Allelopathy in the Management of Plant-Parasitic Nematodes 1 J
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Journal of Nematology 28(1):8-14. 1996. © The Society of Nematologists 1996. Allelopathy in the Management of Plant-Parasitic Nematodes 1 J. M. HALBRENDT 2 Abstract: There are numerous reports of nematicidal chemicals in crude plant homogenates, leach- ates, and decomposing residues. These compounds are usually assumed to be secondary metabolites, which serve as chemical defenses against disease and parasites. When such compounds are released into the rhizosphere, they are known as allelochemicals. The possibility exists to exploit allelochem- icals for nematode control, and there have been many attempts to use this approach either by rotation, intercropping, or green manure treatments. Results have met with mixed success. Proof of allelochemical activity in field situations is difficult to obtain, but it is evident that some rotation crops are significantly better at reducing nematode populations than others. Rotations with non-host plants may simply deny the nematode population an adequate food source for reproduction (passive suppression), whereas allelopathic crops kill nematodes by the production of toxic compounds (ac- tive suppression). Progress toward sustainable agriculture should benefit from studies on allelopathic nematode control. However, grower acceptance of new plant-rotation strategies are based on eco- nomic and logistical considerations as well as efficacy. A potential practical application of allelopathic nematode control that involves using rapeseed as a green manure crop to reduce populations of Xiphinema americanum sensu lato in temperate orchards is presented. Key words: allelopathy, amendment, Brassica, glucosinolate, green manure, isothiocyanate, man- agement, nematode, rapeseed, rotation, thioglucosidase, Xiphinema. The term allelopathy was coined in 1937 Use of the term allelopathy is somewhat by Molisch to designate plant-plant and controversial. Various disciplines have ex- plant-microorganism biochemical interac- panded the meaning to include plant- tions. In a review of the subject, Rice stated insect and plant-higher animal biochemi- that Molisch intended the term to include cal interactions (32,39), and Lovett and both inhibitory and stimulatory interac- Ryuntyu (20) proposed that chemical com- tions. Rice (30) defined allelopathy as "any munications between plants and between direct or indirect, harmful or beneficial ef- plants and other organisms also be consid- fect by one plant (including microorgan- ered as examples of allelopathic interac- isms) on another through production of tion. Einhellig (11) suggested that allelop- chemical compounds that escape into the athy should be considered an umbrella environment." Many studies of allelopathy term to describe all of these interactions, have focused only on interactions in which including the chemical defenses of plants. one organism is detrimentally affected by Some authors have proposed that the term the association, and only infrequently has "chemical ecology" encompasses all bio- reference been made to possible beneficial chemical interactions between organisms or stimulatory interactions. In the fifth and that use of the term allelopathy should edition of 'The Dictionary of Scientific and be restricted to only plant-plant interac- Technical Terms' by McGraw-Hill (2), al- tions (27). However, through usage, the lelopathy is defined simply as "the harmful broad definition of allelopathy has become effect of one plant or microorganism on widely accepted. another owing to the release of secondary A major principle that separates allelop- metabolic products into the environment." athy from other types of interactions such as competition is that allelopathy is caused by allelochemicals. Allelochemicals are Received for publication 13 September 1995. plant metabolites or their products that are 1 Symposium paper presented at the Annual Meeting of released into the microenvironment. AI- the Society of Nematologists, 5-9 August 1995, Little Rock, AR. Paper number 2029 in the Department of Plant Pathol- lelopathic compounds are released ogy, The Pennsylvania State University. through volatilization, exudation from ~Associate Professor, Department of Plant Pathology, The Pennsylvania State University, P.O. Box 309, Biglerville, PA roots, leaching from plants or residues, 17307-0309. and decomposition of residues (11,27,39). Allelopathy in Nematode Management: Halbrendt 9 Allelopathic interactions have been rec- have since shown that various species and ognized for centuries, with one of the ear- cultivars of marigold can effectively con- liest recorded examples in agriculture dat- trol nematodes on agronomic crops when ing from 285 BC (28,30). However, it was grown in