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What Is Lettuce Drop: Recognizing Sclerotinia Symptoms In Lettuce

What Is Lettuce Drop: Recognizing Sclerotinia Symptoms In Lettuce


By: Mary Ellen Ellis

If your lettuce leaves in the garden are wilting and yellowing with brownish decaying spots, you may have sclerotinia lettuce disease, a fungal infection. This kind of infection can destroy entire heads of lettuce, making it inedible, but cultural practices or fungicides can help you limit the damage.

What is Lettuce Drop?

Lettuce drop is a disease caused by a fungal infection. There are two species of fungus that can cause the disease, one of which only attacks lettuce, peppers, basil, cauliflower, legumes, and radicchio, called Sclerotinia minor. The other species, Sclerotinia sclerotiorum, may infect hundreds of different plants, including many that may be in your garden.

As with most fungal infections, lettuce sclerotinia favors humid, wet environments. A lot of rain, lack of airflow between plants, and leaves touching damp ground can all make lettuce beds more susceptible to the infection.

Sclerotinia Symptoms

The symptoms of this disease vary a little depending on the infecting species. Both species cause the lettuce leaves to wilt, beginning with those touching the soil. They also cause brown spots of decay on the leaves. Eventually, usually when the lettuce plant is nearly mature, the entire plant will collapse.

Plants infected by S. sclerotiorum may also develop decay on higher leaves because the fungus produces airborne spores. These lettuce plants may develop soft rot on upper leaves along with white fungal growths. On plants infected by either species, you may also see black growths called scerlotia.

Treating Lettuce Drop

Treating lettuce drop is most often a matter of cultural control, although you can also use fungicides to treat it. Fungicides have to be applied at the base of young plants to stop the spread of the disease. If you do not want to use chemical controls, there are other things you can do to manage lettuce drop.

Management necessitates that you take all reasonable measures to make sure your lettuce plants stay dry. Make sure your bed drains well and water early in the morning so the soil can dry out throughout the day. It’s also important to avoid over-fertilizing with nitrogen, which promotes fungal growth. If you do see infection in your plants, remove the diseased leaves and plants and destroy them. At the end of the season you can plow infected plant matter under, but it needs to be at least ten inches deep.

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Downy Mildew

Downy mildew is a fungus that can affect up to 80 percent of acreage once established. Mature leaves are often most affected by this disease. Lettuce develops a yellow area on the upper side of the leaves and a white or gray fluff on the underside.

Leaf Spot

This disease comes from weeds and typically affects less than five percent of acreage. Because of the warm weather, leaf spot is commonly a fall disease.

Lettuce Mosaic Virus

Wrapper leaves affected by lettuce mosaic virus may appear dull, are folded backward, and may have more marginal serration. Romaine lettuce leaves typically show the same symptoms in addition to leaf blistering. Butter head lettuce experiences stunting and severe chlorosis.

Tomato Spotted Wilt Virus

Transported by thrips, this virus was first spotted in 2004 and will probably increase in the coming years.

Bottom Rot

Bottom rot generally affects iceberg, Boston, and Bibb varieties of lettuce and can be observed during warm weather. Although loss from this disease is typically less than five percent of acreage, it can affect up to 100 percent in rare cases. Lettuce suffering from bottom rot usually loses a couple of wrapper leaves.

Drop

Drop generally affects between one to two percent of acreage yearly, but in general it only becomes an issue when the land is unable to be flooded in the summer. The disease first attacks older leaves, which experience a progressive wilt, and then younger and outer leaves collapse around the plant.

Bacterial Diseases

These diseases include bacterial corky root rot, bacterial soft rot, and a number of diseases that can affect roots, leaves, and even the entire plant.


Moisture Stress

Insufficient soil moisture or overly wet soil can both result in poor growth and wilting. Lettuce suffering from drought stress wilts quickly and fails to put on new growth. Wet and soggy soil causes the plant's roots to drown and rot. Leaves may begin to yellow and wilt, or the whole plant may become stunted. Planting in rich, moist soil that drains well and covering the bed with mulch helps prevent drought stress. Provide plants with about 1 inch of water weekly but avoid overwatering that leads to waterlogged conditions.


