AGFACTSAGFACTSAGFACTS Field lettuce

productionAgfact H8.1.40, First edition May 2004Tony NapierDistrict Horticulturist, Yanco

ORDER NO. H8.1.40 AGDEX 268/11

INTRODUCTIONLettuce, (Lactuca sativa L.), is an annual of theAsteraceae family and native to the Mediterranean. Itis thought to have originated in Europe and Asia andhas been cultivated for at least 2500 years. Thecultivated lettuce Lactuca sativa is a popular saladvegetable and is grown on all continents throughoutthe world including Australia.

PRODUCTIONApproximately 6000 hectares are grown each yearthroughout Australia, producing 135,000 tonnes forboth the fresh and processing markets. TheAustralian lettuce industry is estimated to be worthapproximately $100 million with $8 million goingto export. NSW is the third largest producing statewith about 27,000 tonnes produced annually fromaround 1100 hectares.

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Australian lettuce production by state (2002)

State Area Production(Hectares) (Tonnes)

Victoria 2188 32,970

Queensland 1805 52,552

New South Wales 1011 27,188

Western Australia 538 14,136

South Australia 277 6234

Tasmania 151 1923

Australia 5970 135,015

Source: ABS 2002

Lettuce is grown in most districts of NSW with thebulk of field production centred in the three maingrowing areas of Sydney Basin, Murrumbidgee andCentral West.

Sydney BasinLettuce is sown and harvested all year round usingboth field and hydroponic systems. Productiondeclines through summer due to the warm conditions.About 520 hectares are sown each year by directseeding and by transplants.

MurrumbidgeeApproximately 320 hectares is sown from earlyFebruary through to late July for harvesting fromApril to the end of October. Production throughsummer is not possible due to high temperatures.Most crops are direct seeded with no hydroponicproduction.

Central WestLettuce is only scheduled for harvesting during springand autumn. Production outside these times is difficultdue to harsh climatic conditions. Most crops are sownby transplants with no hydroponic production.Approximately 180 hectares are grown each year.

LETTUCE TYPESThere are four main types of lettuce.

CrispheadMore commonly known as iceberg which forms atight firm head of crisp leaves. At maturity mostvarieties are mid green in colour and about 1 kg inweight. Crisphead lettuce is the major type grown inAustralia and also the most difficult to grow.

ButterheadA heading type like the crisphead but with the leavesmore loosely folded on top of one another. Headsizes range from 0.3�0.5 kg for hydroponically growncultivars to 0.5�0.7 kg for field grown cultivars. Theinside leaves, because of their lack of light, are creamor butter coloured. The outer leaves are brownish ora darker green than the crisphead and tend to bruiseand tear more easily.

RomaineAlso known as Cos, this lettuce is upright in form andgrows up to 30 cm tall. The tightly folded leaves aregreenish white in the centre and medium green on theoutside. Their flavour is sweeter than the other typesof lettuce.

LooseleafAlso known as leaf, speciality, fancy or Europeanlettuce. Looseleaf lettuce is marketed as either a wholeproduct or as an ingredient of ready-made saladmixes. This type does not form hearts and comes in arange of colours with various types of mottling orpatterns. Looseleaf lettuce is generally grown inhydroponic systems and is considered the easiest typeof lettuce to grow.

HYDROPONIC LETTUCECoastal areas of NSW have seen a large increase inproduction of hydroponically grown lettuce over thepast few decades. Looseleaf and Butterhead are themain types of lettuce grown in these systems.Hydroponically grown looseleaf lettuce offers theadvantage of producing a saleable product in half thetime it takes to produce a Crisphead type. This Agfactdoes not cover issues of hydroponically grown lettucebut only the issues of field grown lettuce.

FIELD LETTUCETwo different methods are used to produce lettuceunder field conditions in NSW. One method usestransplants and overhead irrigation while the other isby direct seeding and furrow irrigation. In theMurrumbidgee district most growers prefer to directseed and furrow irrigate due to a heavier soil type. Inthe Sydney Basin and the Central West where the soiltype is lighter, most growers prefer to use transplantsand overhead irrigation.

SEEDBED PREPARATIONBed size width varies with soil type and irrigation

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method. In the Murrumbidgee district where furrowirrigation is used the bed width ranges from 1.4 to 1.6m from centre to centre. In other districts whereoverhead irrigation is used the bed width ranges from1.6 to 1.8 m from centre to centre.

Field lettuce needs to be sown on raised beds to aidwater infiltration and retention and allow adequate soilaeration and drainage. Beds should be cultivated andformed to produce a fine soil tilth in which the seedcan germinate readily without being affected by soilcrusting. Lettuce is a shallow-rooted crop, and a fineseedbed is needed to allow the roots to establish andgrow.

SOWINGLettuce crops are seeded or transplanted with 2 to 4rows per bed. The most common method for directseeding is establishing 2 rows per bed under furrowirrigation. When direct sowing, the seed can bepelleted or unpelleted depending on the grower�spreference and type of seeding equipment used.Unpelleted seed is sown at high rates of about 0.3kg/ha resulting in seed intervals of about 60 mm. Thecrop is later thinned to about 300 mm per row whenthe plants reach about the 6 leaf stage. Where bed

centers are 1.5 m apart with two rows per bed youcan expect a maximum plant population of about45,000 per hectare. Pelleting individual lettuce seedspermits the use of precision planters when seeding.This allows for lower seeding rates, more evenlyspaced plants and less labour costs to thin the crop.

