Commercial Recommendations Extension Bulletin E-1746, October 1983 (new)

Bernard H. Zandstra, Department of not cover the heads well, leaving them peratures of 70° to 80° F. Tempera­ Horticulture; Darryl D. Warncke, subject to sun scald. Leaves are fre­ tures above 85°F result in sun scald Department of Crop and Soil quently quite ribby (i.e., very large (wrapper leaves break down and stick Sciences; Melvin L. Lacy, Depart­ midribs) which results in cracked to the head) and tipburn. Tempera­ ment of Botany and Plant Pathology midribs at harvest. Ithaca is very tures below 30° F will cause severe susceptible to brown rib. frost damage to the heads. Production Raleigh is a recently released varie­ ty that has done well in Michigan A good yield of head lettuce is 800 Water and Irrigation trials for 3 years. It produces large, 50-lb. boxes per acre; under ideal con­ firm, uniform heads with good flavor. Lettuce deteriorates rapidly under ditions, production can exceed 1,000 Large wrapper leaves give protection wet soil or very humid air conditions. boxes per acre. Other types of lettuce from sun scald. It has some tolerance Several foliar and root diseases thrive yield 600 to 800 25-lb. boxes per acre. to corky root rot. Ribs may become in a damp atmosphere. However, brittle and crack as heads become lettuce seeds need an adequate supply Use oversize. Because of its large size, of moisture to germinate uniformly, Raleigh may be a substitute for Ithaca. and the plants need a constant supply Approximately 80% of the head let­ Growers should try it on a trial basis of water throughout their growth and tuce grown in Michigan is packed in only. development to produce marketable cartons for the fresh market; 20% is heads. Balancing the need for mois­ shredded for use in restaurants. Montello, Green Lake, and Ithaca are also suitable for production on ture and the susceptibility to too Types and varieties soil. much water is probably the most dif­ Butterhead: Bibb, Summer Bibb, ficult aspect of lettuce production in Several types of lettuce (Lactuca Buttercrunch, White Boston Michigan. sativa) are commonly grown commer­ Romaine: Parris Island, Lettuce seeds should be planted in cially: crisphead (iceberg), butterhead Valmaine, Rubra (red) moist soil. If the weather is hot at (bibb, Boston), romaine (cos), loose- seeding, irrigate with Vz inch of water Leaf: Domineer, Grand Rapids, leaf, and stem lettuce. Endive and immediately after seeding, and main­ Waldmann's Green escarole (Cichorium endivia) are close­ tain a moist soil surface until the ly related to lettuce and grow similar­ Stem lettuce: Celtuce seedlings emerge. ly in the field. Endive: Salad King, Green Curled After thinning", lettuce grown on The following crisphead varieties Escarole: Broad Batavian, Florida deep muck usually does not require are suitable for production on muck Deep Heart more irrigation before harvest. For soil: lettuce grown on lighter soils, and Montello forms uniform medium- Climatic Requirements during extended hot periods without large heads. Medium-size wrapper rain on muck soils, subsequent irriga­ leaves provide partial to good protec­ Lettuce requires a moderate climate tions may be needed. Irrigate with 1 tion for the head. The midribs are to grow well. It is sensitive to both inch of water every 10 to 14 days fairly smooth with little ribbiness, but high and low temperatures, and wet before heading and every 7 to 10 days they are quite subject to cracking. and dry conditions. Lettuce seed ger­ during heading. Irrigate in the morn­ Mature heads are firm with a light minates at temperatures as low as ing so that leaves dry out quickly. Be green basal color. Flavor is acceptable 40°F, but germinates best at 60° to conservative, and do not irrigate if to good. It has some tolerance to 70°. However, it is subject to thermo- rain is expected. corky root rot. dormancy at temperatures above Lettuce is often seeded on the outer Green Lake is a sister line of 80°F; i.e., the seed will not germinate, edges of raised beds, to avoid a Montello. The heads tend to be slight­ or will germinate erratically. saturated root zone in case of heavy ly larger but less firm than Montello. Lettuce grows best in sunny, dry rain or rain following irrigation. This Wrapper leaves provide good cover weather, with daytime high tem­ planting system also allows for better over the head. Flavor, midrib char­ acteristics, basal color and tolerance to corky root rot are similar to The authors acknowledge the help of Robert E. Lucas, Department of Crop and Montello. Soil Sciences (retired) and Edward J. Grafius, Department of Entomology, in Ithaca forms large firm heads with preparing this publication. good flavor. The wrapper leaves do

