ORGANIC farmers could soon be using their first broad-spectrum herbicide in
the shape of a fungus-derived protein that attacks broadleaved weeds.
The protein, taken from the soil fungus Fusarium oxysporum, kills
the leaves of many broadleaved weeds, but doesn’t damage crops such as corn,
says Bryan Bailey, a plant pathologist with the US Department of Agriculture in
Beltsville, Maryland. The protein does not kill the weeds outright but they are
stunted until new leaves grow back. “If you can delay the weed problem, the crop
will outgrow it and shade it out,” Bailey says.
Organic farmers already employ pesticides and fertilisers that are certified
as “organic”, but there are no herbicides cleared for use on their crops. They
have to rely on crop rotation or mechanical methods such as tilling to keep
weeds at bay.
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The protein, called Nep1, triggers a hypersensitivity reaction which plants
normally use to kill cells surrounding the site of an infection. But Nep1 sends
this reaction into overdrive, and the weeds kill off entire leaves. The finding
will appear in the next issue of Weed Science.
But not all plants are susceptible to the protein. Nep1 seems to mainly
target dicotyledons—plants such as dandelions that have two seed leaves.
Single-leafed monocotyledons such as corn and other grasses don’t appear to have
cell receptors for the protein and are immune to its effects.
Because Nep1 is soaked up through the leaves’ stomata, it must be combined
with a suitable surfactant so that it sticks to leaves. At present no
surfactants have been certified as organic, but Bailey says it should be
possible to make one. And although he says there is no reason to think the
protein might be toxic, it will have to be tested. Animals may be allergic to
it, for example. If such problems can be overcome, he expects the product to
reach the market in five years.
Rob Durgy of the Northeast Organic Farming Association of Connecticut says
that if researchers solve the surfactant problem, the herbicide would probably
be certified organic, in the same way that the bacteria-derived Bt pesticide
is.
But some organic farmers will resist using even a natural herbicide. “We
certainly discourage farmers from using inputs unless all else fails. We don’t
like to think of farmers solving their problems by buying something in a bag,”
says Anne Mendenhall, director of Demeter Association, an organic certifier
based in Aurora, New York, that advocates using completely natural methods in
farming.