Lettuce germinates best at low temperatures between0ºC to 10ºC, and depending on variety, germination isinhibited when temperatures reach 25ºC to 35ºC. Thisis known as thermo-inhibition, with the seedgerminating when returned to more suitabletemperatures. Unfortunately, continued exposure tohigh temperatures may induce a secondary dormancy,called thermo-dormancy which means the seed willnot germinate even when returned to non-inhibitingtemperatures. Thermo-inhibition and thermo-dormancy is a common cause of germination failureduring summer. Germination of lettuce is alsopromoted by light and inhibited by dark; which isknown as photo-dormancy. As light is essential, theseeds need to be planted very shallow to allow goodgermination.

Transplanting overcomes many establishmentproblems including germination failure and the costof thinning. The cost of transplants varies from $20

Transplanting four rows of lettuce to a bed.

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to $40 per 1000 with some growers producing theirown. Transplants are planted on beds, usually with 3or 4 rows per bed, depending on bed width. Theplants are spaced about 300 mm apart along therows, giving a plant population of about 60,000/ha

NUTRITIONLettuce is a rapid growing plant that needs a well-balanced nutritional program to produce a highquality, high yielding crop. Field lettuce grows best onwell-drained fertile soils with a pH between 6.0 and7.0. Where soil pH is too low (too acid), theapplication of lime or dolomite is necessary to allowbetter uptake of nutrients. For optimum resultsincorporate the lime or dolomite into the soil at least 6weeks before planting.

A soil test will help determine the soil nutrient statusand should be the basis of any fertiliser program. Afull soil test costs around $100 per sample and analysisresults will be received a few weeks after sending. Thefollowing recommendations are based on the nutrientrequirements to produce a 50 t/ha lettuce crop.

Nitrogen (N)A soil test measures nitrate nitrogen and where levelsare very low a maximum fertiliser rate of 200 kg/haof N is recommended. As soil nitrate levels increasethe fertiliser rate should be reduced. When soil nitratelevels are high (50 ppm or more), fertiliserapplications should be no more than 50 kg/ha of N.Nitrogen is easily leached from the soil by irrigationand rain and is not recommended for all nitrogen tobe applied as a base application. As a general guide,apply one third as a base application and apply theremainder in even ammounts during the crop�s life.Monitoring sap nitrate throughout the season mayhelp with this decision making. Side dressing ofnitrogen should be drilled in when cultivating forweeds or spun over the top and irrigated. Thenumber of side dressings required varies from one insoils with high nitrate levels to three or four evenlysplit over regular intervals where a high amount ofnitrogen is required. When high inputs of nitrogen arerequired it is recommended that the fertiliser be in thenitrate form rather than the ammonium form toreduce the chance of ammonium toxicity (jelly butt).Care needs to be taken not to apply excess nitrogen tominimise the incidence of jelly butt, excessive growthand outer leaf burn.

Phosphorus (P)Where a soil test shows very low phosphorus levels, amaximum fertiliser rate of 100 kg/ha of P is

recommended. As soil phosphorus levels increase, thefertiliser rate should be reduced. A rate of only 20kg/ha of P is required when soil tests show highlevels of phosphorus. Phosphorus is stable in the soiland does not move much after it has been applied;therefore it can be applied weeks before the crop isplanted. The entire recommended rate should beapplied as a base dressing. Broadcasting phosphorusfertilisers is common but banding it below the seedline is preferable, allowing roots to quickly find it.There is of little value in applying phosphorusfertilisers as a side dressing after a crop has alreadybeen established.

Potassium (K)Where a soil test shows very low potassium levels, amaximum fertiliser rate of 150 kg/ha of K isrecommended. As soil potassium levels increase, thefertiliser rate should be reduced. A rate of only20 kg/ha is required when soil tests indicate high levelsof potassium. Potassium is more stable in the soil thannitrogen but can be leached over the season.Application recommendations are the same asnitrogen with one third as a base application and theremainder applied evenly over the crop�s life.

OtherCalcium, magnesium and molybdenum become lessavailable to lettuce as soil pH decreases. The amountof lime needed to correct pH varies with soil type.Soil testing will help with this decision making. Heaviersoil types will need higher rates of lime to increase pHthan lighter soils.

Animal ManureAnimal manure can be used for growing lettuce andshould be incorporated into the soil at least 4 to 6weeks before sowing. This is important, especiallywith fowl manure to avoid ammonium toxicity (jellybutt). It is recommended that fresh manure be cured(dried) before use to avoid plant burn and Bottomrot (Rhizoctonia) problems. Animal manures helpincrease the soil�s organic matter, improving soilstructure and water and nutrient holding capacity.Animal manures are generally low in N:P:K content,with fowl manure generally the highest atapproximately 2.1% N, 1.6% P and 1.0% K. Anapplication rate of about 7 t/ha (approximately20 m³) makes a useful base dressing for lettuce crops.

