CLIMATE AND VARIETY SELECTION
Lettuce is a cool season crop with optimal growing temperatures from 23ºC during the day to around 7ºC at night. Temperature and day length experienced throughout the growing period has the greatest influence on how a variety performs. Lettuce grown over the hotter months matures much quicker than in the colder months. Long periods of high light intensities can also affect lettuce as it has an internal counter that keeps track of the number of daylight hours the plant receives. Once a critical number of hours are received, the plant will bolt.
Bolting is the term used for the initiation and development of the flower stalk. The exact number of hours varies from cultivar to cultivar. Lettuce varieties can be classified as either hot or cold weather varieties (or somewhere in between) depending on what temperature and time of the year best suits them. Lettuce varieties are adapted to specific planting periods to help overcome the range of climatic conditions therefore planting a variety in its correct slot will help reduce these symptoms. Disease resistance is an important consideration when selecting varieties. Some varieties are more susceptible to certain diseases than others; e.g. Salinas types tend to be more susceptible to dry leaf spot, a bacterial disease prevalent in wet weather or with overhead irrigation.
Varietal resistance to downy mildew, corky root, black root and big vein are available. Different varieties can show reduced or increased disease levels due to some physical characteristic of the lettuce. Cos type lettuce can be more prone to Botrytis infection as it is a more upright lettuce than an iceberg type lettuce. In windy conditions the taller varieties are more likely to be damaged at the base of the stem which allows entry of the fungus. Some iceberg lettuce varieties have more leaves hanging very close to the ground, which can create a warm moist environment allowing fungi such as Botrytis and Sclerotinia to invade the leaves.
Choosing a variety is difficult and it is hard for growers to keep abreast of all the new varieties released each year. Lettuce varieties are adapted to specific planting periods and planting them out of their time slot will result in a less than desirable product. Whether growing for the fresh market or for processing will also have an influence on what variety is grown. When considering a new variety, trial it alongside your standards so you can assess its performance against your current varieties.
WEED CONTROL
Weeds reduce available light, moisture and nutrition to the crop. Lettuce does not compete well and poor weed control will result in reduced quality and yield. Hand hoeing, inter-row cultivation and herbicides are all used to help control weeds. Weed management starts with planting into a weedfree seedbed. This can be achieved by planting into a bed that has been freshly cultivated and bed formed.
If the paddock has a high weed seed load then it is recommended you pre-irrigate to germinate an initial flush of weeds. These weeds can then be controlled by a shallow cultivation or the use of a knockdown herbicide. The use of pre-emergent herbicides is the standard method for controlling broadleaf weeds in lettuce when sowing by transplants. There are only a few registered herbicides in NSW that control broadleaf weeds in lettuce. These chemicals will only work if applied before the weeds have emerged and are ineffective against established weeds. It is best to minimise the time between the final cultivation (or knockdown herbicide) and the application of these pre-emergent herbicides. A delay of more than a few days could mean no control on newly germinated weeds. There are no selective herbicides that will control emerged broadleaf weeds in lettuce. If broadleaf weeds germinate after sowing then a shallow inter-row cultivation or hand hoeing are the only control options. Avoid cultivating the soil too early, as disturbing the soil will stimulate more weed germination and prematurely upset the pre-emergent chemical barrier. Timing it with the first fertiliser side dressing should work. Be careful not to leave the cultivation too late or there may be insufficient space between rows and plant damage could occur. Care also needs to be
taken as lettuce is a shallow-rooted crop and cultivation may cause damage to feeder roots close to the surface. Grass weeds alone do not pose much of a problem as most are controlled at sowing by the pre-emergent herbicides and then by the inter-row cultivation used to control the broadleaf weeds. If grass weeds need controlling, there are post emergent herbicides registered in NSW. Soil fumigation is also an option to control weeds and should be applied at least two to three weeks prior to planting. If soils are heavy (i.e. high in clay content), very high in organic matter or remain below 150C following application, the plant-back period will need to be longer. Fumigating also controls nematodes, fungi and soil borne insects. Care is needed with fumigation as overuse can lead to soil biodegradation and reduced pest control. When direct seeding, registered pre-emergent herbicides are available, but many growers elect not to use them. Crops are direct seeded into a weed free seedbed at a high sowing rate. When the crop reaches about the 6 leaf stage it needs to be thinned and interrow cultivated. Hand hoeing to thin the lettuce crop and weed along the plant line takes about 60 man hours per hectare where 2 rows of lettuce are sown on 1.5 m beds. Use of precision seeding can speed the thinning operation and help reduce labour costs. To achieve a consistent maximum yield of high quality lettuce, growers will need to irrigate their crops. Lettuce is a shallow rooted crop that has difficulty in extracting water from depth. Furrow and overhead irrigation of lettuce are the two preferred options with growers in NSW. Drip irrigation is not widely used for lettuce in NSW but would be very suitable. The most critical stage for lettuce is during germination, seedling establishment and the last few weeks before harvest. Growers should use moisture monitoring equipment (e.g. tensiometers) to help schedule irrigation requirements. A lettuce crop requires about 3 to 4 megalitres of irrigation water from sowing to harvest. The crop is sensitive to salinity and plant growth may be affected if levels are too high. The recommended threshold to avoid yield loss is when the conductivity of the irrigation water reaches 900 µS/cm. When levels increase to 1400 µS/cm, a yield loss of 10% may be expected and when levels reach 2100 µS/cm, a yield loss of 25% may be expected. These values are a guide only and vary with soil type, leaching potential, irrigation method and age of plant.
