Tim C. Thoden

Organic amendments and their influences on plant-parasitic and free-living nematodes: a promising method for nematode management?

The use of organic soil amendments, such as green manures, animal manures, composts or slurries, certainly has many advantageous aspects for soil quality and is suggested as a promising tool for the management of plant-parasitic nematodes. However, during a recent literature survey we also found numerous studies reporting an increase of plant-parasitic nematodes after the use of organic amendments. Therefore, we critically re-evaluated the usefulness of organic amendments for nematode management and suggest possible mechanisms for a stimulation of plant-parasitic nematodes, as well as mechanisms that might be causing a reduction of plant-parasitic nematodes. In addition, we also elucidate a possible mechanism that might be responsible for the observed overall positive effects of organic amendments on crop yields. It is likely that a significant part of this is, inter alia, due to the proliferation of non-pathogenic, free-living nematodes and their overall positive effects on soil microbial populations, organic matter decomposition, nutrient availability, plant morphology and ecosystem stability.

Effects of pyrrolizidine alkaloids on the performance of plant-parasitic and free-living nematodes.

BACKGROUND Chemical nematicides such as methyl bromide have for decades played a significant role in the management of plant-parasitic nematodes. Their application is problematic because of negative environmental impacts, and therefore methyl bromide was phased out in Europe in 2005. A possible alternative to synthetically derived nematicides is seen in the use of plants and/or their secondary metabolites. These plants could either be used as nematicidal green manure or as a source for nematicidal extracts. This study aimed to evaluate the effects of 1,2-dehydropyrrolizidine alkaloids (PAs), a group of secondary plant metabolites found in hundreds of plant species throughout the world, on the performance of plant-parasitic and free-living nematodes. RESULTS PAs induced nematicidal, ovicidal and repellent effects on different plant-parasitic and free-living nematodes. There was no conclusive ranking in toxicity for the different structural types of PAs tested. However, the effects were often more pronounced for the tertiary than for the oxidised form of PAs. Further, large differences were observed in the susceptibility of different nematode species to PAs. CONCLUSIONS PAs do affect several performance parameters and developmental stages of nematodes. Therefore, PA-producing plants such as species of Crotalaria, Ageratum or Senecio might be promising candidates for nematode management strategies. [Correction made here after initial online publication].

Pyrrolizidine alkaloids of Chromolaena odorata act as nematicidal agents and reduce infection of lettuce roots by Meloidogyne incognita

1,2-dehydropyrrolizidine alkaloids (PAs) represent a class of secondary plant compounds that are active in defence against herbivory. They are present in Chromolaena odorata, one of the most invasive weeds of Asia and Africa. In vitro studies demonstrate that pure PAs from C. odorata roots have nematicidal effects on the root-knot nematode Meloidogyne incognita, even at concentrations of 70-350 ppm. In vivo experiments show that mulch or aqueous crude extracts from C. odorata roots reduce the infection of lettuce by M. incognita. Thus, the use of PA-containing plants appears to be a valuable element for integrated nematode management.

Effects of plants containing pyrrolizidine alkaloids on the northern root-knot nematode Meloidogyne hapla

Abstract1,2-Dehydropyrrolizidine alkaloids (PAs), known to be nematotoxic in vitro, represent a class of secondary plant metabolites from hundreds of plant species worldwide. Pot experiments with the commercially available PA-containing plants Ageratum houstonianum, Borago officinalis, Senecio bicolor, and Symphytum officinalis demonstrate that Meloidogyne hapla is not per se repelled by these plants as all species were infested with nematodes. However, the development of M. hapla juveniles was completely suppressed on A. houstonianum and S. bicolor. Soil in which A. houstonianum and S. bicolor were cultivated and incorporated contained 200–400 times less nematodes than soil treated with Lycopersicon esculentum. Depending on their qualitative composition of PAs at least some of these plants thus appear to be valuable tools for integrated root-knot nematode management.

Plants producing pyrrolizidine alkaloids: sustainable tools for nematode management?

1,2-dehydropyrrolizidine ester alkaloids (pyrrolizidine alkaloids; PAs) are a class of secondary plant metabolites found in hundreds of plant species. In vitro, PAs are known to affect plant-parasitic nematodes. Thus, PA-producing plants might be used in nematode management. So far, however, Crotalaria is the only PA-producing plant that has been used for nematode control. Consequently, we reviewed the literature on other PA plants and their effects on nematodes. This included the analysis of in vitro experiments with PA plant extracts, host tests with PA plants, as well as experiments in which PA-producing plants were used as different types of soil amendments (mulch, in situ mulching, extracts). Our results clearly show that, in addition to Crotalaria, certain other PA-producing plant species have the potential to be used in sustainable nematode management. This is especially true for management of sedentary endoparasitic nematode species. Promising applications include the use of PA plant extracts and mulches, as well as the use of PA plants for in situ mulching.

A field study on the host status of different crops for Meloidogyne minor and its damage potential on potatoes.

For several years, a new species of root-knot nematode, Meloidogyne minor, has been reported from parts of The Netherlands, Belgium, UK and Ireland. So far, this species causes most problems on golf courses but has also been reported from a potato field in Zeijerveld (The Netherlands) where it caused strong growth reduction on potato plants, but no damage to potato tubers. As The Netherlands is a potato-producing country, field experiments were set up to evaluate the potential risks this species poses. We tested the host status of some common crops for M. minor under field conditions and, more importantly, also tested its potential to harm potato production in terms of quantity as well as quality. In a 2-year field experiment (2007-2009), the host status of potato (cv. Bartina), rye, annual ryegrass, sugar beet, and maize was tested in the first growing season. Afterwards, these plots were used to evaluate the damage potential of M. minor on two commonly cultivated potato cultivars (cvs Asterix and Markies). In general, only potato seemed to be a good host for this nematode species with a Pf∕Pi-ratio about 1.5. Reproduction was observed mostly on roots but also on tubers, which increases the spread of M. minor by seed potatoes. However, there was no reduction in potato production, neither in yield nor in tuber quality. No significant reproduction could be observed on the other plants (Pf∕Pi values close to zero). From these results one might conclude that this nematode will not become a major threat to European agriculture. However, care has to be taken as within additional glasshouse experiments potato tubers were susceptible for damage caused by M. minor. Thus, further studies on the general biology and ecology of M. minor are needed to make a better risk assessment on this new nematode pest.