Mycorrhization Mitigates Disease Caused by " Candidatus Liberibacter solanacearum" in Tomato

Abstract

Disease caused by the bacterial pathogen "Candidatus Liberibacter solanacearum" (Lso) represents a serious threat to solanaceous crop production. Insecticide applications to control the psyllid vector, Bactericera cockerelli Šulc (Hemiptera: Triozidae) has led to the emergence of resistance in psyllids populations. Efforts to select natural resistant cultivars have been marginally successful and have been complicated by the presence of distinct Lso haplotypes (LsoA, LsoB) differing in symptoms severity on potato and tomato. A potentially promising management tool is to boost host resistance to the pathogen and/or the insect vector by promoting mycorrhization. Here we tested the hypothesis that mycorrhizal fungi can mitigate the effect of Lso infection on tomato plants. The presence of mycorrhizal fungi substantially delayed and reduced the incidence of Lso-induced symptoms on tomato as compared to non-mycorrhized plants. However, PCR with specific Lso primers revealed that mycorrhization did not prevent Lso transmission or translocation to newly formed leaves. Mycorrhization significantly reduced oviposition by psyllids harboring LsoA and survival of nymphs from these eggs. However, mycorrhization had no effect on oviposition by psyllids harboring LsoB or the survival of nymphs from parents harboring LsoB. These findings indicate the use of mycorrhizal fungi is a promising strategy for the mitigation of disease caused by both LsoA and LsoB and warrants additional field testing.

Keywords: Bactericera cockerelli; Candidatus Liberibacter solanacearum; Rhizophagus irregularis; mycorrhizae-induced resistance; tomato.

Figure 1

Influence of mycorrhization and Lso infection on tomato shoot dry weight (A), root dry weight (B), height (C), and number of leaves (D). NM: non-mycorrhized plants, RI: mycorrhized plants. Growth data are pooled across experiments. For shoot dry weight biomass there was a significant Lso treatment-by-mycorrhization treatment interaction effect, and bars marked with different letters are significantly different (p < 0.05) according to multiple comparisons using Student’s t test. For all other variables, there were significant mycorrhization treatment and Lso treatment main effects (p < 0.05). Multiple comparisons (p < 0.05) were used to test for differences among Lso treatments. Results were as follows: RI > NM, root dry weight: no-psyllid ≥ LsoA ≥ LsoB, plant height: no-psyllid = LsoA > LsoB, leaf number: no-psyllid > LsoA > LsoB.