SPRAY APPLICATION
To control weeds, insect pests and diseases, growers often need to apply the appropriate spray treatments. Factors influencing the biological efficacy of these spray treatments include the dosage rate, spray coverage, timing and product efficacy. Timing and spray coverage are the factors which growers have the most control over. Timely spray application can only be achieved with regular and thorough crop scouting. Crop scouting involves close inspection of some twenty to forty individual lettuce plants throughout the crop and recording the incidence of pests and diseases. This way the grower can be better informed and able to apply sprays when the pest is most vulnerable and before a problem gets worse. (i.e. target heliothis when at early instar stages) To achieve the maximum spray coverage growers should use the most suitable boom types. Control Droplet Application sprayers and air-shear sprayers have given better droplet penetration and spray coverage than conventional booms. Their superior spray coverage is due to the air stream causing agitation of plant foliage increasing droplet penetration. Modifying a conventional boom with inter-row droppers can greatly improve spray coverage and penetration on lettuce. Nozzles should be aimed in towards the base of lettuce plants helping to increase spray coverage in and under lower leaves. All spray equipment needs to be regularly calibrated and checked for wear. Poorly maintained equipment is often the cause of spray failures. Only with correctly calibrated equipment can growers achieve good results.
DISEASE CONTROL
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 mildew
Cause: Caused by the fungus Bremia lactucae.
Symptoms: Light green patches develop on leaves, which eventually turn yellow then brown. A fluffy growth can often be seen on the underside of the leaves associated with the patches.
Source of infection: The primary source of infection is from the seed, spores in debris, spores from wild lettuce weeds and from nearby fields. Good weed control and irrigation is needed for even yields. Fitting a conventional boom with droppers and extra nozzles can improve spray coverage. Once established, spores from diseased lettuce within the crop spread the disease further, helping carry the fungus from season to season.
Control: Using resistant varieties is the best method of controlling this disease. If growing a variety that is susceptible to Downy mildew then a strategy of using appropriate fungicides is the best option. Both protective and curative (systemic) fungicides are available.
Sclerotinia
Cause: Caused by the soil borne fungi Sclerotinia minor and Sclerotinia sclerotiorum.
Symptoms: This disease is often called lettuce drop. The early signs of the disease are wilting and browning of leaves followed by eventual death. In NSW, Sclerotinia minor is the most common type of sclerotinia.
Source of infection: Both species produce hard, black reproductive survival structures called sclerotia. These can be seen on the lower leaves and at the base of the lettuce when infected by S. minor and towards the top of the lettuce when infected by Sclerotinia sclerotiorum. The sclerotia survive from season to season in the soil. The sclerotia of Sclerotinia minor germinate in the soil and infect the lower leaves of the lettuce. S. Sclerotiorum infect sclerotiniathe lettuce in two ways. Sclerotia germinate in the soil and infect the lower leaves (similar to Sclerotinia minor) or the germinating sclerotia eject spores into the air. These spores may then infect the top of the lettuce, usually where some previous damage has occurred (i.e. damage from frost or insects). Excess rainfall or irrigation can predispose lettuce to sclerotinia.
Control: Rotation with non-host crops helps in controlling both types of fungi but fungicide applications are also generally required. To control Sclerotinia minor, dipping the trays in fungicide followed by spraying at the base of the plant will give good protection. If Sclerotinia sclerotiorum is a problem then further applications are necessary to cover the foliage and protect the leaves from infection.
Grey Mould
Cause: Caused by the fungi Botrytis cinerea.
Symptoms: The symptoms are similar to Sclerotinia with the lower leaves wilting and eventually the whole plant dies. The fungal growth (hyphae) of Botrytis looks grey whereas the hyphae of Sclerotinia are white.
Control: The fungicides used for the control of sclerotinia also control Botrytis. Anthracnose
Cause: Caused by the fungi Microdochium panattonianum.
Symptoms: The symptoms are leaf spotting, often along the midrib of the lettuce. First seen as tiny water soaked spots which enlarge to brown circular to angular spots with a reddish edge about 4 mm in diameter.
Source of infection: The fungus survives as sclerotia in soil and as hyphae in infected crop debris. This disease needs free moisture for its spores to germinate. The spread of the disease is favoured by cool-wet conditions.
Control: Controlling this disease is assisted by crop rotation and ploughing in debris as soon as possible after harvest. Removal of weed hosts is also important. Avoiding overhead irrigation in the late evening may assist in control.
