New research is showing some practical tips
for countering the biggest pest threat to the
potato crop without using pesticides. Adam
Clarke reports from the fields of Shropshire
Incorporating cover crops can give good non-
chemical control of the potato crop's most
serious pest with 60-85% reduction seen in
potato cyst nematodes (PCNs).
Research focused on growing a number of
brassica species ahead of potato planting has
shown a reduction in PCN numbers.
Brassica cover crops such as Indian mustard,
when macerated and ploughed in, decompose
and produce a biofumigation effect.
Those used to produce this biofumigation effect
have been selected for their high levels of
glucosinolates. When combined with a certain
enzyme and water in the soil, glucosinolates
produce substances toxic to nematodes known
as isothiocyanates (ITCs).
Work at Harper Adams University – in
conjunction with Agrovista and Barworth
Agriculture – has worked on which brassica
species give the best level of ITCs for PCN
suppression.
"Brassicas usually contain six or more
glucosinolates that produce a range of ITCs and
modern biofumigation crops are bred for the
highest levels possible," says Matthew Back, the
university's PCN expert.
Dr Back and his team focused on Indian
mustard and also oil radish, white mustard and
rocket, all brassica species that contain high
levels of glucosinolates.
"Plants are naturally high in glucosinolates, but
they are completely inert until cells are
damaged, when they combine with the enzyme
myrosinase in the presence of moisture to
produce volatile substances called
isothiocyanates," he explains.
"It looks like we need to focus our
attention on summer-established Indian
mustard and oil radish for PCN control."
Matthew Back, Harper Adams University
It is that substance that provides the biocide
effect that suppresses PCN, but can also control
potato skin blemishes such as rhizoctonia and
black dot, which reduce tuber quality and
saleable yield.
The project research shows that the Indian
mustard has displayed the greatest efficacy
against PCN, with its high proportion of the
most potent glucosinolate – sinigrin.
Dr Back says this may not be suited to all
situations as the mustards are susceptible to
clubroot and verticillium wilt, so may be more
of a hindrance in close rotations of oilseed rape
or other brassica crops.
"Radish thrives in the summer, but also
tolerates overwintering and isn't susceptible to
clubroot, so could be the better option in those
situations," he says.
He also points out that white mustard contains
more of the enzyme myrosinase, so has the
potential to be used in mixtures to increase the
enzyme activity when crops are incorporated.
Rocket has shown that it has winter hardiness,
but is the least effective for PCN control and has
moved further down the pecking order when
considering a cover crop mix.
"It looks like we need to focus our attention on
summer-established Indian mustard and oil
radish for PCN control," says Dr Back.
But perhaps the most exciting discovery from
the project, conducted by PhD student Bruno
Ngala, was evidence that the viability of
nematode eggs within the characteristic cysts
that lay dormant in the soil is decreased
without the maceration and incorporation.
Dr Back explains that glucosinalates can be
leached from roots, but the enzyme myrosinase
cannot, and his theory is that micro-organisms
in the soil are producing myrosinase to use
glucosinolates as a food source.
Although only a theory, it could mean that
growers could get a level of control from
merely growing the crop.
"Our own work has thrown up some good
results, but we need to take it forward and
further new projects looking into
biofumigation have begun this year to reflect
that," says Dr Back.
A three-year project, funded by the Potato
Council, will investigate the suitability of
species around Great Britain, incorporation
methods and cover crop nutrition.
The Potato Council research and development
manager Sue Cowgill says that the trials will
take place in Shropshire, eastern England and
Scotland.
"We need to understand which cover crops
perform best in which region and how their
performance is influenced by crop agronomy
and incorporation techniques," she says.
Additional, more fundamental work funded by
the horticulture and potato initiative (HAPI)
will kick off in the new year, looking into the
impact of those factors on glucosinalates and
other chemicals that the cover crops produce.
Distributor and adviser Frontier is also
investigating incorporation efficiency.
"It is no silver bullet for PCN control and there
are still questions to be answered, but it's
certainly going to give us a benefit as part of an
integrated strategy," adds Dr Back.