Program Areas

There are three major fungal diseases of lettuce found in Connecticut: Sclerotinis drop, bottom rot, and gray mold. All are favored by moist conditions, although bottom rot is favored by warm and moist conditions and the others are favored by cool and moist conditions. All produce fungal growth under wet conditions that allow the diseases to be distinguished.

There are also yellows and viral diseases of lettuce. Lettuce mosaic and aster yellows are discussed here.

Fungal Diseases

Sclerotinia Drop

Sclerotinia Drop is caused by the fungi Sclerotinia sclerotium and S. minor. These fungi can affect lettuce and many other plants, including almost all vegetables except corn. Sclerotinia drop is a serious disease, and was first reported in the 1890's in Massachusetts. It is now believed to be found worldwide wherever there is cool, moist weather and lettuce is grown. Infection of the plants occurs mainly as they near maturity, but may occur at any time during the season. Under moist conditions, the entire plant may collapse in two days.

Symptoms. Usually, the first symptom that is noticed is wilting of the outermost leaves. Before the leaves wilt, however, a water soaked area caused by the fungus as it begins to grow, appears on the stem near the soil. The fungus will grow from this point down into the roots, and up through the rest of the stem. As the fungus grows into each leaf, the base of the leaf rots. This causes the leaves to droop and wither, and their tips to touch the soil or rest on the leaves below. As the fungus grows up the plant, each leaf is affected in turn. The inner leaves usually remain moist enough for the fungus to completely invade them, and reduce them to a slimy mass. Under moist conditions, a snowy white mass of fungus, resembling spider webs, is produced over the entire head. Black structures, as small as a mustard seed or as large as a bean, may be formed in this web of fungal growth, usually on the undersides of the leaves touching the soil.

Identification of disease. Snowy white web-like fungal growth is present.

Prevention. Use long rotations away from lettuce, beans, celery, or carrots. The small grains are non-hosts for this fungus and are often included in rotations. Plant in well- drained soil and/or use raised beds. Steam greenhouse soil for one hour at 131o F or for 36 hours at 113o F. Space plants widely and avoid overhead irrigation to keep the soil surface dry. Flooding soil for 23 to 45 days destroys the resting structures of the fungus. Removing infected plants from small plantings is effective in preventing spread of the disease to other plants. Trim outer leaves after harvest, before packing, to avoid rot of the plants in storage. Refrigerate plants after harvest. Immediately plow debris under after harvest. See current recommendations for chemical control measures. It is important to time chemical applications carefully in order to combat this disease.

Bottom Rot is caused by the fungus Rhizoctonia solani, which affects lettuce, escarole, endive, potato, pepper, eggplant, radish, cucumber, and many other fleshy plants. This worldwide disease was first identified on lettuce in 1900 in Massachusetts greenhouses. It is now a greenhouse and field disease and is favored by warm, wet conditions. Plants are usually affected when they are nearly mature.

Symptoms. The first symptom seen from above is usually wilting of the outer leaves. Before this happens, the fungus enters the plant through lower leaves which are touching the soil. Slightly sunken spots, rust-colored to chocolate brown, appear on the leaf petioles and midribs. These spots can be very small or can grow rapidly to cover the entire petiole/midrib area. While these spots are being formed, they may ooze a light brownish or amber colored liquid. If conditions are unfavorable for the fungus, the rust colored spots on the petioles will dry and turn chocolate brown. Under warm, wet conditions, the fungus will continue to grow upward into the leaf blades, and destroy them as it grows from leaf to leaf. The entire head may become a slimy brown mass that soon dries and becomes darker. The stems are usually the last part of the head to decay. Tan to brown web-like fungal growth is usually easily seen on the infected head tissues. Small, irregularly shaped cinnamon brown to dark brown lumpy structures, known as sclerotia, may be seen on the head and on the soil under it. The fungus also provides a path for the entry of secondary rot bacteria.

