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

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

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.

References.

• 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

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