Nutritional DisordersTipburnTipburn is considered a problem of calciumdeficiency. Most vegetable growing soils have enoughcalcium with the problem usually being associatedwith water stress. Tipburn is the breakdown of leaf

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margins, particularly on the inner heart leaves and isdifficult to see during harvest. Where this disorder is aproblem, it is common practice to apply weekly foliarsprays of calcium nitrate. However, using lesssusceptible varieties and good soil moisturemanagement will give better tipburn control thanusing foliar supplements later on.

Ammonium toxicity (Jelly Butt)Jelly Butt can be a problem when high inputs ofnitrogen are used in the ammonium form. Theammonia released in the soil becomes toxic andcauses poor emergence of seedlings, followed bywilting and death. Symptoms in older lettuce are seenin the central root tissue with browning and tissuebreakdown. Jelly Butt is common where heavyapplications of fowl manure are applied on wet soils.Fowl manure needs to be incorporated into the soil atleast 4 to 6 weeks before sowing.

Suggested Fertiliser ProgramThe base fertiliser application will depend on soilfertility with between 300 and 900 kg/ha of 8:11:10compound (or similar) usually being required. Ifapplying fowl manure at 20 m³/ha the base fertiliserapplication rate can be reduced by one third. Ensurethat the total phosphorus requirements are applied atthis stage. Banding the base application under the seedor transplant line will give the best results. Ifbroadcasting prior to hilling-up, you will need toincrease the base application by 50 to 75%.

Side dressing of nitrogen should start at about the 6leaf stage and can be timed with weed cultivation. Arate of about 100 kg/ha of ammonium nitrate(nitram) should be applied at each application with thenumber of applications depending on total nitrogenrequirements. Intervals of at least 2 weeks are neededbetween each application. Yellowing of the olderleaves is a good indication of nitrogen deficiency. Ifextra potassium is required then the first side dressingof nitram can be substituted with nitrate of potash.Muriate of potash is also used, as it is one of thecheapest forms of potassium available.

CLIMATE AND VARIETALSELECTIONLettuce is a cool season crop with optimal growingtemperatures from 23ºC during the day to around7ºC at night. Temperature and day length experiencedthroughout the growing period has the greatestinfluence on how a variety performs. Lettuce grownover the hotter months matures much quicker than inthe colder months. Long periods of high light

intensities can also affect lettuce as it has an internalcounter that keeps track of the number of daylighthours the plant receives. Once a critical number ofhours are received, the plant will bolt. Bolting is theterm used for the initiation and development of theflower stalk. The exact number of hours varies fromcultivar to cultivar. Lettuce varieties can be classified aseither hot or cold weather varieties (or somewhere inbetween) depending on what temperature and timeof the year best suits them. Lettuce varieties areadapted to specific planting periods to help overcomethe range of climatic conditions therefore planting avariety in its correct slot will help reduce thesesymptoms.

Disease resistance is an important consideration whenselecting varieties. Some varieties are more susceptibleto certain diseases than others; e.g. Salinas types tendto be more susceptible to dry leaf spot, a bacterialdisease prevalent in wet weather or with overheadirrigation. Varietal resistance to downy mildew, corkyroot, black root and big vein are available. Differentvarieties can show reduced or increased disease levelsdue to some physical characteristic of the lettuce. Costype lettuce can be more prone to Botrytis infection asit is a more upright lettuce than an iceberg type lettuce.In windy conditions the taller varieties are more likelyto be damaged at the base of the stem which allowsentry of the fungus. Some iceberg lettuce varietieshave more leaves hanging very close to the ground,which can create a warm moist environment allowingfungi such as Botrytis and Sclerotinia to invade theleaves.

Choosing a variety is difficult and it is hard forgrowers to keep abreast of all the new varietiesreleased each year. Lettuce varieties are adapted tospecific planting periods and planting them out oftheir time slot will result in a less than desirableproduct. Whether growing for the fresh market or forprocessing will also have an influence on what varietyis grown. When considering a new variety, trial italongside your standards so you can assess itsperformance against your current varieties.

WEED CONTROLWeeds reduce available light, moisture and nutrition tothe crop. Lettuce does not compete well and poorweed control will result in reduced quality and yield.Hand hoeing, inter-row cultivation and herbicides areall used to help control weeds.

Weed management starts with planting into a weed-free seedbed. This can be achieved by planting into a

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bed that has been freshly cultivated and bed formed.If the paddock has a high weed seed load then it isrecommended you pre-irrigate to germinate an initialflush of weeds. These weeds can then be controlledby a shallow cultivation or the use of a knockdownherbicide.

The use of pre-emergent herbicides is the standardmethod for controlling broadleaf weeds in lettucewhen sowing by transplants. There are only a fewregistered herbicides in NSW that control broadleafweeds in lettuce. These chemicals will only work ifapplied before the weeds have emerged and areineffective against established weeds. It is best tominimise the time between the final cultivation (orknockdown herbicide) and the application of thesepre-emergent herbicides. A delay of more than a fewdays could mean no control on newly germinatedweeds. There are no selective herbicides that willcontrol emerged broadleaf weeds in lettuce.

If broadleaf weeds germinate after sowing then ashallow inter-row cultivation or hand hoeing are theonly control options. Avoid cultivating the soil tooearly, as disturbing the soil will stimulate more weedgermination and prematurely upset the pre-emergentchemical barrier. Timing it with the first fertiliser sidedressing should work. Be careful not to leave thecultivation too late or there may be insufficient spacebetween rows and plant damage could occur. Carealso needs to be taken as lettuce is a shallow-rootedcrop and cultivation may cause damage to feederroots close to the surface.

