6
The effect of fibre hemp (Cannabis sativa L.) on selected soil-borne pathogens
C.J. Kok, G.C.M. Coenen and A. de Heij
Research Institute for Plant Protection (IPO-DLO), P.O. Box 9060, NL 6700 GW Wageningen, The Netherlands
Kok, C.J., G.C.M.Coenen and A de
Heij, 1994. The effect of fibre hemp (Cannabis sativa L.) on selected
soil-borne pathogens. Journal of the International Hemp Association 1: 6-9.
The effect of the cultivation of fibre hemp
(Cannabis sativa L.), cv Kompolti Hibrid TC, on the population dynamics of the soil-borne
pathogens Verticillium dahliae (Fungi), Meloidogyne chitwoodi and Pratylenchus
penetrans (Nematoda) was studied in greenhouse and field experiments. Fibre hemp
suppressed the population of M. chitwoodi strongly. A reduction in population
of V. dahliae was also noted. Removal of stubble of hemp from soil after harvest
was found to inhibit formation of survival structures (microsclerotia) of V. dahliae.
Introduction
Dutch agriculture faces an
increasing problem with soil-borne pathogens (including nematodes) due to intensive
growing of a limited number of crops in short rotations. Government plans to reduce
the use of soil disinfectants by 50 % by the next century may well aggravate the
situation. A possible solution to the problem is the introduction of new crops in
the rotation.
It is generally assumed that long rotations are
less susceptible to soil-borne diseases than short rotations. However, this
assumption may not apply to soil-borne pathogens with a wide host range (polyphagous
pathogens). Several fungi (e.g. Verticillium dahliae and Rhizoctonia
solani) and nematodes (e.g. Pratylenchus spp. and Meloidogyne spp.) have
a very large number of host plants. Introduction of new crops of unknown host
status into a rotation may cause or aggravate problems with these pathogens.
Therefore, the population dynamics of important polyphagous pathogens on new crops must be
assessed before a safe introduction can be made.
Fibre hemp is one of the potential new crops
under investigation in the Netherlands. Even though hemp is an ancient crop, there
are few reports on soil-borne diseases in this crop and the general impression is that
cultivation of hemp poses few problems with plant diseases (Termorshuizen 1991).
Soil-borne diseases however often remain undetected due to the inconspicuous
symptoms. Furthermore, the absence of damage in a host plant does not mean that the
pathogen cannot multiply on the host. Therefore, the population dynamics of some
important soil-borne pathogens on hemp were investigated.
The pathogens selected in this study were the
fungus Verticillium dahliae and the nematodes Meloidogyne chitwoodi and Pratylenchus
penetrans. These pathogens are potentially very harmful to potato, which is the
major crop in arable farming in the Netherlands. V. dahliae is a major factor in
the early dying syndrome in potato, which can lead to severe yield reduction. This
pathogen has extremely persistent survival structures (microsclerotia), that can endure in
soil for over 12 years. Control of this pathogen is therefore very difficult (Heale
1988).
The Columbian root-knot nematode M.
chitwoodi has a wide host range including many representatives of the Gramineae,
which are commonly used in rotational programmes with potatoes (Santo et al. 1980).
Low population levels of M. chitwoodi already cause major crop losses of potato
because of reduced tuber quality and heavy infestations can substantially decrease
yields. Tuber infestation by M. chitwoodi causes necrotic areas and other
blemishes, rendering the tuber unsaleable (Viglierchio 1987).
P. penetrans, the root-lesion nematode,
has an extremely wide host range. This species can cause severe growth reduction and
yield loss in potato production (Mai et al. 1977, Santo 1989). Furthermore, P.
penetrans increases the effect of V. dahliae in the early dying syndrome of
potato (Kotcon et al. 1985, Rowe et al. 1985).
Materials and Methods
The investigations described in this
paper were carried out using the Hungarian fibre hemp cultivar Kompolti Hibrid TC.
This cultivar was chosen for its good agricultural properties and prior use as a standard
in a cluster of research projects on fibre hemp as an arable crop for paper production in
the Netherlands ("National Hemp Programme").
The effect of cultivation of fibre hemp on the
selected pathogens was assessed by comparing the population before sowing and after
harvest of the crop. M. chitwoodi and V. dahliae were studied in field and
greenhouse experiments. The reproduction of P. penetrans on hemp was studied
in the greenhouse only. The effect of fibre hemp on changes in population of the
pathogens was compared with the effect of crops with known host status.
The greenhouse experiment with V. dahliae
was carried out using a organic sandy soil to which microsclerotia of V. dahliae
were added (2,500/ml soil). In 400 ml pots, hemp, wheat and pea were grown (5 pots
per treatment). A control treatment contained no plants (fallow). Wheat and
pea are resistant and susceptible, respectively, to V. dahliae. Afterwards,
above-ground parts of the plants were removed and the population of V. dahliae was
estimated.
