PROTECTED LETTUCE


       - avoiding pesticide residue problems

October 2002
(Web Version amended November 2003)

Every care has been taken in the preparation of this leaflet to ensure the accuracy of
information and recommendations given. All applications of crop protection chemicals must
be made in accordance with label recommendations, which should be consulted before
spraying. Use of some fungicides not supported by label recommendations may be
permissible under the Specific Off-Label Approval (SOLA) arrangements. In these cases,
use is at risk of the user and the Pesticide Safety Directorate cannot accept any liability for
any loss or damage caused by such use.

Leaflet written by Dan Drakes and Tim O’Neill, ADAS Horticulture.                    (Revised
November 2003)

The help of lettuce growers and consultants in the preparation of this leaflet is
gratefully acknowledged.
- best practice to avoid pesticide residue problems

    Background and objective                                       1

    Identification of the common diseases                          1

    Fungicide use on lettuce                                       2
    - Use the minimum treatment necessary for commercially         2
       economic control
    - Which products can be used?                                  2
    - Illegal fungicide use                                        3

    Reducing the risk of residue problems                          3
    - Practical pointers                                           3
    - Crop management                                              5
    - Specific rules for dithiocarbamates                          6
    - Bromide residues following soil sterilisation with methyl    6
       bromide                                                     6
    - Residue sampling and testing

    Summary of useful information                                  7
    - Recent and forthcoming changes in fungicide approvals        7
    - Fungicides approved on lettuce and MRL values                8
    - Definition of terms                                          9
    - Websites and contact points                                  9
Background and objective

PSD continues to monitor UK glasshouse lettuce for pesticide residues during the winter
period. Latest survey results (winter 2002/2003) showed that 14% of samples tested
contained pesticide residues exceeding the MRL. In addition, 5% contained residues of
pesticides not registered for use on protected lettuce. In both categories the majority of
pesticides recorded were fungicides.

It is illegal to use on UK grown protected lettuce any pesticide which is not specifically
approved (either with an on-label Approval, a Specific Off-Label Approval, or through the
Off Label Extension of Use Arrangements) for use on that crop. The maximum fine for
illegal use is £5,000.

The objective of this leaflet is to provide practical guidance to minimise pesticide residues in
winter lettuce, and in particular how to minimise the risk of exceeding Maximum Residue
Levels (MRLs). Good agricultural practice (GAP) is the key to achieving this objective.
This leaflet should enable growers to achieve GAP. It identifies where and how in the past
residue problems have commonly arisen and explains the way these can be avoided or at
least minimised. It also provides details of various websites and contact points for you to
obtain the most up to date information.

Identification of the common diseases

Fungicides are used against a range of fungal diseases, the three most common being downy
mildew, Botrytis and Rhizoctonia bottom rot. It is crucial to identify diseases correctly if
inappropriate and irrational use is to be avoided but it needs to be acknowledged that with
these three diseases it will sometimes be necessary, when risk is high, to apply fungicide
preventively. Careful examination of affected plants, looking for the characteristic features
(see below), will help to identify the disease. If in doubt, contact your crop consultant, or
send a sample to a recognised plant clinic.

•   Botrytis
    Reddish brown rot at the stem base causing yellowing and greying of the leaves and
    eventual wilting and collapse of the entire plant which may become severed from the
    root system. Damaged leaves may be directly infected. Characteristic greyish mould
    and spore masses invariably develop on affected tissues.

•   Downy mildew
    Conspicuous angular yellow patches on the upper leaf surfaces with white, downy fungal
    growth on the underside. Infected leaves become necrotic and decayed. Generally
    more common in later summer and autumn, though it can be found at any time of year.

•   Rhizoctonia bottom rot
    Dark water-soaked looking spots on leaves in contact with the soil surface. This may
    progress to cause the lowest whorl of leaves to become a slimy black mass. The fungus
    then spreads to leaves immediately above, appearing as foxy-brown flecks and spots,

    especially on the midribs. Sometimes, cobweb-like threads can be seen extending from
    one leaf to the next. Brown resting structures (sclerotia) may develop on affected

•   Sclerotinia rot
    Very soft water-soaked rot of the stem at soil level with dramatic wilting and collapse of
    the foliage. Dense white fluffy growth develops on affected tissues in which the
    characteristic black resting structures (sclerotia) are formed. These sclerotia are up to
    10 mm long for Sclerotinia sclerotiorum but are much smaller, about 1 mm long, for S.

•   Phoma rot
    Dryish, orange-brown (less often, dark brown-black) rot of the stem base, which
    usually rots through the stem causing the plant to collapse. No fungal growth is visible,
    unlike rhizoctonia or botrytis. A leaf spot (often at the edge of the leaf) is sometimes
    seen on the lower leaves - dark brown-black (1-2 cm in diameter) and with age these
    fall out leaving a ragged edge.