Grass weeds alone do not pose much of a problemas most are controlled at sowing by the pre-emergentherbicides and then by the inter-row cultivation usedto control the broadleaf weeds. If grass weeds needcontrolling, there are post emergent herbicidesregistered in NSW.

Soil fumigation is also an option to control weeds andshould be applied at least two to three weeks prior toplanting. If soils are heavy (i.e. high in clay content),very high in organic matter or remain below 150Cfollowing application, the plant-back period will needto be longer. Fumigating also controls nematodes,fungi and soil borne insects. Care is needed withfumigation as overuse can lead to soil biodegradationand reduced pest control.

When direct seeding, registered pre-emergentherbicides are available, but many growers elect not touse them. Crops are direct seeded into a weed freeseedbed at a high sowing rate. When the crop reachesabout the 6 leaf stage it needs to be thinned and inter-row cultivated. Hand hoeing to thin the lettuce cropand weed along the plant line takes about 60 manhours per hectare where 2 rows of lettuce are sownon 1.5 m beds. Use of precision seeding can speedthe thinning operation and help reduce labour costs.

Cultivating and renovating furrows.

Hand cultivation is still an option for smaller directseeded plantings.

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IRRIGATIONTo achieve a consistent maximum yield of high qualitylettuce, growers will need to irrigate their crops.Lettuce is a shallow rooted crop that has difficulty inextracting water from depth. Furrow and overheadirrigation of lettuce are the two preferred optionswith growers in NSW. Drip irrigation is not widelyused for lettuce in NSW but would be very suitable.The most critical stage for lettuce is duringgermination, seedling establishment and the last fewweeks before harvest. Growers should use moisturemonitoring equipment (e.g. tensiometers) to helpschedule irrigation requirements.

A lettuce crop requires about 3 to 4 megalitres ofirrigation water from sowing to harvest. The crop issensitive to salinity and plant growth may be affectedif levels are too high. The recommended threshold toavoid yield loss is when the conductivity of theirrigation water reaches 900 µS/cm. When levelsincrease to 1400 µS/cm, a yield loss of 10% may beexpected and when levels reach 2100 µS/cm, a yieldloss of 25% may be expected. These values are aguide only and vary with soil type, leaching potential,irrigation method and age of plant.

SPRAY APPLICATIONTo control weeds, insect pests and diseases, growersoften need to apply the appropriate spray treatments.Factors influencing the biological efficacy of thesespray treatments include the dosage rate, spraycoverage, timing and product efficacy. Timing andspray coverage are the factors which growers have themost control over. Timely spray application can onlybe achieved with regular and thorough crop scouting.Crop scouting involves close inspection of sometwenty to forty individual lettuce plants throughout thecrop and recording the incidence of pests anddiseases. This way the grower can be better informed

and able to apply sprays when the pest is mostvulnerable and before a problem gets worse. (i.e.target heliothis when at early instar stages)

To achieve the maximum spray coverage growersshould use the most suitable boom types. ControlDroplet Application sprayers and air-shear sprayershave given better droplet penetration and spraycoverage than conventional booms. Their superiorspray coverage is due to the air stream causingagitation of plant foliage increasing dropletpenetration. Modifying a conventional boom withinter-row droppers can greatly improve spraycoverage and penetration on lettuce. Nozzles shouldbe aimed in towards the base of lettuce plants helpingto increase spray coverage in and under lower leaves.

All spray equipment needs to be regularly calibratedand checked for wear. Poorly maintained equipment isoften the cause of spray failures. Only with correctlycalibrated equipment can growers achieve goodresults.

DISEASE CONTROLMore detailed information on disease control can befound in the �Integrated Pest Management in Lettuce:Information Guide� (see references on page 15).

Disease reduces both the yield and quality of lettuce.The three main types of disease in lettuce are fungal,bacterial and viral.

Downy mildewCause: Caused by the fungus Bremia lactucae.

Symptoms: Light green patches develop on leaves,which eventually turn yellow then brown. A fluffygrowth can often be seen on the underside of theleaves associated with the patches.

Source of infection: The primary source of infection isfrom the seed, spores in debris, spores from wildlettuce weeds and from nearby fields. Once

Good weed control and irrigation is needed for evenyields.

Fitting a conventional boom with droppers and extranozzles can improve spray coverage.

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established, spores from diseased lettuce within thecrop spread the disease further, helping carry thefungus from season to season.

Control: Using resistant varieties is the best method ofcontrolling this disease. If growing a variety that issusceptible to Downy mildew then a strategy of usingappropriate fungicides is the best option. Bothprotective and curative (systemic) fungicides areavailable.

SclerotiniaCause: Caused by the soil borne fungi Sclerotinia minorand S. sclerotiorum.

Symptoms: This disease is often called lettuce drop. Theearly signs of the disease are wilting and browning ofleaves followed by eventual death. In NSW, S. minor isthe most common type of sclerotinia.