Identification of disease. Tan to brown web-like fungal growth on plant.

Prevention. Provide good drainage and weed control. Growing on a 4-inch-high and 6-inch-wide ridge may be helpful in preventing this disease, because there is less contact of the bottom leaves with soil. Avoid irrigation near harvest. Avoid rotating potatoes and other very susceptible crops with lettuce. Plow rather than disk, to bury the lettuce debris better. The deeper the sclerotia are buried, the shorter time they will survive. Resistance is not available for this disease, although plants with erect architecture may be less likely to become infected. See current recommendations for chemical control measures. Careful placement and timing of the fungicides is extremely important. A few varieties are tolerant to this disease.

Gray mold is caused by the fungus Botrytis cinerea, which infects lettuce and many other plants. Few vegetables are not hosts of Botrytis. One of the two most important diseases of head lettuce on the New York market, and particularly a problem in greenhouse lettuce, this fungus is everywhere and occurs wherever lettuce is grown. The fungus can grow in a wide range of temperatures, but grows best in cool (65o to 75o F), moist weather.

Symptoms. Disease starts at the oldest or damaged leaves, and progresses upwards. As they are invaded by the fungus, inner leaves first become water soaked, then grayish green or brown, and finally turn into a brownish-gray, slimy mass. The fungus can also grow up the stem and rot out the inside of a head, causing the plant to collapse before any symptoms are visible outside. If lettuce is allowed to flower, the flowers can be infected during and after the flowering period.

An ashen-gray layer of fuzzy, fungal growth is produced over all diseased parts of the plant, especially in parts that stay moist for long periods of time, such as the bottoms of leaves. The spore masses look like bunches of grapes under magnification. Black, flat or cylindrical structures may also form on or within the decayed tissue. The plant will eventually dry and wither.

Identification of disease. Look for ashy gray spores on plant parts.

Prevention. In the greenhouse, keep the humidity low, the temperature warm and sterilize soil before planting. Keeping the plants as dry as possible helps prevent infection, so avoid overhead watering. Water early in the day to allow plants to dry thoroughly. In the field, plant in well-drained soil, and orient rows with the prevailing winds to keep air circulating. Avoid planting near buildings or large trees that shade the plants, to keep the stems from twisting as they grow and thus becoming damaged. Keeping plants healthy helps prevent infection. Since infection often occurs from damaged tissue, it is important to control other diseases, and avoid excessive nitrogen and low calcium. Avoid bruising and other injury during harvest, and keep harvested plants refrigerated between 32o and 36o F. Remove or plow under debris after harvest.

There are no varieties that are resistant to this disease. See current recommendations for chemical control measures.

Yellows and Viral Diseases

Lettuce Mosaic

Lettuce mosaic is caused by the lettuce mosaic virus (LMV). This virus affects all types of lettuce and many other hosts, including other greens, pea, spinach, aster, marigold, sweet pea, zinnia. Weeds such as chickweed, groundsel, cheeseweed, henbit, lambsquarters, milk-thistle, ox tongue, shepherds purse, sowbane, sow-thistle, and scarlet pimpernel are hosts as well. It is one of the most common and damaging diseases of lettuce. It was first recognized in 1921 in Florida and is now found in most of the world. The virus can infect old and young plants and causes decreased vigor and stunting of the plants.

Symptoms. Symptoms of this disease can vary considerably, depending on the age of the plant at infection, the variety and the temperature. Plants infected as seedlings are very stunted. Leaves of infected plants are often irregularly shaped and mottled yellow and green. Mottling is most noticeable on leaf lettuce. It is easiest to see on cool cloudy days, and when holding the leaf up to the light and looking through it. When plants are in the early-rosette stage, the veins appear clear and the smaller veins have brown flecks on them. In very sensitive lettuce varieties, there may be browning of the veins or on the edges of the leaves. The leaves may die. Leaf margins may become ruffled and distorted, and the leaf tips roll back. This downward curling of the leaf tips can be diagnostic in older plants of the crisphead variety.