The population of V. dahliae was
assessed with a bioassay, using the potato as test plant. Rooted potato cuttings
were planted in 400 ml pots containing the soil to be tested. The pots were placed
on a greenhouse bench at 19 °C and watered sparingly. At two to three days
intervals, the number of leaves showing wilt symptoms was determined. The disease
progress was monitored until the first potato plants died (60 days after planting).
The cumulative disease score was calculated per plant and taken as the measure of the soil
population of V. dahliae.
Formation of microsclerotia on crop residues of
fibre hemp such as plant tops and stubble was studied in some detail. Since
microsclerotia were very rarely observed on plant parts that were vital (green) at
harvest, the effect of pulling stubble directly after harvest on the formation of
microsclerotia was determined. In a field infested with V. dahliae, plant
tops were collected and green stubble was removed from the soil after harvest. Three
weeks later, when the stubble had died, another batch of stubble was collected in the
field. On both occasions, the plant material was left to dry on top of moist soil in
a greenhouse, to simulate drying in the field. Once they were air-dried, the number
of stubbles containing microsclerotia was assessed.
The reproduction of M. chitwoodi on hemp
was studied in a greenhouse and in the field. In the greenhouse experiment
reproduction of the nematode was determined after 1 generation. Three week old hemp
seedlings were inoculated with 200, 400, 800, 1,600 or 3,200 nematodes (hatched
second-stage juveniles) in 400 ml pots in 5 replications. After 1 generation cycle
of the nematodes (7 weeks at 20 °C), the number of newly formed eggs on the roots was
determined by the method of Vrain (1977). The susceptibility of hemp was also tested
in the field. On a sandy soil heavily infested with M. chitwoodi (mean
population 1500 eggs+larvae per 100 ml soil) fibre hemp was compared to five other
crops. All crops were grown in 6 x 6 m plots. Before sowing and after harvest
of the plants, the centre of the plot (1 x 1.5 m) was sampled with an auger (100 random
samples, in total ± 3 litre soil per plot). The population of M. chitwoodi
was determined with an Oostenbrink elutriator ('s Jacob and van Bezooijen 1984). The
experiment was set up as a randomized block design with 4 replications.
The reproduction of P. penetrans on
fibre hemp and, for comparison, sugar beet (Beta vulgaris L.) or fiddleneck (Phacelia
tanacetifolia Benth.) was determined in a greenhouse experiment. P. tanacetifolia
and sugar beet are susceptible and resistant, respectively, to P. penetrans.
The test plants were grown in 400 ml pots containing potting mixture and coarse river sand
(1:2, on volume base). The plants were inoculated with a nematode suspension or with
nematode-infested oat root pieces. P. penetrans originated from a greenhouse
culture maintained on oats (courtesy of Mr. van Bezooijen, Agricultural University
Wageningen). Nematodes for inoculum preparation were extracted from oat roots in a
mist chamber ('s Jacob and van Bezooijen 1984). Nematodes were inoculated in
suspension (470 per plant) or in infected roots (210 per plant). The experiment was
stopped at senescence of the hemp plants, 2 months after inoculation. Nematodes were
extracted from the root system in a mist chamber and from the soil with an Oostenbrink
elutriator. The experiment was carried out in a completely randomized design on a
greenhouse bench at 20 °C. Per inoculum type, 5 replicates per plant species were
taken.
Results
-Verticillium dahliae
The effect of the plants tested on the
resulting disease index of the potato test plants is shown in Table 1. Fibre hemp
gave a disease index in the bioassay that was not significantly different from the
resistant host plant wheat and the fallow treatment. The susceptible host plant pea
caused a significantly higher disease index. This experiment was repeated with
similar results.
Table 1. Effect of plant species on the Verticillium dahliae disease index of potato test plants in a greenhouse. | ||
Plant species | Cumulative disease score1,2 |
|
None (fallow) Wheat (Triticum aestivum) Fibre hemp (Cannabis sativa L.) Pea (Pisum sativum) |
55 36 58 104 |
(a) (a) (a) (b) |
1 mean cumulative
number of leaves with wilt symptoms per potato test plant 2 means followed by a different letter are significantly different (LSD-test, p <0.005) |
In the field, no microsclerotia were observed on the plant tops. Stubble harvested before it became senescent showed a significantly reduced occurrence of microsclerotia of V. dahliae , compared to stubble which was senescent (Table 2, p <0.05, fisher's exact test).
Table 2. Occurrence of microsclerotia on green and senescent stubble of fibre hemp (Cannabis sativa L.) from a field infested with Verticillium dahliae. | |
Stubble | % stems with microsclerotia |
green senescent |
17 60 |
-Meloidogyne chitwoodi
M. chitwoodi showed a low reproduction
on hemp in the greenhouse, at all densities of inoculum (Table 3).
