Agriculture Reference
In-Depth Information
dicotyledenous plant that also attacks other
species. It appears that this hemp parasite is
the same species that parasitizes tomato and
melon, as well as rape and sunflower. A number
of other non-cultivated plants also host broom-
rape, although few further details are available.
Ongoing work at the Universities of Nantes
and Jussieu hopes to provide further informa-
tion over the coming years.
OTHER PESTS
.
Hemp is usually little affected by
insects. Various studies have shown the effect
of cannabinoids as insect repellents. Some
attacks by larvae of the insect genus
Tipula
are
seen occasionally. These larvae destroy plants
by attacking the base of the stem. In 1993,
France witnessed a spectacular invasion of lar-
vae of the cabbage moth (
Mamestra brassicae
),
with almost three-quarters of fields requiring
insecticide treatment. While the apparent dam-
age appeared impressive at the end of May and
the beginning of June, there was no significant
difference in the yield between those fields that
were and were not treated. Some attacks on
the cotyledons by insects of the genus
Psylliodes
(flea beetles) have also been reported. Finally,
the Eurasian corn borer (
Ostrinia nubialisa
)
also finds refuge among hemp without provok-
ing any significant damage.
Game can sometimes cause damage, as is
the case for large populations of rabbits living
close to hemp fields and using these as a prom-
ising food source. Wild boar can also cause cer-
tain problems for cultivators at the start of the
vegetation phase or just before the hurd bundles
are collected, when their foraging for earth-
worms and insect larvae can produce holes that
interfere with the harvesting process.
Seeds can be treated with bird repellant to
help reduce the attractiveness of hemp seed.
The seeds can prove just as attractive at har-
vesting time, when clouds of starlings (
Sturnus
vulgaris
) can cause heavy losses (as in the
Rhine valley in 2003) through the consump-
tion and shelling of hemp seed a few days
before it is due to be harvested.
Slugs can provoke some losses. While it
may be worth being particularly vigilant on the
edge of woodland and beside paths, it is rarely
necessary to intervene, as the loss of a few
plants is unlikely to impact on the final yield.
The management of risks is similar to that
required for other crops.
DISEASES
.
Breeding over the centuries has
never led to the commercialization of a pure
line variety. The varieties supplied to farmers
are heterogeneous populations in terms of
their resistance to pathogens. Consequently,
only a small fraction of plants are actually
affected by a health threat. Thus, each year,
a variable percentage of plants die during the
vegetation phase, forming 'dead roots'. Roots
suffering from a range of diseases can be
seen, including
Botrytis cinerea, Sclerotinia
sclerotinium
and
Rhynchosporium secalis
.
Other pathogenic agents are probably also
present, although their effect on yield is
negligible.
Although a number of plant diseases have
been reported on hemp, major disease out-
breaks are uncommon. The most important dis-
ease of hemp is grey mould, caused by the
fungus
B. cinerea
. This disease attacks hemp
stems under conditions of cool to moderate
temperature and high humidity. In severe cases,
B. cinerea
can destroy a
Cannabis
crop com-
pletely within a week. Hemp canker, a fungal
disease caused by
S. sclerotiorum
(Lib.) Mass.,
attacks hemp stems, resulting in wilt and stem
breakage. Under cool, wet growing conditions
in the Netherlands, severe hemp yield reduc-
tions have been reported recently due to
B. cin-
erea
and
S. sclerotiorum
. Fungicide applications
every 10-14 days were required to keep the
crop disease free. Seed and soilborne diseases
such as
Pythium
and
Fusarium
are commonly
controlled by seed treatment with fungicides
prior to planting (Dempsey, 1975). Many other
minor diseases have also been reported on
hemp.
Further information can be found on the
Oregon State University website (
http://
Climatic accidents
HAIL
.
A distinction must be made between
losses during the vegetative phase (before flow-
ering) and those sustained after flowering.
Spectacular as they may be, losses sus-
tained during the growth period will impact
negatively on hurd production, whereas seed
