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Mid-summer potato field: growers are looking for ways to reduce pesticide use. Photo: BPC |
Growers asked to use less sulphuric acid
Growers for one of the UK's leading potato companies
are being encouraged to switch from sulphuric acid to alternatives. Geest QV,
which supplies Marks and Spencer, has been monitoring chemical use on its potato
crops with a view to finding alternatives and success has been found for the
chemicals including sulphuric acid and aldicarb.
Bruce Coulson, an agronomist for Geest QV, explained that
Marks and Spencer had put sulphuric acid on an amber list, which meant that
growers were actively discouraged from using it as a desiccant in potato
production. ‘I will be asking growers how many of their fields had sulphuric
acid last year,’ Mr Coulson explained. ‘I will talk to them and find out if,
for example, 70% of the fields were involved, perhaps that could be brought down
to 30% this season.’
Sulphuric acid is still the most widely-used desiccant on
potatoes. It is a hazardous pesticide to use, warning notices must be given to
owners of neighbouring land and unprotected people must be kept out of treated
areas for at least 96 hours after treatment. ‘Diquat (Reglone), glufosinate
ammonium (Harvest) and carfentrazone-ethyl (Spotlight 24 EC) are also used by
growers as dessicants. They are used in much smaller amounts than sulphuric
acid.’ At least 220 litres of sulphuric acid are used on each hectare. By
comparison, said Mr Coulson, only about four litres of diquat are used per
hectare.
Regular crop monitoring
As well as sulphuric acid, Geest QV is encouraging
growers to monitor their blight spray programmes more carefully by using weather
stations. ‘During days when temperatures and humidity are high – ideal
blight conditions – we would recommend growers to spray more frequently at
seven to ten day intervals,’ said Mr Coulson. ‘However, when the conditions
are warm and dry, they could extend the blight spray intervals to 14 days.’
Growers are also asked to monitor the crop regularly so that
they use the appropriate blight sprays for the growth stage of the plant. For
example, when the plant is growing rapidly at the beginning of the season,
systemic fungicides are recommended. When the crop is at full canopy, contact
fungicides are more appropriate.
‘On average, blight spraying costs between £20 and £40 a
hectare per spray,’ said Mr Coulson. ‘By using these methods, growers have
the opportunity to reduce blight sprays by between 10% and 20% over a season.’
All Geest QV's new potatoes are sold fresh, so there is no
need for storage. But UK main crop potatoes are stored from September and
October for up to six months. Geest QV main crop growers store these potatoes at
low temperatures to prevent them from sprouting and avoid using chemical sprout
suppressants.
Growing without aldicarb
Even though potato plantings are sensitive to market
demand and can fall dramatically, pesticide use can increase. For example, Crop
Protection Association data for the 2000/2001 crop year showed that plantings
fell by 23% on the previous year to 181,000 hectares. Despite this, overall
herbicide usage rose by 14% and blight spray usage went up by 16% compared with
1999/2000.
High pest pressures and the need for several applications of
insecticides and molluscicides meant the area treated rose by 37% to 422,000
hectares. Some of this usage will have been aldicarb, one of the most
controversial chemicals in agriculture.
But Bruce Coulson maintained that most potato growers could
produce healthy crops without applying aldicarb. Among its uses as a nematicide,
aldicarb controls free living nematodes which harbour the tobacco rattle virus
causing spraing, seen as red or brown corky lesions in the flesh of the potato
tuber when it is cut. ‘Aldicarb still has a use on its label for spraing
control in potatoes,’ Mr Coulson said.
‘All the other nematicides have recommendations to control
potato cyst nematode (PCN), but not the free living nematode. So this is one
area where aldicarb still has the recommendation on the label but the others do
not.’ But Mr Coulson explained that as very few of the Geest QV main crop
pre-pack potatoes were grown on sandy soils where the free living nematode is
prevalent, most growers in this situation could avoid using aldicarb.
For potato cyst nematode – one of the crop’s main pests
– growers could find alternatives. While these remain largely chemically
based, the adoption of integrated control measures, including the use of
resistant varieties and rotation, can help to reduce chemical dependence.
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Mike Storey of the British Potato Council. Photo: BPC |
Industry body explains the challenges
While the British Potato Council (BPC) – which
represents the industry – accepts there are alternatives to aldicarb, it says
there is a view that the chemical controls PCN better on some soil types.
‘That view is held by chemical company advisers based on their experience,’
said Dr Mike Storey, the BPC's director of research and development.
Dr Storey explained there were two species of PCN – one
with a golden hue, Globodera rostochiensis; and another with a whitish
colour, Globodera pallada. Of the potato varieties grown commercially in
the UK, Maris Piper – the most popular with consumers – has resistance to
the golden nematode populations, so the past cropping patterns on farms have
self selected for the white species. The increased reliance on Maris Piper has
therefore led to an increase in the white cyst nematode.
Potato varieties lack nematode resistance
‘We have no commercially available varieties with
full resistance to the white cyst nematode,’ said Dr Storey. ‘So we rely on
those varieties with partial resistance combined with crop rotations and
chemicals. Aldicarb is one of the main chemicals in the armoury which growers
rely on for controlling both species, particularly the white.’ A fumigant
called Telone (1,3-dichloropropene) is also available, but this has to be
injected and some soil types are unsuitable.
In 2002, about 27% of the potato crop was treated with
nematicides including aldicarb, said Dr Storey. During the 1997/98 crop year, a
survey found that 64% of 484 sites in England and Wales had PCN: in the same
period, the industry suffered £43 million in terms of yield lost to PCN.
What makes the PCN so damaging is its persistence: the female
thrives by extracting nutrients from the developing potato plant. She dies along
with the plant and her body forms a pin-head sized leathery cyst containing
between 200 and 300 eggs which usually persist in the soil for between ten and
15 years. When a new potato is planted, chemicals in the roots stimulate the
eggs and the cycle begins again. ‘Some of the eggs die off naturally,’ said
Dr Storey. ‘But in a rotation of between four and seven years, the population
will not have declined sufficiently for some to hatch and start moving towards
the crop.’
Aldicarb is a microgranule incorporated into the ground when
seed potatoes are planted through March and April. It is applied by trained
operators using special granule applicators mounted on the back of a tractor.
‘Growers will make an informed judgement about using any chemical based on the
number of nematodes in their field samples, the yield they want from the crop
and their customers’ requirements,’ Dr Storey added.
With farmers facing increasing cost pressures, and consumers
wanting food without pesticide residues, these initiatives to find safer
alternatives to the most hazardous pesticides used in potatoes are rising to the
challenge of pesticide reduction. Farmers can only take further steps when the
tools for pesticide reduction are available to them.
[This article first appeared in Pesticides News No. 64, June 2004, page 4-5]