•   Ring spot
    Small (3-5 mm diameter) brown spots with a yellow halo on the leaves. These may fall
    out giving a “shot hole” appearance. The midrib may be attacked causing sunken brown
    lesions similar to slug damage. Often associated with wet areas and drips.

Fungicide use on lettuce

Use the minimum treatment necessary for commercially economic control

By following good crop management practices (see page 3) the need for fungicide treatment
is kept to the minimum necessary and yet will still provide effective disease control. In the
summer months it is often unnecessary to apply fungicides, other than perhaps a treatment at
planting for rhizoctonia. Inevitably, for winter lettuce production fungicide treatment is
essential. There are fewer restrictions on fungicide use during propagation than after planting
out, and application early on in the life of the crop often gives better disease protection.

Which products can be used?

The approval status of pesticide products permitted on specific crops changes frequently, so
it is essential to have the most up to date information. Recent changes are shown on page 7.
Information about products approved for use in the UK on protected lettuce can be
obtained from:

•   The Pesticides Safety Directorate (telephone: 01904 455775; fax 01904 455733;

•   A BASIS-qualified crop consultant specialising in protected lettuce

•   Horticultural Development Council headquarters (HDC levy payers only) for a
    Factsheet (07/01) which includes a comprehensive list of approved fungicides (as at
    October 2001) with full details of all product restrictions and example programmes
    (telephone: 01732 848383; email:

• Central Science Laboratory          (CSL)       Liaison   website   (subscription   required,

Approval documents for products with a Specific Off-Label Approval (SOLA), must be
obtained and complied with. SOLAs can be downloaded from the PSD website
( or by contacting ADAS Horticulture (Tel: 01954 268205). Use
of products under a SOLA is entirely at the grower's own risk.

There is a useful website (, provided by PSD,
which gives up to date information on products for which approval has been revoked.

Illegal fungicide use

Over the last five years, residues of several pesticides which are not permitted on protected
lettuce in the UK, have been detected. These include:

         NEVER approved on lettuce in                 NO LONGER approved on
         the UK                                       lettuce in the UK

         Sumisclex                                    Ronilan
         Bravo 500

Growers who use any of these products on protected lettuce grown in the UK face a
maximum penalty of £5000 and PSD have successfully prosecuted those who have been
found guilty of illegal use.

Reducing the risk of residue problems

Practical pointers
This table lists some of the ways in which errors are made that sometimes result in pesticide
residue problems.

Potential problem area                                 Solution


            1. Using a product no longer           Rigorously check that each product you use
               approved                            on lettuce has a current approval

            2. Exceeding the maximum rate         Follow the label instructions exactly. Do not
               stated or the total number of      exceed the maximum rate or number of
               treatments (it is not only illegal treatments
               to do so but unlikely to improve
               disease control)

            3. Exceeding the total number of       Remember that Favour 600 SC and Fubol
               dithiocarbamate (DTC) sprays        Gold WG contain DTCs. Remember that
               permitted                           the maximum number of treatments
                                                   permitted is different for thiram in the winter
                                                   and summer months

Spray safely

            1. Wrongly calibrated spray            Accurately and regularly calibrate your
               equipment                           spray equipment

            2. Spray overlap causing double        Carefully note and mark the edge of the
               application                         spray width when spraying

            3. Sprayers contaminated with          Thoroughly clean the sprayer immediately
               residues from a previous            after use. Products containing tolclofos-
               treatment                           methyl are particularly prone to come out of
                                                   suspension and may be more difficult to
                                                   completely remove from the sprayer

            4. Spraying on mixed cropping          Use a dedicated sprayer for lettuce, or
               nurseries                           ensure that the sprayer is thoroughly cleaned
                                                   before spraying lettuce

            5. Sprays or dusts drifting from       Choose nozzle size and spray pressure
               adjacent crops                      which will achieve the spray cover required
                                                   with minimal drift. Do not spray in draughty
                                                   or windy conditions


            1. Cutting the crop before the         Ensure that there is a reliable, secure and
               necessary harvest interval has      practical management system for always
               elapsed (either in error or         achieving harvest interval compliance.
               because of rapid maturity)

            2. When extra weight is needed,          Avoid the temptation and delay harvest until
               retaining basal leaves which may      normal trimming can take place
               have a high residue level

Crop management

Attention to the key items of crop management reduces the need for fungicide treatment,
and consequently the risk of residue problems:-

•   Hygiene: Clean up after previous crops. Dispose of debris safely to prevent carry
    over of disease. This is particularly important for downy mildew and Sclerotinia.

