops as well as many toxic plants. The name of the family comes from the Latin Solanum "the nightshade plant", but the further etymology of that word is unclear...
family, such as black nightshade
Solanum americanum
American nightshade is a herbaceous flowering plant of wide though uncertain native range. The certain native range encompasses the tropics and subtropics of the Americas, Melanesia and Meganesia...
. In the spring, conidia are produced. Conidia are splashed by water or by wind onto an uninfected plant. The conidia infect the plant by entering through small wounds, stomata, or direct penetration. Infections usually start on older leaves close to the ground. The fungus takes time to grow and eventually forms a lesion. From this lesion, more conidia are created and released. These conidia infect other plants within the same growing season. Every part of the plant can be infected and form lesions. This is especially important when fruit or tubers are infected.
In general, development of the pathogen can be aggravated by an increase in inoculum from alternative hosts such as weeds or other solanaceous species. Disease severity and prevalence are highest when plants are mature.
Environment
Alternaria solani spores are universally present in fields where host plants have been grown.Free water is required for Alternaria spores to germinate; spores will be unable to infect a perfectly dry leaf.
Alternaria spores germinate within 2 hours over a wide range of temperatures but at 26.6-29.4ºC (80-85ºF) may only take 1/2 hour. Another 3 to 12 hours are required for the fungus to penetrate the plant depending on temperature. After penetration, lesions may form within 2-3 days or the infection can remain dormant awaiting proper conditions [15.5ºC (60ºF) and extended periods of wetness]. Alternaria sporulates best at about 26.6ºC (80ºF) when abundant moisture (as provided by rain, mist, fog, dew, irrigation) is present. Infections are most prevalent on poorly nourished or otherwise stressed plants.
Management
Cultural Control- Clear infected debris from field.
- Water plants in the morning so plants are wet for the shortest amount of time.
- Use a drip irrigation system so leaves never get wet.
- Use mulch so spores in soil cannot splash onto leaves from the soil.
- Rotate to a non-solanaceous crop for at least three years.
- Control wild population of Solanaceae.
- Close monitoring of field, especially in warm weather, to have the highest yields with the lowest fungicide input.
- Choose resistant cultivars.
- Buy disease free seed.
- Increase air circulation in rows. .
Chemical Control
This disease can be prevented with some fungicides, including azoxystrobin
Azoxystrobin
Azoxystrobin is a fungicide commonly used in agriculture. The substance is used as an active agent protecting plants and fruit/vegetables from fungal diseases.-Origin:...
, potassium bicarbonate
Potassium bicarbonate
Potassium bicarbonate , is a colorless, odorless, slightly basic, salty substance...
, hydrogen dioxide as well as the biological control agent Bacillus subtilis
Bacillus subtilis
Bacillus subtilis, known also as the hay bacillus or grass bacillus, is a Gram-positive, catalase-positive bacterium commonly found in soil. A member of the genus Bacillus, B. subtilis is rod-shaped, and has the ability to form a tough, protective endospore, allowing the organism to tolerate...
.
Economic Significance
Early blight caused by A. solani is the most destructive disease of tomatoes in the tropical and subtropical regions. Each 1% increase in intensity can reduce yield by 1.36%, and complete crop failure can occur when the disease is most severe. Yield losses of up to 79% have been reported in the U.S., of which 20-40% is due to seedling losses (i.e., collar rot) in the field.A. solani is also one of the most important foliar pathogens of potato worldwide. Primary damage is caused by premature defoliation of the potato plants, which results in decreased tuber quality and yield reduction. In the U.S., yield loss estimates attributed to foliar damage can reach 20-30%. In storage, A. solani can cause dry rot
Dry rot
Dry rot refers to a type of wood decay caused by certain types of fungi, also known as True Dry Rot, that digests parts of the wood which give the wood strength and stiffness...
of tubers and reduce storage length, which both of which diminish the quantity and quality of marketable tubers.
Because A. solani is one of numerous tomato/potato pathogens that are typically controlled with the same products, accurately estimating both the total economic loss and the total expenditure on fungicides for control of early blight is difficult. Best estimates suggest that total annual global expenditures on fungicide control of A. solani is approximately $77 million: $32 million for tomatoes and $45 million for potatoes.