Acquired resistance to systemic fungicides of Septoria nodorum and Cercosporella herpotrichoides in cereals

Since the introduction of the systemic fungicides in the sixties, the phenomenon of resistance to these compounds has become a serious problem in the control of fungal plant diseases. The aim of this study was to contribute to the knowledge about the consequences of fungicide applications in cereals with respect to the emergence of fungicide resistant strains of Septorianodorum , the causal agent of glume blotch of wheat, and of Cercosporellaherpotrichoides , the causal agent of eyespot of wheat, barley and rye.CHAPTER 3.An accurate and reproducible inoculation method was used to inoculate wheat seedlings with spore suspensions of S. nodorum . The effect of postinfectional applications of the systemic fungicides carbendazim and edifenphos with respect to the development of symptoms and the length of the latent period was studied. Both fungicides suppressed the development of symptoms and elongated the duration of the latent period, the extent to which depending on the concentration of each fungicide used.CHAPTER 4.It is suggested that alternate or combined use of two systemic fungicides with a different mode of action can reduce the enrichment of specific resistant forms in a fungal population. To verify these statements, model experiments were carried out under controlled conditions.During 9 passages on living wheat plants, inoculated with spore suspensions of S. nodorum , the fungicides carbendazim and edifenphos were applied either alone or as a mixture in each passage, or applied alternately from passage to passage. After 9 passages, disease control by either fungicide alone was reduced. This tendency was not so pronounced in that variant where the fungicides were applied alternately and nearly absent in that variant where a mixture was applied.Comparable results were obtained in experiments on agar media. A stable edifenphos resistant strain was selected in that variant, where S. nodorum in each passage was cultured on plates with edifenphos.With C. herpotrichoides , experiments were carried out on agar media only, using carbendazim and nuarimol. Here, a strong effect of the alternate exposure of the fungus to these two compounds was present. since the fungus was hardly able to grow on plates with a moderate dosis of carbendazim, after first being cultured on plates with nuarimol.CHAPTER 5.The effect of field applications of thiophanate methyl and edifenphos on the frequency of resistant spores in field isolates of S. nodorum was examined, during five vegetation periods between 1973 and 1977. Two types of carbendazim resistant strains were present in S. nodorum . Weakly carbendazim resistant strains were inhibited at 5 μg/ml carbendazim in vitro and occured at a frequency of about 1: 7x10 ⁶in populations from unsprayed plots. Highly carbendazim resistant strains occurred at a frequency of about 100 times that low and were hardly inhibited at 1000 μg/ml carbendazim in vitro . In each year, there was a significant increase in the frequency of weakly carbendazim resistant spores after thiophanate methyl applications in the field. but the absolute figures remained rather low. In no case, an isolate with more than 1:10 ⁴weakly resistant spores was found.There was no clear yearly increase in the frequency of highly carbendazim resistant spores after thiophanate methyl application.Edifenphos resistant spores were present in field isolates of S. nodorum . Their frequency- about 1:10 ⁷in isolates from unsprayed plots-, increased significantly after edifenphos application in the field, but the absolute figures remained rather low.During three years, field trials were carried out with C. herpotrichoides to examine the effect of carbendazim application in the field on the frequency of carbendazim resistant spores in field isolates. Their frequency in isolates from unsprayed plots was about 1: 7x10 ⁸. This frequency increased after carbendazim applications, but mostly on an insignificant scale only.CHAPTER 6.Fungicide sensitive and resistant strains of S. nodorum were compared with respect to their mycelial growth rate, their sporulation capacity and their pathogenicity on agar medium and/or living wheat plants.Weakly carbendazim resistant strains showed a significantly faster mycelial growth on agar media with carbendazim, than sensitive ones, but were almost completely inhibited at 5 μg/ml carbendazim. This growth reduction was accompanied
by an increase in the number of spores, produced per mm ²mycelium.Highly carbendazim resistant strains were able to grow and produce spores at 1000 μg/ml carbendazim in vitro . Resistant strians were present with a mycelial growth and spore production equal to that of sensitive ones. Pathogenicity tests with sensitive and highly carbendazim resistant strains on living wheat plants showed the same tendency, as far as the development of symptoms and sporeproduction is concerned.Edifenphos resistant strains of S. nodorum were compared with sensitive ones in tests on agar media and on wheat plants. Resistant strains were strongly inhibited at 200 μg/ml edifenphos in vitro . Edifenphos resistant strains were present with an equal fitness as sensitive ones, when grown in the absence of edifenphos, on agar media and on living wheat plants.The tests with C. herpotrichoides were performed with sensitive mother strains and the corresponding carbendazim. resistant daughter strains. These resistant strains were able to grow and produce spores even at 1000 μg/ml carbendazim in vitro . They produced more spores per mm ²mycelium when cultured in the presence of carbendazim than in its absence. The properties of carbendazim resistant strains in vitro and in vivo did not differ very much from those of sensitive strains, in the absence of the fungicide.CHAPTER 7.The competitive ability of resistant strains was studied by inoculating wheat plants with different mixtures of resistant and sensitive spores. Carbendazim resistant strains of S. nodorum and C. herpotrichoides could maintain themselves in the mixed population for three passages on wheat plants, even in the absence of any fungicide selection pressure.In case of a postinfectional fungicide application, only carbendazim resistant strains could be reisolated.Edifenphos resistant strains disappeared from the respective populations in the absence of the fungicide, due to a smaller relative reproduction ratio.