Cooperative Extension Service • Michigan State University air circulation around the base of the symptoms disappear. On new mucks hand, using a small hoe or com­ plants, which helps avoid foliar testing below 20 ppm copper (Cu), parable tool. At 18 inches between diseases and bottom rot. apply 3 to 5 lb. copper per acre (12 to rows and 12 inches between plants, 20 lb. copper sulfate) in the broadcast there is a potential for 29,000 plants or Soil Requirements fertilizer each year until a total of 30 about 1,200 cartons of head lettuce to 40 lb. total copper has been applied. per acre. Lettuce grows well on both well- Cu deficiency should not subsequent­ Sow seeds V* to Vi inch deep into drained muck and mineral soils. It is ly be a problem. moist soil. During hot, sunny weather, usually grown on muck in Michigan Lettuce is susceptible to tipburn (see sow in the late afternoon or evening because muck maintains uniform below). To help avoid tipburn, spray so that seeds germinate before moisture levels better than mineral lettuce plants twice weekly during hot thermodormancy sets in. soil. Heavy soils hold moisture too weather with 10 lb. calcium chloride Sowing begins as soon as fields can long and result in root and bottom or 15 lb. calcium nitrate in 50 to 100 be worked in the spring. Plant twice a rots. gallons of water per acre. Direct the week until May 10 and 3 times a week Lettuce should be rotated with other spray into the center of the plant. thereafter until August 1. At each crops to avoid a buildup of soil-borne Once heads form, this will not help. date, plant an area of land that will diseases. Leafy type lettuce and endive and provide enough lettuce for 2 days of escarole are more susceptible to tip- harvest. After sowing, irrigate with Vi Fertilization burn than are heading types, and inch of water, unless soil is uniformly calcium sprays will be more effective moist. An 800-box per acre crop of head on them. lettuce uses approximately 125 lb. N, Harvest and Post Harvest 50 lb. P20B, and 200 lb. K20 from the soil. To maintain soil nutrient levels, Spacing and planting Head lettuce takes 55 to 75 days add 100 lb. N, 75 lb. P205, and 200 lb. from sowing to maturity, depending K20 per crop. On muck soil, maintain Land to be planted to lettuce should on temperatures. Lettuce sown in a pH of 5.2-6.0. On mineral soils, be plowed shortly before planting, to April matures in about 75 days; May maintain a pH of 6.0-6.8. a depth of 8 to 10 inches. Plowing too or August—70 days; July—60 to 65 Phosphorus and potassium ferti­ early results in loss of soil moisture. days; June—55 to 60 days. lizer rates should be based on soil test Plow deep enough to turn all trash Lettuce is cut and packed in the results. The following recommenda­ completely under. Disc or roll the field. If possible, harvest in the morn­ tions can be followed if a soil test is land to make a firm, smooth seedbed ing to avoid field heat. Head lettuce is not available. and sow immediately. If planting on packed 18 or 24 heads to a 1% bushel To prepare muck soil for planting beds, form the beds and sow in the carton, weighing about 50 lb. full. A lettuce, broadcast 300 lb. 0-0-60 (180 same operation or the same day. field is mature when most of the lb. K20); band at seeding (2 inches to Plant head lettuce in rows 15 to 18 heads have reached desirable size and the side and 2 inches below the seed) inches apart, with 12 to 14 inches be­ are solid. Most commercial fields are 300 lb. 8-32-16 and 25 lb. manganese tween plants in the row. Large-headed harvested only once. sulfate (24 lb. N, 96 lb. P205, 48 lb. varieties, such as Ithaca, need the To maintain quality, lettuce should K20, 6 lb. Mn). At thinning, sidedress wider spacing. Leaf lettuce may be be vacuum cooled for 30 minutes. If with 50 lb. N. An alternative method planted 6 to 10 inches apart. It takes % lettuce is dry, it should be sprinkled is to broadcast 1,000 lb. 5-20-20 before to Vz lb. of seed to plant an acre of with water before vacuum cooling. It planting, sidedress with 50 lb. N after lettuce. Planters that drop 3 to 4 seeds can be stored for 2 to 3 weeks at 32 to thinning, and apply manganese as per hill use about % lb. of seed per 34°F and 95% R.H. (relative humid­ foliar spray. acre in 18 inch rows with 12 inches ity). Lettuce should be kept dry and On mineral soil, broadcast 700 lb. between hills. not iced in transport or storage. 