Table 3. Reproduction of Meloidogyne chitwoodi on fibre hemp, (Cannabis sativa) cv. Kompolti Hibrid TC at different levels of inoculum in a greenhouse experiment. | ||
Inoculum1 | Final population2 | |
200 400 800 1,600 3,200 |
0 5.6 2.0 0 13.2 |
|
1 second-stage
juvenile/plant 2 eggs/g root after 1 generation of the nematode |
In the field experiment, the reproduction of M. chitwoodi on hemp was so low, that the final population was much lower than the initial population. The effect of fibre hemp did not differ significantly from that of fallow field conditions (Table 4).
Table 4. The effect of several plant species on the population of Meloidogyne chitwoodi after harvest in a field experiment. | ||
Host plant | Population after harvest1,2 |
|
Fibre hemp (Cannabis sativa L.) Bean (Phaseolus vulgaris L.) Wheat (Triticum aestivum L.) Pea (Pisum sativum L.) Potato (Solanum tuberosum L.) Grass (Lolium multiflorum Lamk.) None (fallow) |
10 3 130 220 5,900 12,000 3 |
(a) (a) (b) (b) (c) (d) (a) |
1
number of larvae / 100 ml of soil 2 means followed by the a different letter are significantly different (LSD test after log transformation, p <0.05). In cooperation with plant protection service (PD), centre for agrobiological and soil fertility research (AB-DLO), and research station for arable farming and field production (PAGV). |
-Pratylenchus penetrans
The final population of P. penetrans (adults +
juveniles) is given in Table 5. Since there was no significant effect of the
inoculation method, the results are summarized per host plant.
Table 5. Effect of different host plants on the final population of Pratylenchus penetrans in a greenhouse experiment. | ||
Host plant | Total number of P. penetrans 1,2 |
|
Sugar beet (Beta Vulgaris) Fiddleneck (Phacelia tanacetifolia) Fibre hemp (Cannabis sativa L.) |
89 1,252 2,541 |
(a) (b) (b) |
1 sum of
nematodes in roots and soil 2 means followed by a different letter are significantly different (LSD-test after log transformation, p <0.05) |
Fibre hemp sustained a large population of P. penetrans, not statistically different from that of the susceptible host P. tanacetifolia. The resistant host sugar beet had a much lower final population.
Discussion
Fibre hemp (cv. Kompolti Hibrid TC)
was shown to be resistant to V. dahliae and M. chitwoodi, but susceptible to
P. penetrans, under experimental conditions. Furthermore, it is suggested
that pulling of the stubble of fibre hemp directly after harvest will reduce the formation
of new microsclerotia of V. dahliae. However, the experiments were conducted
under greenhouse conditions or in field trials at a single location and in one year.
This can make generalization of the results precarious. Inclusion of crops of known
host status in the experiments reduces this problem. In this way it is possible to
draw conclusions on the host status of fibre hemp by comparison with the other species.
Fibre hemp can possibly be used to suppress M.
chitwoodi. In the field experiment strong suppression was found and in the
laboratory experiment reduction of the population of M. chitwoodi was evident even
at very low inoculum levels. This is an important result, because of the wide host
range and the high damage potential of this nematode.
The population suppression of V. dahliae
by hemp was similar to that on wheat or in the fallow treatment. But above-ground
parts of fibre hemp may contribute to the population of V. dahliae, if the stubble
is allowed to die off naturally. Pulling the stubble inhibited the formation of
microsclerotia. This measure can further reduce the population of V. dahliae
after the cultivation of fibre hemp.
It seems that fibre hemp is susceptible to P.
penetrans. The results of the greenhouse experiment call for caution in
introducing fibre hemp in the crop rotation at sites where problems with P. penetrans
occur and susceptible crops are grown. However, since this conclusion is based on
greenhouse experiments only, further studies are necessary to assess the risks under field
conditions.
The host suitability of a large number of hemp
cultivars to the root-knot nematode Meloidogyne hapla was tested by De Meijer
(1993). This author found varying resistance levels, ranging from highly resistant
to moderately susceptible host quality. The standard cultivar, Kompolti Hibrid TC,
had an average reproduction rate of M. hapla, both in a laboratory and a field
trial. Similar results were obtained by Kok and Coenen (unpublished results).
However, since some sources of hemp germplasm showed very high levels of resistance
(De Meijer 1993), it should be possible to develop fibre cultivars with good yield
properties and a high resistance level to M. hapla.
Modern European fibre hemp cultivars are
thought to be relatively closely related. Landraces, ornamental cultivars and drug
varieties however seem to be more distantly related (De Meijer and van Soest 1992).
Therefore, generalization of the results to the latter types of hemp is precarious.
The results presented indicate that fibre hemp
reacts differently to various soil-borne diseases and that the notion that fibre hemp will
cause no problems with diseases in crop rotation is not generally justified.
Cultivation of fibre hemp is likely to reduce the population of M. chitwoodi and of
V. dahliae. Problems with P. penetrans however could be increased by
fibre hemp. The results presented here are, however, more of a qualitative than of a
quantitative nature. Long-term field studies are necessary to accurately assess the
quantitative impact of the cultivation of fibre hemp on soil-borne diseases.
References