When plants are older, the yellow green mottling may be difficult to see, although it may be visible on the edges of the leaves. Plants are often a uniform dull pale green to slightly yellow and severely stunted. Outer leaf tips roll downward. The serrations of the younger leaves may be particularly prominent. The plants appear flatter than healthy plants. They may fail to head, produce a small, loose head or, if infected later, may produce a deformed head.

Vector. This disease is transmitted by the green peach aphid in a nonpersistent manner. The aphid acquires the virus from infected plants almost immediately. But, it is only able to infect healthy plants for a short time, usually a few days to a week. The green peach aphid (Myzus persicae) transmits more than 100 viruses to many different plants. The aphids can be pale green to pink and may or may not have wings. In temperate regions, these aphids spend the winter on woody plants (e.g. peach trees) and summers on nonwoody plants.

Prevention. Use certified disease-free seed. Seed is the most important way this virus starts in a field. Control weed hosts. Plow under debris as soon as possible. Resistance is available to this disease, but resistant varieties are not commonly grown in the U.S. because the seed certification program has been very successful. See current recommendations for the chemical control of aphids.

Aster Yellows

This disease is caused by the aster yellows phytoplasma. The phytoplasma has a wide host range, which includes lettuce, many other members of the aster family and 47 other families. Carrot, escarole, endive and celery are other important vegetable hosts. Weed hosts include thistle, fleabane, sow thistle, wild lettuce, plantain, wild chicory, dandelion and galinsoga. Ornamental hosts include gladiolus, poppy, chrysanthemum, phlox and veronica. This disease was first described in 1916. It occurs anywhere lettuce is grown, causing bitter, stunted and disfigured heads.

Symptoms. The young heart leaves become white to yellow and fail to develop normally remaining as short, thickened stubs in the middle of the head. Light brown to pink latex spots collect on the undersides of the midribs of the leaves. When young plants are infected, outer leaves become yellow and twisted. These plants may be severely stunted. Heads often taste bitter. Bushy outgrowths may be present on the flowering stalks, and the plants may be sterile or abort seeds.

Vector. This disease is transmitted from one plant to another by the six-spotted leafhopper and the aster leafhopper (Macrosteles fascifrons). The aster leafhopper, a grayish-green insect about 1/8" long, is found throughout the U.S. The adults must feed on an infected plant for about eight hours before they acquire enough of the phytoplasmas to infect a healthy plant. After the leafhopper has acquired the phytoplasma, it is able to transmit it to healthy plants for the rest of its life.

Prevention. Control weed hosts!This is the most important way this disease starts in an area. The insect that spreads the disease may spend the winter on wheat, rye, barley and some grasses. Do not plant lettuce near other stands of diseased lettuce. Plant far from grains. Control the insect on the grains, especially near grain harvest time. See current recommendations for the control of the six-spotted leafhopper and the aster leafhopper.

  • Davis, R.M. 1997. Aster yellows. in Compendium of Lettuce Diseases. R. M. Davis, K. V. Subbarao, R. N. Raid, E. A. Kurtz, eds. APS Press, St. Paul, MN .
  • Ellis, P.R., and J.A. Hardman. 1992. Pests of Umbelliferous Crops in Vegetable Crop Pests, Chapter 8. R.G. McKinlay, ed. CRC Press, Inc, Boca Raton, FL.
  • Koike, S.T. 1997. R.M. Davis, K.V. Subbarao, R. N. Raid, F.A. Kurtz, eds. Gray Mold in Compendium of Lettuce Diseases, APS Press, St. Paul, MN p.22.
  • McKinlay, R.G., Spaull, A.M., Straub, R.W. 1992. Pests of Solanaceous Crops in Vegetable Crop Pests, Chapter 8. R.G. McKinlay, ed. CRC Press, Inc, Boca Raton, FL.
  • Moline, H.E. and W.J. Lipton. 1987. Market diseases of beets, chicory, endive, escarole, globe artichokes, lettuce, rhubarb, spinach, and sweetpotatoes. U.S. Department of Agriculture, Agricultural Handbook No. 155, 86 p.
  • Raid, R. N. 1997. Bottom Rot in Compendium of Lettuce Diseases. R. M. Davis, K. V. Subbarao, R. N. Raid, F.A.Kurtz, eds. APS PRess, St. Paul, MN pp.15-16.
  • Sherf, A.F. and A. A. MacNab. 1986. Vegetable Disease and Their Control. John Wiley and Sons, New York.
  • Subbarao, K. V. 1997. Drop in Compendium of Lettuce Diseases. R.M. Davis, K.V. Subbarao, R. N. Riad, F.A. Kurtz, eds. APS Press, St. Paul, MN. pp.19-21.
  • Zerbini, F. M. and R. L. Gilbertson. R. M. Davis, K. V. Subbarao, R. N. Raid, E. A. Kurtz, eds. 1997. Lettuce Mosaic in Compendium of Lettuce Diseases. APS Press, St. Paul, MN. pp. 43-44.