Source of infection: Both species produce hard, blackreproductive survival structures called sclerotia. Thesecan be seen on the lower leaves and at the base of thelettuce when infected by S. minor and towards the topof the lettuce when infected by S. sclerotiorum. Thesclerotia survive from season to season in the soil. Thesclerotia of S. minor germinate in the soil and infectthe lower leaves of the lettuce. S. sclerotiorum infectsthe lettuce in two ways. Sclerotia germinate in the soiland infect the lower leaves (similar to S. minor) or thegerminating sclerotia eject spores into the air. Thesespores may then infect the top of the lettuce, usuallywhere some previous damage has occurred (i.e.damage from frost or insects). Excess rainfall orirrigation can predispose lettuce to sclerotinia.

Control: Rotation with non-host crops helps incontrolling both types of fungi but fungicideapplications are also generally required. To controlS. minor, dipping the trays in fungicide followed byspraying at the base of the plant will give good

protection. If S. sclerotiorum is a problem then furtherapplications are necessary to cover the foliage andprotect the leaves from infection.

Grey MouldCause: Caused by the fungi Botrytis cinerea.

Symptoms: The symptoms are similar to Sclerotiniawith the lower leaves wilting and eventually the wholeplant dies. The fungal growth (hyphae) of Botrytislooks grey whereas the hyphae of Sclerotinia arewhite.

Source of infection: The small black sclerotia and sporesproduced from thisdisease are the initialsources of infection. Thesclerotia is long lived andfound in crop trash and inthe soil. Grey mould isfavoured by cool dampconditions.

Downy mildew showing the browning of the leaf andwhite coloured fungal growth.

Lettuce paddock with plant losses due to sclerotinia.

Botrytis is similar tosclerotinia but producesgrey to brown colouredfungal growth.

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Control: The fungicides used for the control ofsclerotinia also control Botrytis.

AnthracnoseCause: Caused by the fungi Microdochium panattonianum.

Symptoms: The symptoms are leaf spotting, oftenalong the midrib of the lettuce. First seen as tiny watersoaked spots which enlarge to brown circular toangular spots with a reddish edge about 4 mm indiameter.

Source of infection: The fungus survives as sclerotia insoil and as hyphae in infected crop debris. This diseaseneeds free moisture for its spores to germinate. Thespread of the disease is favoured by cool-wetconditions.

Control: Controlling this disease is assisted by croprotation and ploughing in debris as soon as possibleafter harvest. Removal of weed hosts is alsoimportant. Avoiding overhead irrigation in the lateevening may assist in control.

Septoria SpotCause: Caused by the fungi Septoria lactucae.

Symptoms: The symptoms are first seen on the olderleaves as brown spotting.

Source of infection: The fungus is initially seed borne andcan be carried from lettuce to lettuce by spores. Thisdisease establishes best in wet conditions. Infectedlettuce debris and weeds such as prickly lettuce(Lactuca serriola) may also harbour the disease.

Control: Planting disease-free seed or seed that hasbeen hot water treated will aid in reducing this disease.Using crop rotations is very important. Rapidbreakdown of crop debris to reduce the carry-overof diseased trash is also necessary.

Bacterial Leaf SpotCause: Caused by the bacterium Xanthomonas campestrispv. Vitians.

Symptoms: Seen as large brown to black circular areasthat start as small translucent spots. When the lesionscoalesce the leaf blade collapses. Bacterial leaf spotmay be confused with Septoria and Anthracnose.

Source of infection: This disease can be seed-borne andspread from infected plants by wind and rain oroverhead irrigation.

Control: Using disease free seeds and variety resistanceare the best control methods for this disease.

Varnish SpotCause: Caused by the bacterium Pseudomonas.

Symptoms: Symptoms include shiny browndiscolouration, often along the midrib of inner leaves.Not always visible from the outside of the lettuce butwhen the outer leaves are removed, the rotting can beseen.

Source of infection: The bacteria survive in crop debris,other hosts and in the soil. They are spread by rainand wind. Insects can cause injuries through which thebacteria may enter the plant.

Anthracnose produces brown spots, often along themidrib of the lettuce.

Septoria symptoms are similar to downy mildew butwithout the white coloured fungal growth.

Bacterial leaf spot lesions look dark and greasy.

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Control: Knowledge about this disease is limited, hencecontrol recommendations are difficult.

Bacterial Soft RotCause: Caused by the bacterium Erwinia.

Symptoms: Symptoms include soft rotting and slime onthe head. Infection starts as translucent spots whichturn to a dark brown colour.

Source of infection: Erwinia bacteria survive in lettucedebris and infection can occur through damaged planttissue. The damage may be caused mechanically, byinsects or by frosts. An ideal condition for infection ishot, wet weather. The disease is often transferredfrom lettuce to lettuce by contaminated cutting knivesor spread by contaminated washing water.

Control: Reducing injury to the lettuce will assist incontrolling this disease.

Big VeinCause: Big Vein Virus

Symptoms: The lettuce develops leaves with large veinsthat appear stiff with ruffled margins. Depending onthe severity of infection, some hearts will make itthrough to harvest, whereas other plants remainstunted, or often don�t form hearts. Cooler weatherand low light levels favour lettuce big veindevelopment.

Source of infection: The soil borne fungus, Olpidiumbrassicae spreads this virus. The fungus itself causes noproblems to the lettuce crop and can survive in thesoil for many years.