8-16-32 and sidedress with 50 lb. N at It is important to have uniform thinning; or broadcast 300 lb. 0-0-60 plant spacing and growth to obtain Physiological Disorders and band 300 lb. 8-32-16 at seeding uniform head lettuce at harvest. The and sidedress with 50 lb. N at most consistent uniformity has been Bolting (seeders) is a result of thinning. obtained by planting pelleted seed lettuce maturing during hot, dry Apply 1 lb. boron (B) (10 lb. borax or with a precision planter and irrigating weather. Usually the seedstalks do not 5 lb. Solubor) per acre in the broad­ after sowing to get uniform emerge from the heads, resulting in a cast fertilizer each year before plant­ emergence. condition called internal seedstalk. ing. If the pH is between 6.0 and 6.4, Lettuce should be planted so that it The heads appear normal externally add 6 lb. manganese (Mn) (24 lb. is easy to thin it to the desired spac­ but have an elongated core inside and manganese sulfate) in the banded ing. Plant seeds 2 or 3 inches apart, are not acceptable in the trade. fertilizer; if the pH is above 6.4, add 8 then thin to stand when lettuce is 1 Bolting is most likely to occur during lb. Mn. If Mn deficiency symptoms inch high. An alternative system is to the hot weather between July 15 and appear during the growing season, drop 3 or 4 seeds every 12 to 14 August 10. Production on moist muck apply 6 lb. manganese sulfate per acre inches, and then thin to 1 plant after land and irrigation during hot, dry in a foliar spray 2 or 3 times until emergence. Lettuce is thinned by periods will help avoid seeder forma-

2 tion. All varieties recommended for twisted and rough, and usually do not in the morning so the leaves .dry Michigan will bolt given favorable fill out. The disease is caused by a rapidly. Apply fungicides on a regular conditions. mycoplasma (similar to a virus) trans­ basis. Tipburn is a physiological disorder mitted by aster leafhoppers. To con­ Bottom rot (Rhizoctonia solani) is a of lettuce caused by uneven distribu­ trol the disease, spray insecticides on major cause of crop loss in Michigan. tion of calcium in the plant. It usually the field and borders to control the The organism causes damping off in occurs during hot weather when let­ leafhoppers. Also, kill off weeds such seedlings early in the season. If infec­ tuce is growing rapidly. Small, brown as wild lettuce, pineapple weed, tion occurs later, the organism enters spots first appear along the outer horseweed, and Queen Anne's lace the plant through leaves touching the margins of outer head leaves, then that serve as alternate hosts for the soil and moves up the stem into other coalesce and form a brown fringe disease and leafhoppers. Chop and leaves. Tissues break down, becoming around the leaves. Soft rot often sets plow or disc into the ground any brown and slimy. The disease spreads in after initial breakdown of tissue by plants left in the field after harvest, to rapidly with wet soil and humid air tipburn. Heads with internal tipburn remove sources of infection. conditions. If the air and soil dry out, appear normal but are unsalable. Leaf A late infection of aster yellows, the infected areas may also dry out, lettuce, butterhead, and endive are which causes pink latex spots on leaving the plant erect. more susceptible to tipburn than is internal leaves, is sometimes confus­ To avoid bottom rot, plant on the crisphead lettuce. Calcium sprays (see ed with tipburn. edges of beds to maximize air move­ above) and good water management "Drop" (Sclerotinia sclerotiorum) is ment and water drainage around the are somewhat helpful in preventing a common