Septoria Spot
Cause: Caused by the fungi Septoria lactucae.
Source of infection: The fungus is initially seed borne and can be carried from lettuce to lettuce by spores. This disease establishes best in wet conditions. Infected lettuce debris and weeds such as prickly lettuce (Lactuca serriola) may also harbour the disease.
Control: Planting disease-free seed or seed that has been hot water treated will aid in reducing this disease. Using crop rotations is very important. Rapid breakdown of crop debris to reduce the carry-over of diseased trash is also necessary.
Bacterial Leaf Spot
Cause: Caused by the bacterium Xanthomonas campestris pv. Vitians.
Symptoms: Seen as large brown to black circular areas that start as small translucent spots. When the lesions coalesce the leaf blade collapses. Bacterial leaf spot may be confused with Septoria and Anthracnose.
Control: Using disease free seeds and variety resistance are the best control methods for this disease.
Varnish Spot
Symptoms: Symptoms include shiny brown discolouration, often along the midrib of inner leaves. Not always visible from the outside of the lettuce but when the outer leaves are removed, the rotting can be seen.
Source of infection: The bacteria survive in crop debris, other hosts and in the soil. They are spread by rain and wind. Insects can cause injuries through which the bacteria may enter the plant.
Control: Knowledge about this disease is limited, hence control recommendations are difficult.
Bacterial Soft Rot
Cause: Caused by the bacterium Erwinia.
Symptoms: Symptoms include soft rotting and slime on the head. Infection starts as translucent spots which turn to a dark brown colour.
Control: Reducing injury to the lettuce will assist in controlling this disease.
Big Vein
Cause: Big Vein Virus
Symptoms: The lettuce develops leaves with large veins that appear stiff with ruffled margins. Depending on the severity of infection, some hearts will make it through to harvest, whereas other plants remain stunted, or often don’t form hearts. Cooler weather and low light levels favour lettuce big vein development.
Source of infection: The soil borne fungus, Olpidium brassicae spreads this virus. The fungus itself causes no problems to the lettuce crop and can survive in the soil for many years.
Control: There is no chemical control available for lettuce big vein. Irrigation management to reduce waterlogging will help reduce the likelihood of this disease. Where possible, growers should use varieties that show a useful degree of resistance.
Lettuce Necrotic Yellows Virus
Cause: Lettuce Necrotic Yellows Virus
Symptoms: This disease causes yellowing of the lettuce accompanied by stunting of infected plants. Older plants may develop the yellowing on one side only. Plants infected when young may turn brown and die.
Source of infection: The sowthistle aphid (Hyperomyzus lactucae) transmits this disease from the sowthistle (Sonchus oleraceus) to the lettuce plants. This aphid normally prefers to feed on sowthistle weeds but occasionally feeds on lettuce plants. The aphid transmits the virus while feeding on the lettuce plant.
Control: Insecticides are available but the aphids are very difficult to control because they tend to only visit the crop for a short period before moving on to neighbouring sowthistles. The best method of control is to remove all sowthistles in the vicinity of lettuce crops.
Cause: Caused by the Tomato Spotted Wilt Virus (TSWV)
Symptoms: The main symptoms on lettuce are yellowing, often associated with brown spotting and a distinct russetting on the leaves. The central leaves may be distorted. Plants do not recover once infected and heart formation is often incomplete. Seedlings can be infected with the virus and not show symptoms till the plants mature.
Source of infection: Tomato Spotted Wilt Virus has numerous hosts and can be transmitted from plant to plant by thrips. The main thrips species that transmit TSWV are tomato thrips, onion thrips and western flower thrips. The spread of TSWV occurs when juvenile thrips feed on TSWV infected plants, and fly into lettuce crops as adults. Secondary spread occurs by adult thrips moving within the crop.
Control: Controlling thrips numbers is the only way of reducing the incidence of the disease. If using an IPM program, beneficial insects may keep their numbers down. When their numbers increase, and/or high levels of the virus are present, insecticides are needed for control.
Lettuce Mosaic Virus
Cause: Caused by the Lettuce Mosaic Virus
Symptoms: This virus causes mottling and mosaic symptoms on leaves. Growth is restricted if the plant is infected when young. When older plants are infected, the symptoms can be very similar to necrotic yellows, producing a yellow mottling on the leaves. However leaves may have a downward curling symptom as well.
Source of infection: The main source of this virus is through contaminated seed. Lettuce mosaic virus can also be transmitted by numerous types of aphids. Aphids can transmit the disease from an alternate host outside the crop or from an infected lettuce within the crop.
Control: Because lettuce mosaic virus is predominantly a seed-borne disease, seed companies grow their seed crops away from normal growing areas to reduce the chance of mosaic virus developing in their crops. Controlling aphids with registered insecticides will help to reduce the spread of the disease. Narrow spectrum chemicals are recommended to preserve beneficial insect activity.