Control: There is no chemical control available forlettuce big vein. Irrigation management to reducewaterlogging will help reduce the likelihood of thisdisease. Where possible, growers should use varietiesthat show a useful degree of resistance.

Lettuce Necrotic Yellows VirusCause: Lettuce Necrotic Yellows Virus

Symptoms: This disease causes yellowing of the lettuceaccompanied by stunting of infected plants. Olderplants may develop the yellowing on one side only.Plants infected when young may turn brown and die.

Source of infection: The sowthistle aphid (Hyperomyzuslactucae) transmits this disease from the sowthistle(Sonchus oleraceus) to the lettuce plants. This aphidnormally prefers to feed on sowthistle weeds butoccasionally feeds on lettuce plants. The aphidtransmits the virus while feeding on the lettuce plant.

Varnish spot causing rotting on themidrib of inner leaves.

Soft rot causeshearts to turnbrown and slimy.

Big vein symptoms are unmistakable with the characteristic large veins.

Lettuce necrotic yellows causes yellowing and darkmarkings on leaves associated with stunting of the plant.

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Control: Insecticides are available but the aphids arevery difficult to control because they tend to only visitthe crop for a short period before moving on toneighbouring sowthistles. The best method of controlis to remove all sowthistles in the vicinity of lettucecrops.

Tomato Spotted Wilt VirusCause: Caused by the Tomato Spotted Wilt Virus(TSWV)

Symptoms: The main symptoms on lettuce areyellowing, often associated with brown spotting and adistinct russetting on the leaves. The central leaves maybe distorted. Plants do not recover once infected andheart formation is often incomplete. Seedlings can beinfected with the virus and not show symptoms till theplants mature.

Source of infection: Tomato Spotted Wilt Virus hasnumerous hosts and can be transmitted from plant toplant by thrips. The main thrips species that transmitTSWV are tomato thrips, onion thrips and westernflower thrips. The spread of TSWV occurs whenjuvenile thrips feed on TSWV infected plants, and flyinto lettuce crops as adults. Secondary spread occursby adult thrips moving within the crop.

However leaves may have a downward curlingsymptom as well.

Source of infection: The main source of this virus isthrough contaminated seed. Lettuce mosaic virus canalso be transmitted by numerous types of aphids.Aphids can transmit the disease from an alternate hostoutside the crop or from an infected lettuce within thecrop.

Control: Because lettuce mosaic virus is predominantlya seed-borne disease, seed companies grow their seedcrops away from normal growing areas to reduce thechance of mosaic virus developing in their crops.Controlling aphids with registered insecticides willhelp to reduce the spread of the disease. Narrowspectrum chemicals are recommended to preservebeneficial insect activity

INSECT PEST CONTROLMore detailed information on insect pest control canbe found in the �Integrated Pest Management inLettuce: Information Guide� (see references page 15).

Most pests of lettuce are common pests of othervegetable and field crops. The insects either damagethe lettuce plants by physical damage or bytransmitting diseases. Pests are typically managed bycalendar spraying, strategic spraying or integrated pestmanagement.

Calendar sprayingCalendar spraying is the term used for a set programof sprays, usually using older cheaper broad spectrumchemicals. Unfortunately resistance has developed tomany of the older insecticides by some insect pests.Although calendar spraying has the advantage of

Control: Controlling thrips numbers is the only way ofreducing the incidence of the disease. If using an IPMprogram, beneficial insects may keep their numbersdown. When their numbers increase, and/or highlevels of the virus are present, insecticides are neededfor control.

Lettuce Mosaic VirusCause: Caused by the Lettuce Mosaic Virus

Symptoms: This virus causes mottling and mosaicsymptoms on leaves. Growth is restricted if the plantis infected when young. When older plants areinfected, the symptoms can be very similar to necroticyellows, producing a yellow mottling on the leaves.

TSWV symptoms show leaf spotting with some dwarfingof lettuce.

Mosaic infected lettuce have mottled leaves with rufflededges.

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being predictable, it often means sprays are appliedwhen either not necessary or not at the optimal time.This strategy is not recommended.

Strategic SprayingStrategic spraying involves scouting the crop on aregular basis to determine the level of pest activity.When pests are detected and their numbers reach alevel that will cause damage, insecticides are used forcontrol. Broad spectrum insecticides are also morecommonly used due to their cheaper price.

Integrated Pest Management (IPM)This option is a pest management system where all thepest and beneficial insects are considered inmanagement decisions. Crops are scouted on a regularbasis to determine the number of both pests andbeneficial insects. While trying to reduce insect pestnumbers, every effort is made to try and preservebeneficial insects as they eat and kill the pest insects. Allavailable tools for managing pests are consideredincluding, cultural practices, beneficial insects,biological insecticides, new generation narrowspectrum insecticides and broad-spectrum insecticides.Where possible, new generation narrow spectruminsecticides are preferred over broad-spectruminsecticides to encourage beneficial insect activity.

HeliothisHeliothis is the most common caterpillar pest oflettuce with two species found in Australia

Helicoverpa armigeraProbably the most difficult caterpillar to control as ithas developed resistance to a range of insecticides.H. armigera are usually locally bred pests that emergeas moths in late spring from over-wintering pupae.There are usually 3 or 4 generations each year witheach successive generation larger in number than theprevious. H. armigera populations decrease in autumnas more of the mature larvae delay their developmentinto a pupa over the winter months.