An integrated approach for sustainable
production
For one Shropshire grower it was either get on
top of the farm's nematode population, or park
up the potato kit for good and cover crops have
helped slash PCN levels.
Neil Furniss of M E Furniss and Sons, based in
Caynton, near Newport, grows about 50ha of
Marfona and Maris Piper potatoes for the pre-
pack market on his 410ha of sandy loam soils,
so skin finish is paramount.
The farm has been growing potatoes for 40
years and as PCN populations have built up,
yield and quality was being compromised from
nematode feeding.
"It reached the stage in 2003 when we had to do
something about it or the farm would cease to
grow the crop," says Mr Furniss.
The farm has invested heavily in expensive
chemical fumigation with Telone (1,3-
dichloropropene) before it was banned and
now metam-sodium, which is carried out by
specialist contractors.
Mr Furniss also uses broadcast and
incorporated applications of Vydate (oxamyl)
ahead of planting to ensure that there isn't any
multiplication in the nematode population after
fumigation.
Having moved from a one-in-four to a one-in-
six rotation, the farm has also been using
biofumigation cover crops for four years, with
advice of local specialist PCN agronomist
Andrew Wade.
"PCN has needed an integrated management
approach on our farm using nematicides,
biofumigants and volunteer management
throughout the rotation," says Mr Furniss.
The cover crop mix is broadcast in July after
winter barley behind a Gregoire Besson
Discordon and rolled, followed by an
application of pig slurry from the farm's pig
unit.
The crop is then flailed, rotovated and ploughed
all within an hour to ensure maximum efficacy
and Mr Furniss prefers to do it at mid- to late-
flowering, which is normally early November.
"I don't want to make a mess if it's too wet, so
we would wait until the spring if conditions
turned against us."
Mr Wade blends the seed mix, which includes
5kg/ha of Caliente 199 mustard and 10kg/ha of
Bento fodder radish, both of which suppress
nematodes in the soil.
"Both are good for fumigation, with the Caliente
strong on PCN and the radish giving some extra
benefit on free living nematode (FLN) control
too," he says.
In addition to nematode suppression, Mr
Furniss is also seeing great benefits for soil
quality and nutrition from the "green manure"
effect.
"The crop is sucking up nitrogen in the autumn
and that is being released in the following
spring for the potatoes to utilise and we have
seen visual differences in the crop," explains
Mr Furniss.
Unconventional way to smash growers'
biggest pest threat
Potato cyst nematode (PCN) is the potato
growers' biggest pest threat, costing the UK
industry in excess of £50m annually.
A dwindling conventional chemistry set and
lack of resistant varieties is only making control
of this pest more challenging.
With PCN present on two-thirds of potato
growing land in the UK, Dr Back says reliable
control alternatives need to be sought.
"It's very unlikely that conventional fumigant
metam-sodium will be re-registered when it's
reviewed after 2014.
"In addition, the hazardous nature of the two
nematicide options Nemathorin (fosthiazate)
and Vydate (oxamyl) leaves a big question mark
over those products too," he explains.
With limited chemistry to control the two
major species of PCN, Globadera pallida and
Globadera rostochiensis, resistant varieties
would offer a useful tool.
There are varieties that offer full resistance to G
rostochiensis, which stops the multiplication of
nematode populations within the soil.
However, there is only partial resistance to the
more widespread and aggressive G pallida.
On a global scale, biofumigation by
incorporating cover crops attracted interest
when the fumigant methyl bromide was
banned in the wake of the 1987 Montreal
Protocol – designed to protect the ozone from
further damage.
But much of the early research and advice on
using cover crops to control soil pathogens
came from North America, Australia and Italy
and was not relevant to UK conditions.
"Seed companies came over making claims
about these crops and when they didn't work
here, the UK industry lost confidence," says Dr
Back.
"We need to cut through the snake oil and
produce some advice for potato growers on
how to make them work for PCN control in our
climate and rotations."
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