Effects of granular nematicides on the infection of potatoes by Rhizoctonia solani

T.W. Hofman

Research output: Thesisinternal PhD, WU

Abstract

The granular nematicides aldicarb, oxamyl and ethoprophos often are applied to control plant parasitic nematodes. However, the use of these pesticides may have some disadvantages. In field trials, they increased stem infection of potatoes caused by <u>Rhizoctonia solani</u> Kühn and incidence of black scurf (sclerotia of <u>R. solani</u> ) on tubers. This thesis, reports about possible mechanisms involved in the increased infection in nematicide treated fields. The effects of granular nematicides were studied on:<br/>- the pathogenicity of <u>R. solani</u> (Chapters 2 and 3)<br/>- the susceptibility of the host plant (Chapter 3)<br/>- the microbial antagonism to <u>R. solani</u> (Chapter 2)<br/>- the mycophagous soil fauna, i.e. nematodes (Chapters 5 and 7) or springtails and mites (Chapters 6 and 7)<br/>Ethoprophos was fungitoxic to <u>R. solani</u> . On Czapek Dox agar (CDA), the EC <sub><font size="-1">50</font></sub> of the compound was 49 mg l <sup><font size="-1">-1</font></SUP>and at 100 mg l <sup><font size="-1">-1</font></SUP>there was a total growth inhibition. Oxamyl showed a slight fungitoxicity. At 100 Mg 1-1 oxamyl in CDA, the growth of <u>R. solani</u> was reduced by 26 %. Aldicarb did not influence the growth of <u>R. solani</u> on CDA, but caused a slight growth stimulation on potato dextrose agar (PDA).<p>The carbamate compounds aldicarb and oxamyl are systemically transported in plants. The presence inside the plant might, therefore, have some effect on the host-plant resistance to <u>R. solani</u> . The organophosphorus compound ethoprophos is not known to have a systemic activity, so it differs from the other two nematicides with respect to the uptake by the host plant. It was therefore, less likely that the increased infection by <u>R. solani</u> in nematicide-treated soil, was due to a reduced host-plant resistance. Laboratory experiments supplied the evidence that the nematicides did not reduce host-plant resistance.<p>Ethoprophos caused an initial growth inhibition of potato sprouts, but immediately after their emergence, growth was promoted. In laboratory experiments carried out at 15 °C in a growth chamber, the emergence of sprouts was delayed by three days, when tubers were planted in ethoprophos-treated soil. However, most probably due to the fungitoxicity of ethoprophos, the appearance of lesions on the stems was delayed by nine days in this treatment. Oxamyl showed similar effects to ethoprophos, but to a lesser extent, while aldicarb did neither influence sprout development nor the appearance of lesions.<p>The stages in the infection process, including growth of runner hyphae, branching of hyphae on the stem surface, formation of infection cushions and the relationship between size of the infection cushions and size of lesions, were not influenced by any of the nematicides.<p>The previous observations made an indirect effect highly evident. In a study on the infection process was found that the size of lesions was proportional to the size of the infection cushions (dense masses of mycelium from which <u>R. solani</u> penetrates the plant). Any agent that changes the size of infection cushions will alter, therefore, the severity of the disease.<p>By far the most important mycoparasite of <u>R. solani</u> in Dutch potato fields is <u>Verticillium biguttatum</u> , so the study on effects on mycoparasites was focussed on this fungus. The growth of <u>V. biguttatum</u> was strongly stimulated by aldicarb and ethoprophos on potato dextrose agar plates covered with mycelium of <u>R. solani</u> . The mechanism involved was not studied. In the nematicide-treated fields, the incidence of <u>V. biguttatum</u> on stolons was increased. This may be due to an increased availability of substrate (i.e. mycelium of <u>R. solani</u> ) or a reduced activity of the mycophagous soil fauna, as well as from a direct stimulation by the nematicide. At the recommended dosages of aldicarb, oxamyl or ethoprophos, an effect on soil fungistasis was not found, neither in laboratory experiments nor in field experiments.<p>The laboratory experiments indicated that the most probable explanation for the increased disease severity of <u>R. solani</u> in nematicide-treated fields is a suppression of the mycophagous soil fauna. In the experimental fields, the most abundant mycophagous nematodes were <u>Aphelenchus avenae</u> and <u>Aphelenchoides</u> spp. These nematodes could be reared on petri-dishes covered with mycelium of <u>R. solani</u> . In the rhizosphere of plants of which the foliage was removed, numbers of <u>A. avenae</u> increased dramatically. Outside the rhizosphere, only very low numbers of <u>A. avenae</u> and <u>Aphelenchoides</u> spp. were found. It was assumed that the large increase was due to a rapid development of the microflora in the rhizosphere of dying roots. Aldicarb, oxamyl and ethoprophos greatly limited this increase.