Helicoverpa punctigeraThis native budworm is also prevalent on lettuce andhas tended not to develop resistance to insecticides.H. punctigera is the dominant species in spring butgenerally numbers diminish in summer. H. punctigeraare believed to over-winter in the arid regions, flyingin early spring (usually during September), often withspring storm fronts.

In temperate areas both H. armigera and H. punctigeramoths have usually finished laying eggs by the end ofApril with only very low numbers of larvae seen bythe end of May. In sub-tropical areas, Heliothis maycontinue breeding throughout the winter months.

A female moth may deposit a thousand eggs, usuallylaid singly, on different leaves or plants. When the eggshatch, small larvae emerge, about 1.5 mm long. Thelarvae begin feeding almost immediately and usuallyburrow into the heart. After 6 moults, the maturelarvae finish eating, descend the plant and burrow intothe soil to pupate.

Spraying for control should be timed when grubs arejust hatching and before they burrow into the heart.Even though H. armigera have developed resistance toa number of chemicals, they are still susceptible at the1st and 2nd instar stage. Timing sprays for egg hatchwill mean that sprays can be less frequent during thespring and autumn compared to the hotter months ofsummer.

After harvest, cultivate paddocks to a depth of 10 cm(�Pupae Busting�) to kill any pupating larvae. Do this assoon as possible after harvest for maximum benefit.

Other caterpillarsOther caterpillar pests of lettuce include cutworms(Agrotis spp), loopers (Chripodeixis spp) and clustercaterpillars (Spodoptera spp). These can also bedestructive pests in lettuce but are not usually seen inas high numbers as heliothis. Control measures forheliothis will usually also control loopers and clustercaterpillars. Cutworms are more of a problem atestablishment and can be controlled by culturalmethods and insecticides.

AphidsA number of aphid species infest lettuce including theGreen peach aphid (Myzus persicae), Brown sowthistleaphid (Vroleucon sonchi), Potato aphid (Macrosiphumeuphorbiae), Rose aphid (Macrosiphum rosae), andSowthistle aphid (Hyperomyzus lactucae). These aphidsare all vectors of the Lettuce Mosaic virus while onlythe sowthistle aphid is a known vector of theNecrotic Yellows virus. Cool dry weather favoursaphids. Aphids have a wide range of natural enemies,

Heliothis caterpillars feed on leaves and often burrowinto the heart.

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and if narrow spectrum insecticides are used tocontrol other pests, then aphid populations are usuallykept below levels that cause damage from directfeeding.

Aphids can be in either the winged or non-wingedform. When in the winged form they are on themove and when they are in the non-winged form theyare colonising and rapidly increasing in number.Aphids mostly reproduce asexually and are welladapted to rapidly increase in number.

Damage from direct feeding results in leaf curling andwilting. Lettuce will tolerate a moderate level ofdamage depending at the growth stage. Aphidsgenerally tend to feed on the lower side of outerleaves, which are mostly removed at harvest.Beneficial insects usually keep aphids in control. Whenaphid levels increase to damaging numbers use onlynarrow spectrum chemicals for control.

A new and highly invasive aphid is soon expected toarrive in Australia. The Lettuce Aphid (Nasonoviaribisnigri) is presently found in Europe, NorthAmerica, South Asia, and New Zealand. Unlike otheraphids that colonize on the outer leaves, the lettuceaphid prefers to feed in the centre of lettuce heartsand is therefore difficult to control.

ThripsA number of thrips species infest lettuce includingWestern Flower thrips (Frankliniella occidentalis), Onionthrips (Thrips tabaci), Plague thrips (Thrips imaginis) andTomato thrips (Frankliniella schultzei).

Thrips are very small insects about 1 to 2 mm long.They vary in colour and size depending on species andgenerally have a wide host range. Thrips can causemechanical damage to leaves of seedlings, resulting incurled leaves, silvering and wilting. Transmittingdisease is usually the biggest concern with thrips. Somespecies of thrips can transmit Tomato Spotted Wiltvirus which causes wilting and a distinct russetting ofthe leaves.

Yellow sticky traps are a good method of monitoringthrips. If using an IPM program, beneficial insectsmay keep their numbers down. When their numbersincrease, and/or high levels of the virus are present,insecticides are needed for control.

Other sap sucking pestsOther sap sucking pests include leafhoppers andRutherglen bugs. These are occasional pests and canmigrate into lettuce crops when the surrounding cropsand weeds dry off. The main concern is their presencecauses a contamination problem. When these insectsare in very high numbers throughout the hearts, thelettuce becomes undesirable for both fresh andprocessing markets.

YIELD AND HARVESTINGIceberg lettuce is ready for harvesting once a firmheart is formed. This can be as quick as 8 weeks oversummer, if sown as transplants, and as long as 16weeks over winter, if sown by seed. The lettuce is cutclose to the ground and excess leaves are trimmedoff. The heads are washed if needed and packed inthe field. Most iceberg lettuce are cut by hand andloaded onto harvest aids for packing in the field.Growers are moving towards mechanising thisoperation with a number of mechanical harvestersnow operating around Australia.