rotation, interplanted with the not until Molisch theorized that biochemi- crop, or used as a soil amendment (3,29). cal processes might explain these observa- Also, several nematicidal compounds have tions that the principles of allelopathy been isolated from marigold, with alpha- were first established. Only during the past terthienyl being one of the most potent 20 years has there been significant (17,37,38). progress in the study of allelopathic chem- Many crops and weeds have been evalu- istry resulting from new techniques which ated for chemical activity against nema- enable the isolation, identification, and todes. Results from these experiments re- quantitative determination of trace natural vealed that numerous species, represent- products in complex matrices such as soil. ing many plant families, produce It was not until such a firm foundation was nematicidal compounds. However, the ex- established that allelopathy became widely perimental design and sophistication of recognized as a scientific discipline (27,31, the techniques used in these studies varied 39). greatly. Only a few studies have provided unequivocal evidence of allelopathy, with ALLELOPATHY IN PLANT NEMATOLOGY most having provided only circumstantial evidence and needing to be substantiated Plant compounds elicit nematode behav- (3,17,28). iors such as attraction or repulsion from roots and, therefore, allelopathic research PROOF OF ALLELOPATHY may be considered a fundamental compo- Many claims of allelopathic activity have nent of nematological research. Because been based on results of bioassays using nematodes are recognized as economically plant homogenates. Merely showing that important pests, many studies have fo- substances extracted from the roots or cused on nematode suppressive crops as leaves of plants cause adverse effects on potential management tools in cropping nematodes in vitro does not provide un- systems. Such studies have shown that equivocal evidence that these compounds some crops "actively" reduce nematode are causing allelopathy under natural con- populations by the production of nemati- ditions. Plant homogenates contain various cidal compounds while others are simply intracellular metabolites, and it seems un- nonhosts (33). The prospect of exploiting likely that the chemicals extracted from a naturally occurring allelochemicals for plant are the same ones normally released nematode control has advantages over the into the soil. Such work should not be dis- current use of toxic chemicals, and there couraged, but many plant tissues contain have been many attempts to utilize this ap- chemicals with potential allelopathic activ- proach either by rotation, intercropping, ity and whether these same compounds or green manure treatments. are released into the rhizosphere in suffi- The most comprehensively studied ex- cient quantities and with enough persis- ample of allelopathy in nematology is that tence to affect nematodes remains a critical of marigold (Tagetes spp.). Marigold has question in many cases of alleged allelopa- been an important medicinal plant since thy (8). the first century, and often medicinal There is no specific protocol (similar to plants have been shown to have allelo- Koch's postulates for proof of disease) to pathic activity (28). Empirical observations establish proof of allelopathy. Neverthe- indicated that many species of marigold less, to establish a cause-and-effect rela- were resistant to a number of nematode tionship, the following events must occur species (35,36). Numerous experiments in sequence: 1) the allelochemical is pro- 10 Journal of Nematology, Volume 28, No. 1, March 1996 duced, 2) the chemical is transported need to develop safer and more ecologi- through the environment from the plant cally sound pest control strategies. At least to the target organism, and 3) the target two potential benefits may be obtained organism is exposed to the chemical in suf- from allelopathy research. One possibility ficient quantity and for sufficient time to is the discovery of plant compounds that have an effect. Many reports of allelo- may lead to new pesticide chemistry having pathic activity have provided evidence for greater efficacy, specificity, and(or) com- the first and third events, but the critical plete decomposition in soil. A second possi- link between production and exposure has bility is the identification of efficacious rota- only infrequently been established (7). tion or green manure crops which could be Soil is a dynamic and complicated phys- used to control nematodes using conven- ical, chemical, and biological medium, and tional or only slightly modified farming organic compounds released into it may be practices, thus reducing or eliminating the altered and transported by biological and need to apply nematicides. This latter pos- physical