disease of lettuce grown on base of the plants. Space plants as far tipburn. Most varieties currently muck soil, especially during cool, wet apart as feasible. Apply fungicides as grown in Michigan are quite resistant periods. Infected plants collapse sud­ recommended to reduce incidence of to tipburn. denly, from which the common name, infection. Maintain nutrient levels to Brown rib is a physiological "drop," is derived. Infected plants are obtain maximum growth. Do not disorder sometimes called rib blight covered to some extent by cottony, throw soil against the base of the or mahogany rib. In early growth white mycelia, and black, hard over­ heads during tillage or thinning. stages, the cells of the outside petioles wintering bodies, called sclerotia, are turn pithy and brown. As the disorder formed in the cottony mycelial mats. Downy mildew (Bremia lactucae) develops, leaf tissue also shows The same organism causes diseases in may be a problem during cool (below brown streaks, both inside and on the several other , including 65°F) and damp near 100% R.H. surface. Decay caused by soft rot or potatoes, carrots, celery, cole crops, weather. The spores are spread by other organisms then sets in rapidly. tomatoes, beans, and cucurbits. To wind and splashing water, and re­ Brown rib is most severe during avoid "drop," rotate to crops not quire free water on the leaves to ger­ periods of high humidity and tem­ susceptible to Sclerotinia, such as minate. Symptoms first appear on perature, especially when it rains onions, spinach, or small grains. older leaves. Pale green spots appear shortly before harvest. Planting head Chopping crop residue and allowing on the upper surface of the leaves. On lettuce on raised beds and avoiding it to dry before plowing it under will the lower sides of the leaves, a white irrigation during the last 2 weeks reduce inoculum in the soil. New mold appears on the infected spots. before harvest will help reduce the in­ fungicides, such as Ronilan and The spots enlarge and coalesce, but cidence of brown rib. The cultivars Rovral, have given some control of usually do not cross main veins. In­ Great Lakes 659 and Ithaca are quite "drop." fected leaves turn brown, and secon­ susceptible to brown rib. Gray mold (Botrytis cinerea) usually dary infection often sets in. The heads themselves may not be infected, but Ribbiness or ribby lettuce is a begins on older leaves where they unsightly wrapper leaves make them physiological disorder that prevents touch the soil. The infection spreads unsalable. Regular applications of normal head development or results to the stem and then into the head. In­ fungicides will help reduce downy in rough, loose heads. The plants have fected areas are covered with a mildew. Begin spraying when disease bulky, prominent outer leaf ribs brown, watery rot; a fuzzy, gray- symptoms appear. which do not fit tightly around the colored mold develops on the outside head. Ribby lettuce is more of a prob­ of infected tissues. Later in the Lettuce wilt or stunt (Pythium lem during excessively hot days with disease development, small, hard tracheiphilum) is a disease that can warm nights. Shading by windbreaks, sclerotia appear on the heads. Gray cause up to 30% loss of stand in cloudy weather, and low soil fertility mold develops most rapidly in cool lettuce growing on muck. This disease also seem to increase the frequency of (65 to 70°F) and damp (90 to 100% is particularly prevalent in poorly the disorder, which suggests that it R.H.) weather. drained areas and during cool-wet may be caused by low Botrytis cinerea and Sclerotinia weather in May and June. The disease synthesis by the plant. sclerotiorum both form sclerotia infects both the vascular and leaf which remain viable in the soil for tissues, causing wilting and rapid Diseases many years. If the soil is badly in­ death of the whole plant and a jelly­ fested, rotation is of little value in like decay in the center of the tap root. Aster yellows is a major disease of avoiding the disease. To reduce in­ Good drainage and crop rotation will Michigan lettuce. Infected plants cidence of infection, irrigate lettuce help reduce the severity of the have a yellowish-green color and are only when necessary, and then do it disease.