•   Resistant varieties: This is an essential element in the strategy to minimise the risk of
    downy mildew but it will usually be even more effective if integrated with cultural
    measures and rational use of fungicides. Use appropriate downy mildew resistant
    varieties wherever possible. (Unless you are sure, take advice from your crop
    consultant about the most appropriate choice).

•   Healthy transplants: Make sure plants are healthy in the propagation stage. Plant
    only disease-free young plants, and ensure plants are healthy 3 weeks after planting.

•   Fertilisers: Achieving balanced nutrition will give a stronger plant and will help to
    minimise disease risk. Use regular soil analysis.

•   Plant size: Make sure plants are not too big and handle them carefully to prevent
    damage. Sturdy seedlings are less susceptible to disease.

•   Plant establishment: Encourage plants to root out quickly by not letting the
    propagation block dry out.

•   Irrigation: Get the balance right; take note of the weather forecast. Ensure plants do
    not wilt, but avoid prolonged periods of leaf wetness as this encourages disease.

•   Glasshouse environment: Avoid extremes and keep temperatures as even as
    possible. Try to avoid long periods of high humidities. Leave some ventilation on
    whenever possible. When vents have to be closed use ventilation fans to create air

•   Crop monitoring: Spot potential problems early by monitoring crops regularly.
    Accurately identify the cause and take prompt, and appropriate, action when necessary.

Specific rules for dithiocarbamates

Dithiocarbamate fungicides have regularly caused residue problems in the past and their use
is covered by specific rules. The commonly used currently approved fungicides in this group
are Favour 600 SC, Fubol Gold WG, Karamate Dry Flo and Unicrop Thianosan DG.

a) Use of dithiocarbamate fungicides is restricted after planting out

A total of only 2 applications of fungicides from this group may be made to the crop. These
2 applications must be in the first 2 weeks after planting out. The only exception is thiram
used in the winter (November-March maturing crops) when 3 applications are permitted in
the first 3 weeks after planting.

b) Remember that ALL these products have a 21-day harvest interval.

c) There are no restrictions on dithiocarbamate use during propagation

The move by some growers to use “spacer trays” could potentially affect residues in the
harvested produce, depending on how "planting out" is interpreted. For example, the
restriction on dithiocarbamate use restricting application to within 14 days post-planting.
Where spacer trays are used planting out can be delayed by 2-3 weeks. If a
dithiocarbamate is then applied twice during the 14 days the larger plants will intercept more
spray and the last application will be much closer to harvest. This is likely to lead to
residues above the MRL.

Growers using spacer trays, or other similar techniques, are strongly advised to comply with
harvest intervals which are determined from the date of spacing, and not from planting out.

Bromide residues following soil sterilisation with methyl bromide

Inorganic bromide residues above the MRL of 100 mg/kg (Codex) can be a problem in
slow-growing winter lettuce crops. Organic soils and those with high water tables can also
cause problems. The following guidelines will minimise the risk:

•   Do not use methyl bromide to sterilise soil later than July for autumn or winter lettuce
•   Do not use methyl bromide if the water table is likely to rise to within 50 cm (20 in) of
    the soil surface within six months after use
•   Flood after sterilisation with the quantity of water required according to soil type. A
    minimum of 75 l/m2 (20 gal/yd2) should be applied to soil that is already at field capacity

Residue sampling and testing

Regular sampling and testing for pesticide residues is now standard practice for all growers
who supply lettuce to Assured Produce Scheme standards. The following sampling strategy
is recommended:

•   sample size should be a minimum of 10 heads of lettuce, with a minimum total weight of
    1 kg
•   sampling should be done in an "X" or "W" shape through the crop
•   each "lot" to be tested should comprise a single variety from a clearly defined crop area
•   the sample has to be representative of that "lot"
•   do not sample too near to harvest so that action can be taken if MRLs look likely to be

Maintain good records of all pesticide applications, and also the crop growth stage, weather
and house or block number, so that you learn from experience should treatment lead to
residue levels close to the MRL.

Summary of useful information

Recent and forthcoming changes in Fungicide Approvals on lettuce

Product                   Active ingredient       Change
Basilex                   Tolclofos-methyl        For operator safety considerations, label
                                                  changes for this product mean that
                                                  application with hand held sprayers is no
                                                  longer permitted. On many nurseries this will
                                                  prevent use on protected lettuce.

Favour 600 SC             Metalaxyl + thiram      Approval revoked 18 June 2003. Use-up
                                                  period expires on 2 November 2004.

