Inoculum production of Phytophthora medicaginis can be used to screen for partial resistance in chickpea genotypes

Abstract

Phytophthora root rot caused by Phytophthora medicaginis is an important disease of chickpeas (Cicer arietinum) in Australia with limited management options, increasing reliance on breeding for improved levels of genetic resistance. Resistance based on chickpea-Cicer echinospermum crosses is partial with a quantitative genetic basis provided by C. echinospermum and some disease tolerance traits originating from C. arietinum germplasm. Partial resistance is hypothesised to reduce pathogen proliferation, while tolerant germplasm may contribute some fitness traits, such as an ability to maintain yield despite pathogen proliferation. To test these hypotheses, we used P. medicaginis DNA concentrations in the soil as a parameter for pathogen proliferation and disease assessments on lines of two recombinant inbred populations of chickpea-C. echinospermum crosses to compare the reactions of selected recombinant inbred lines and parents. Our results showed reduced inoculum production in a C. echinospermum backcross parent relative to the C. arietinum variety Yorker. Recombinant inbred lines with consistently low levels of foliage symptoms had significantly lower levels of soil inoculum compared to lines with high levels of visible foliage symptoms. In a separate experiment, a set of superior recombinant inbred lines with consistently low levels of foliage symptoms was tested for soil inoculum reactions relative to control normalised yield loss. The in-crop P. medicaginis soil inoculum concentrations across genotypes were significantly and positively related to yield loss, indicating a partial resistance-tolerance spectrum. Disease incidence and the rankings for in-crop soil inoculum were correlated strongly to yield loss. These results indicate that soil inoculum reactions may be useful to identify genotypes with high levels of partial resistance.

Keywords: pathogen proliferation; phenotyping; quantitative resistance; root disease; tolerance.

Figure 1

Results for two recombinant line (RIL) populations, (A) Cicer echinospermum backcross*susceptible (BC*susceptible) and (B) C. echinospermum backcross*tolerant (BC*tolerant), in two Phytophthora medicaginis inoculated single-row experiments, for proportions of plants dead at the final assessment with a development status categorised as dead seedlings (DS, □) or dead podded plants (DP, +). Median values for each category presented and symbols in grey to the right of each population plot are the six selected high disease RIL that were postharvest soil sampled.

Figure 2

Results for selected recombinant inbred lines (RILs) in two Phytophthora medicaginis inoculated single-row experiments, with six low disease and six high disease lines from (A) Cicer echinospermum backcross*susceptible (BC*susceptible) and (B) C. echinospermum backcross*tolerant (BC*tolerant) populations, with the proportion of plants with foliage symptoms (chlorotic plus dead) plotted against the log-transformed postharvest soil P. medicaginis (P.med) DNA concentrations (number of sequence copies/g soil) for individual RIL and high and low disease group averages. Error bar shows LSD (p < 0.05) for disease group analysis. LSD, least significant difference.

Figure 3

Relationships for eight superior recombinant inbred lines (RIL) from two populations (Cicer echinospermum backcross*susceptible (D09008) and C. echinospermum backcross*tolerant (D09024)) and two parents for plots of (A) back-transformed proportion of symptomatic plants vs. normalised yield loss, (B) normalised height reduction vs. normalised yield loss, (C) area of early senescence vs. normalised yield loss, (D) normalised height reduction vs. area of early senescence, (E) in-crop Phytophthora medicaginis (Pm) DNA concentrations vs. normalised yield loss, and (F) postharvest P. medicaginis DNA concentrations vs. normalised yield loss. Fitted regression lines and equations are presented. For (E, F), one RIL was excluded (Ex.) from each regression.