3 Corky root rot is a dry breakdown other crops or weeds anytime from Plant bugs feed by puncturing leaf of lettuce roots. Decomposing lettuce May to September. tissue and injecting toxic saliva, root residue is a major source of the Check lettuce fields for aster leaf- which causes local dead areas. They problem. It is primarily a problem hoppers twice per week, especially are highly mobile, moving into and where lettuce has been grown for after spring storms, and begin insec­ out of fields but are usually kept under many years. Plants from contami­ ticide sprays if they are present. control by insecticide sprays for other nated soil show browning and swell­ Although the leafhoppers do not insects. ing of the roots, which appears to be always carry the aster yellows caused by nematodes. As the problem mycoplasma, there is at present no progresses, the tap root and laterals way to determine their infectiveness Weeds take on a corky, scabby, rough texture in the field. Good weed control is important in which reduces vigor. In advanced Cutworms and armyworms may lettuce production. Several common stages, it causes death of small attack lettuce throughout the growing weeds are alternate hosts of insect seedlings. season. Emerging seedlings may be and disease pests of lettuce. Young To avoid corky root rot, work plant eaten off at the ground by early- lettuce plants are poor competitors residues into the soil immediately emerging larvae. Later generations and many will not survive under after harvest. Do not plant lettuce damage leaves close to the ground weed pressure. Unfortunately, there after lettuce the same year and use and may enter the head. Apply insec­ is a very limited choice of herbicides varieties with some tolerance, such as ticides as soon as damage becomes available to control weeds in lettuce Montello and Green Lake. evident. grown on muck soil. Keep fields Lettuce mosaic virus (LMV) causes Cabbage loopers (Trichoplusia nij clean, cultivate, hand hoe, and use mottling and curling of leaves; plants may be a serious pest on lettuce after herbicides to obtain good weed infected early are stunted and do not mid-July. The adults migrate into control. mature properly. The virus is seed- Michigan from the south, and lay More information on lettuce pro­ borne and is spread in the field by their eggs on the undersides of the duction is available in the following aphids. Since original infection usual­ leaves. The larvae emerge in about 1 bulletins, available from your county ly begins with plants from infected week and begin to feed on the foliage. Extension office or the MSU Bulletin seed, the easiest means of control is to The larvae develop into light-green Office, P.O. Box 231, East Lansing, plant disease-free seed. Most seed caterpillars, % to 1 inch long. They MI 48824. Check with your local Ex­ companies list the level of virus infec­ obtain their name from their looping tension office for prices and tion of their lettuce seed. action as they move along. availability of bulletins. Loopers must be controlled to ob­ Insects tain high quality lettuce. Begin spray­ E-312 — Control of insects, diseases, ing insecticides as soon as eggs or and nematodes on commercial Aster leafhoppers (Macrosteles larvae are present, and continue on a vegetables fascifrons) transmit aster yellows, a regular schedule until the end of the E-433 — Weed control guide for major disease of lettuce and several season. Since loopers migrate into vegetable crops other crops. The leaf hoppers are % Michigan, time of occurrence and E-486 — Secondary and micro- inch long, light green, with 6 black numbers vary greatly from year to nutrients for vegetables and field spots on the front of their heads. They year. crops overwinter on weeds, winter grain, or Aphids cause damage by reducing E-550 — Fertilizer recommendations other plants, or migrate into Michigan the vigor of the plants and by con­ for vegetable and field crops in on storm fronts, usually first appear­ tamination of the lettuce with live Michigan ing in vegetable crops in mid-May. aphids, honey dew, and cast skins. E-1278 — Botrytis diseases: recogni­ They may move into lettuce from Peak numbers occur in August. tion and control

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