Karamate Dry Flo          Mancozeb                Approval revoked on 21 September 2001.
                                                  A "use-up" period expires on 30 September
                                                  2004, after which use will be illegal.

Methyl bromide            Methyl bromide          This product is being phased out.
                                                  Production and supply of methyl bromide is
                                                  due to be prohibited from 1 January 2005.

Terraclor Flo and         Quintozene              Approval revoked on 27 March 2001. The
Terraclor 20D                                     "use-up" period expired on 27 June 2002,
                                                  and their use is now illegal.

Unicrop Zineb             Zineb                   Approval revoked on 14 May 2001. The
                                                  "use-up" period expired on 22 September
                                                  2002, and use is now illegal.

(It should be noted that it will also be illegal to offer for sale lettuce with residues of revoked
pesticides after the “use-up” date has expired. This will mean not applying a pesticide to
any crop which is likely to be harvested after the “use-up” date.)

Fungicides approved on protected lettuce (including products with a SOLA), and
MRL values where set

Active ingredient             MRL (and authority)            Product

Azoxystrobin                  3 mg/kg (EU/UK)                Amistar

Carbendazim                   5 mg/kg (EU/UK)                Bavistin DF

Dicloran                      10 mg/kg (Codex)               Fumite Dicloran Smoke

Fosetyl aluminium             *                              Aliette 80WG
                                                             Fosetyl-AL 80WG

Iprodione                     10 mg/kg (EU/UK)               Rovral WP
                                                             IT Iprodione
                                                             Standon Iprodione 50WP

Mancozeb                      5 mg/kg (EU/UK)                Karamate Dry Flo

Mancozeb                      5 mg/kg (EU/UK)                Fubol Gold WG
+ metalaxyl – M               2 mg/kg (EU/UK)

Metalaxyl                     1 mg/kg (EU/UK)                Favour 600 SC
+ thiram                      5 mg/kg (EU/UK)

Prochloraz                    5 mg/kg (EU/UK)                Scotts Octave

Propamocarb                   10 mg/kg (Codex)               Filex
Hydrochloride                                                Proplant

Pyrimethanil                  *                              Scala

Thiram                        5 mg/kg (EU/UK)#               Unicrop Thianosan DG

Tolclofos-methyl              2 mg/kg (Codex)                Basilex

* Products approved for use on lettuce but without a currently set MRL.
# refers to generic MRL for dithiocarbamates.

Definition of terms

MRL - Maximum Residue Level. This is a statutory limit to provide a check that
products have been used as directed - a reflection of good agricultural practice (GAP). The
units are expressed as milligrams of residue per kilogram of food (mg/kg). They are not
safety limits, but take account of consumer safety and are set at levels that ensure normal
dietary intake of the food product presents no risk to health. It is an offence to offer for sale
any produce where the MRL is exceeded.

ADI - Acceptable Daily Intake. This is the quantity of a pesticide which can be
consumed every day for a lifetime, in the practical certainty, on the basis of known facts, that
no harm will result.

DTC - Dithiocarbamate fungicide (i.e. containing the active ingredients mancozeb, thiram
or zineb) As well as products containing a single active ingredient which is a
dithiocarbamate, certain products such as Fubol Gold WG and Favour 600 SC contain a
mixture of metalaxyl and a dithiocarbamate (mancozeb and thiram respectively). Use of
DTCs on glasshouse lettuce is restricted after planting out (see page 6).

GAP - Good Agricultural Practice. There does not appear to be a widely accepted
definition of GAP but the authors of this publication define it as optimum commercial
production achieved with minimum use of resources and with due regard for the protection
of human health and the environment. Codes of Good Agricultural Practice for the
Protection of Soil, Air and Water, and a Code of Practice for the Safe Use of Pesticides on
Farms and Holdings (Green Code), are available free of charge from DEFRA Publications,
tel: 08459 556000. Quote references: Soil Code PB 0617, Air Code PB 0618, Water
Code PB 0585, Pesticides PB 3528.

Websites and contact points

•   Current EU/UK MRLs can be accessed at:

•   Current Codex MRLs can be accessed at: _q-
    e.htm (note the underscore _ between pest and q)

•   Latest information on product revocations:
    (note the underscore _ between raid and info)

•   For a copy of SOLA approval documents: or contact ADAS Horticulture (Tel:
    01954 268205)

•   A leaflet "Working within the Residue Limits" has been prepared by the Crop Protection
    Association and can be ordered by visiting:

•   HDC levy payers can obtain a detailed Factsheet (07/01: Disease Control in Protected
    Lettuce) and relevant SOLA documents: email (tel: 01732 848383).

•   Pesticide Safety Directorate (PSD), for information on product approvals: Tel 01904
    455775, or email


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