Written by: Pamela S. Mercure, IPM Program Assistant, University of Connecticut

Originally published in: Grower, Volume 98-4, April 1998

Reviewed by: Jude Boucher, UConn IPM, 2012

The information in this document is for educational purposes only. The recommendations contained are based on the best available knowledge at the time of publication. Any reference to commercial products, trade or brand names is for information only, and no endorsement or approval is intended. The Cooperative Extension System does not guarantee or warrant the standard of any product referenced or imply approval of the product to the exclusion of others which also may be available. The University of Connecticut, Cooperative Extension System, College of Agriculture and Natural Resources is an equal opportunity program provider and employer.


Why is my lettuce dying?

QUESTION: I’m noticing my lettuce plants starting to turn brown. Why is my lettuce dying? -Kim T

ANSWER: There are many reasons a lettuce plant may start to die, which we’ve outlined here along with the signs and solutions for each. The more quickly you can work to assess the problem and find a solution, the better the chances of your lettuce recovering.

  • Anthracnose: You can tell a plant is struggling with anthracnose by the watery spots that appear on the outermost leaves. Spots will expand, and when they’re mature, the center of the lesions falls out, leaving infected plants full of holes. Anthracnose is a fungal disease that’s spread through splashing water. If your plants are struggling with anthracnose, be careful not to overwater them, and always water from the base to avoid splashing water on their foliage. Remove and discard any plants that show symptoms, then work to address the conditions that allowed anthracnose to strike. Rotate crops every year and use certified disease-free seeds to lessen the risk of plants contracting anthracnose. Topping your lettuce plot with a layer of mulch can help keep this disease from spreading and also keeps water in the soil instead of letting it splash on your plants.
  • Crowded growing conditions: When lettuce plants are crammed too close together, the plants won’t ever grow to be large, and their leaves will taste bitter. Thin out or reposition your plants to make sure they have 12 to 14 inches of room between head lettuces and six to 10 inches of distance between for looseleaf varieties of lettuce.
  • Damping off:Damping off affects seedlings as they’re just starting out, as early as the germination stage and usually before transplanting into the garden. It’s caused by a combination of environmental factors like excess moisture or high humidity and microbes (Rhizoctonia solani, Pythium species, Sclerotinia species, and Thielaviopsis basicola) and can affect any kind of plant. Seedlings show they’re struggling with damping off with brown spots or white mold on the stem, and there may be mold visible on the soil as well.

    Plants that are affected gradually weaken, fall over, and then die. In some cases, seedlings won’t show any symptoms, but underneath the soil, the roots are black or brown from the disease. There is no way to treat the problem once it’s started other than planting a new batch of seeds and correcting the problems that caused damping off. Never sow your seeds into cold, wet soil. Make sure seedlings have plenty of drainage and aren’t being overwatered. Give young plants plenty of ventilation, too—once seeds have germinated, remove any plastic cover you’re using over their container. If you continue to struggle with damping off, try using a soil-free starting mix. After the first few weeks of their growth cycle, plants are no longer at risk for damping off.