To supply the fresh market, the lettuce is packed intowaxed cardboard cartons. A total of 12 heads arepacked in each carton with the bottom (butt) facingthe top. If supplying a processing market then thelettuce is loosely packed in large bins or crates. Lettucefor processing requires heavier trimming with moreof the wrapper leaves removed at harvest.

Yields can vary from 15,000 to 30,000 cartons perhectare with an average of about 2200 cartons perhectare. Lower yields are experienced when theclimatic conditions become harsh. Yields in theMurrumbidgee district are higher in the warmer

Winged and wingless sowthistle aphids. Typical damage caused by thrips.

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shoulder seasons than during the very cold winters.Unformed hearts, pest and disease are also reasonsfor reduced yields and returns. Prices returned togrowers vary from season to season and currentlyrange from $5 to $12 per carton.

COST OF PRODUCTIONCost of lettuce production varies depending on thetype of growing system used and the amount ofvariable inputs needed. The following table gives anapproximate cost of production for a direct seededcrop yielding 2200 cartons per hectare. The cost isshown on a �per carton� and �per hectare� basis.

These costs are a guide only but give a goodindication of which operations are the mostexpensive. For this example 82% of the cost ofproduction is incurred during and after harvest. Thispercentage will change depending on the distancefrom the markets. Transport cost will be muchcheaper close to the markets but will increase to over$1.00 per carton if carting long distances. Chemicalrates, fertiliser rates, labour costs and irrigationrequirements are other examples where costs varyfrom area to area.

Lettuce production costs

Variable inputs Cost/ha Cost/carton

Seed $225.00 $0.10

Land preparation $526.75 $0.24

Irrigation $69.44 $0.03

Fertiliser $305.35 $0.14

Pest and disease control $572.96 $0.26

Weed control $870.00 $0.40

Cartons $5874.00 $2.67

Harvesting $2134.00 $0.97

Cooling $110.00 $0.05

Freight $1980.00 $0.90

Agent’s commission $2112.00 $0.96

Research levy $88.00 $0.04

Total $14,867.50 $6.76

Source: - NSW Agriculture Farm budgethandbook for NSW vegetables – 2001

Lettuce being harvested and packed in the field.

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STORAGE AND TRANSPORTHarvesting is usually conducted in the cool of themorning before the lettuce temperature becomes toowarm. Once harvested and packed in the field, thelettuce should be cooled to 1ºC to ensure that itarrives at the market in a fresh condition. Prompt andthorough cooling is necessary to remove any field heatfor a longer shelf life. Forced air cooling is preferredto a standard coolroom as it will bring the lettucetemperature down much quicker. Vacuum cooling isalso very efficient on leafy products but not aseffective on bulky vegetables. Lettuce held between0ºC and 1ºC and in high relative humidity (95�97%),have a shelf life of 2 to 3 weeks. If harvesting closeto markets, unrefrigerated trucks are used fortransport. If harvested a long distance from themarket then refrigerated trucks are needed. Harvestingover-mature lettuce or storing without cooling willhasten lettuce breakdown and deterioration.

REFERENCES AND FURTHERREADINGIntegrated Pest Management in Lettuce: Information Guide.NSW Department of Agriculture, 2002Sandra McDougall et.al.ISBN 0 7347 1346 0

Australian Vegetable Growers Handbook: 2nd ed.Scope Publishing, Victoria, 1998Giuliano (John) Salvestrin.ISBN 0 909677 02 6

Lettuce Information Kit (Agrilink)QDPI, 1997Glenis AylingISBN 0 7242 6722 0

Diseases of Vegetable CropsQDPI, 1994Denis PersleyISBN 0 7242 5372 6

Horticulture Australia: The complete reference of theAustralian Horticultural IndustryMorescope Publishing Pty Ltd, 1995Bob CoombsISBN 0 646 17525 4

Australian Lettuce Industry Conference proceedingsProceedings one and twoNSW Department of Agriculture, 2000 and 2002

AcknowledgementsThe assistance and input from the following people isgratefully acknowledged

Mr Andrew Watson, Research Pathologist, NSWAgriculture

Dr Sandra McDougall, Technical Specialist, NSWAgriculture

Mr Andrew Creek, Technical Officer, NSWAgriculture

Twelve heads of lettuce are packed in each carton withtheir butts facing the top.

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DisclaimerThe information contained in this publication is basedon knowledge and understanding at the time ofwriting (May 2004). However, because of advances inknowledge, users are reminded of the need to ensurethat information upon which they rely is up-to-dateand to check currency of the information with theappropriate officer of New South Wales Departmentof Agriculture or the user�s independent adviser.

Always read the labelUsers of agricultural (or veterinary) chemical productsmust always read the label and any Permit, beforeusing the product, and strictly comply with thedirections on the label and the conditions of anyPermit. Users are not absolved from compliance withthe directions on the label or the conditions of thepermit by reason of any statement made or omittedto be made in this publication.

© The State of New South WalesNSW Agriculture 2004

ISSN 0725-7759

This publication is copyright. Except as permittedunder the Copyright Act 1968 (Commonwealth), nopart of the publication may be reproduced by anyprocess, electronic or otherwise, without the specificwritten permission of the copyright owner. Neithermay information be stored electronically in any formwhatever without such permission.

Editing and layout by Bill NoadInformation Delivery ProgramDubbo, May 2004Job No. 4532