<p>In a field that had been fumigated with dichloropropene, the granular nematicides hardly affected stem infection and black scurf incidence. Initially, metabolites that may have been formed after fumigation could have had a toxic effect on <u>R. solani</u> . All soil animals were strongly reduced at the beginning of the growing season in the fumigated field. Later in the season, numbers of <u>A. avenae</u> were much higher in the rhizosphere of plants grown in fumigated soil than in non-fumigated soil. Although the granular nematicides reduced numbers of <u>A. avenae</u> also in the fumigated soil, the remaining population density of <u>A. avenae</u> was still high. In nematicide-treated plots, the disease suppression was, therefore, not much lower than in the untreated plots.<p>To simulate potato stems colonized by <u>R. solani</u> , flax straws colonized by the fungus were buried in the field. Mycophagous nematodes rapidly increased on the straws. In aldicarb-treated plots, only low numbers of mycophagous nematodes were observed on these straws. These data show that grazing of mycophagous nematodes on mycelium and probably also on infection cushions of <u>R. solani</u> will be less in nematicide-treated soil than in untreated soil.<p>In one field trial, the effect of the insecticide lindane on <u>R. solani</u> was assessed to discriminate between the role of mycophagous nematodes and mycophagous microarthropods. In lindane-treated plots, stem infection and black scurf incidence were increased to almost the same extent as in aldicarb-treated plots. Lindane did not have any nematicidal activity. This indicates that at least in this field microarthropods may also reduce <u>R. solani</u> by grazing. Aldicarb, ethoprophos and lindane reduced the numbers of most springtails and mites in potato fields. The most abundant springtails were <u>Tullbergia krausbaueri</u> , <u>Isotoma notabilis</u> and <u>Folsomia fimetaria</u> , and the most abundant mites were <u>Pygmephorus sellnicki</u> , <u>Pygmephorus blumentritti</u> , <u>Coccotydeus</u> sp. and <u>Histiosoma litorale</u> . Numbers of <u>Pygmephorus</u> spp. were not reduced in aldicarb- and ethoprophos-treated plots. <u>T. krausbaueri</u> was reduced in lindaneand ethoprophos-treated plots, but only slightly in aldicarb- treated plots.<p>In the laboratory, <u>T. krausbaueri</u> , <u>F. fimetaria</u> and <u>H. litorale</u> could be reared on petri-dishes covered with <u>R. solani</u> or cultures of <u>R. solani</u> in sterilized soil. <u>Pygmephorus</u> spp. and <u>Coccotydeus</u> sp. could not be reared on these cultures, while <u>I. notabilis</u> was not successfully isolated from the field. In laboratory experiments where the microarthropods were introduced in a soil inoculated with <u>R. solani</u> , <u>T. krausbaueri</u> and <u>F. fimetaria</u> caused a strong reduction of stem infection of potatoes. Lowest numbers of <u>T. krausbaueri</u> and <u>F. fimetaria</u> that significantly reduced disease severity were 9800 and 430 individuals per litre soil, respectively. H. litorale did not survive during the experiment, so it was probably not important in grazing on mycelium of <u>R. solani</u> . In similar inoculation experiments, <u>A. avenae</u> was also found to cause a significant reduction in stem infection when inoculated with 500 nematodes per litre. An <u>Aphelenchoides</u> sp. had no effect on stem infection. Nematodes reproduced within a few days at room temperature, while springtails and mites had a much slower reproduction cycle.<p><u>A. avenae</u> and <u>F. fimetaria</u> seem to be some of the most important mycophagous soil animals that lead to a reduction by <u>R. solani</u> in the field. A smaller contribution in the reduction of disease severity can be expected from <u>T. krausbaueri</u> and other mycophagous soil animals that are only present at low densities. However, some of the species that were not reared in the laboratory on mycelium of <u>R. solani</u> , may also be able to cause a significant reduction of disease severity at low population densities.<p>Any pesticide that reduces the density of mycophagous soil animals will, therefore, facilitate a better development of <u>R. solani</u> . The stimulatory effects of granular nematicides on <u>R. solani</u> on potato seem primarily caused by reduced grazing of the mycophagous soil fauna on the pathogen in nematicide-treated fields.<p><TT></TT>
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
Supervisors/Advisors
  • Dekker, J., Promotor, External person
  • van der Wal, A.F., Promotor
Award date4 Nov 1988
Place of PublicationAmsterdam
Publisher
Publication statusPublished - 1988

Keywords

  • bacteria
  • biological control
  • biological control agents
  • deuteromycotina
  • fungi
  • microbial pesticides
  • nematicides
  • plant pathogenic fungi
  • plant protection
  • potatoes
  • solanum tuberosum
  • viruses
  • moniliaceae

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