  • Heat stress: Most varieties of lettuce you’ll grow in your garden are at their best in cool or moderate weather and will struggle some during the hottest times of the year. When the thermostat goes up, leaf production slows down, and foliage may wilt. In extreme cases of heat, lettuce plants may start to bolt. Look for heat resistant varieties to grow during the summer, or grow lettuce during cooler parts of the year.
  • Lettuce drop: Lettuce drop is a term used to describe the effects of infection by two different fungal pathogens: Sclerotinia minor and Sclerotinia sclerotioru. If your lettuce plants are infected, you may see withering foliage or rot on the plant’s leaves, stems, or in the root system. Plants will grow more slowly than usual, then eventually collapse and die. Make sure your soil offers plenty of drainage and ensure that conditions don’t stay too wet to reduce the chances of your plants getting lettuce drop. Keep leaves as dry as you can by watering plants from the base. Dig up any affected plants and destroy them, then address the moisture problem by increasing drainage in the soil or moving plants to a more hospitable location.
  • Powdery mildew: This fungal disease shows up as white powder on the top and underside of leaves, or you may be able to see the black spores in some cases. In addition to the mildewy appearance, leaves may turn yellow or brown when they’re fighting against powdery mildew. This disease tends to thrive when the weather is humid, and it can be spread by wind. Treatments for powdery mildew include sulfur, neem oil, and potassium bicarbonate as well as fungicides. Often, gardeners remove and destroy plants that have contracted powdery mildew instead of treating them.
  • Problems with water: Your lettuce can start to die if it’s getting either too much or too little water. The plant won’t be growing as it should, and the foliage may turn yellow or wilt. If soil is too moist, plants can get fungal disease, and if the problem persists, the root system can be damaged by root rot. Make sure the soil provides plenty of drainage, and if you’re having to water your lettuce too often, consider adding mulch to help the soil retain water. Lettuce plants should get around an inch of water per week, between rainfall and the water you give them.
  • Root nematodes: These garden pests live in the soil and attack plants by burrowing into their roots. Affected plants will display stunted growth as early as the seedling stage and may develop galls on their roots. In especially bad cases, the foliage above the surface of the soil may start to wilt, change colors, or die. Keep plants strong by making sure they get the right amount of water and fertilizer so they can fight back against root nematodes. Pull up any plants that are left at the end of the season to prevent nematodes from reproducing in their remains, and rotate your lettuce to a new spot in the garden each season to avoid nematode issues.
  • Septoria leaf spot: Plants struggling with septoria leaf spot get small spots on their oldest leaves that eventually turn brown and dry out. In especially bad cases, the spots can combine to form large necrotic areas on the foliage, causing leaves to wilt and eventually, the death of the plant. Septoria leaf spot is caused by a fungus that spreads during wet, humid weather and can hide out in wild lettuces over the winter. Remove all affected foliage from your plants as soon as you notice a problem, and you can also treat plants with fungicidal sprays if necessary. Avoid splashing your lettuce plants with water, instead watering them at the base. Whenever possible, start with seeds that have been certified as disease-free to ensure your seeds aren’t infected. Practice crop rotation to lower the risk of crops being infected with septoria leaf spot.
  • Sunscald: Like people, plants can be burned if they get too much sun. Sunscald can show up as pale blistered spots or brown singed areas on foliage. Plants that are too damaged from sunscald can wither and die. Prevent losing your lettuce to sunscald by hardening off your plants to introduce them to the outdoor sun gradually, and plant them somewhere that offers some shade for protection. If your property doesn’t offer a lot of shade, you can plant lettuces in the shadow of other crops or use shade cloth.
  • Temperature or timing issues: If you planted your seeds when the weather was warm, it may simply be too hot outside for them to germinate. Lettuce has a 99 percent success rate for germination at 77 degrees Fahrenheit, but add just nine degrees to bring the temperature up to 86, and their nearly perfect record for germination drops to 87 percent. If you really want to plant lettuce in the summer, choose heat resistant varieties, and use mulch to help keep the soil cool.


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