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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

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.

        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).

        -Meloidogyne chitwoodi
        M. chitwoodi showed a low reproduction on hemp in the greenhouse, at all densities of inoculum (Table 3).

        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).

        -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.

        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

• 's Jacob , J.J. and van Bezooijen, J., 1984. A manual for practical work in nematology.  Agricultural University, Wageningen, the Netherlands, 77 pp.
• Heale, J.B., 1988.  Verticillium spp., the cause of vascular wilts in many species.  Adv. Plant Pathol. 6: 291-311.
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