Fine Characterization of a Resistance Phenotype by Analyzing TuYV- Myzus persicae-Rapeseed Interactions

Abstract

Turnip yellows virus (TuYV), transmitted by Myzus persicae, can be controlled in rapeseed fields by insecticide treatments. However, the recent ban of the neonicotinoids together with the description of pyrethrinoid-resistant aphids has weakened insecticide-based control methods available to farmers. Since the deployment of insecticides in the 1980s, few research efforts were made to breed for rapeseed cultivars resistant to aphid-borne viral diseases. Thus, only few rapeseed cultivars released in Europe were reported to be TuYV-resistant, and the resistance phenotype of these cultivars was poorly characterized. In this study, several epidemiological parameters (infection rate, latency period, etc.) associated to the TuYV-resistance of the cv. Architect were estimated. Results showed a partial resistance phenotype for plants inoculated at the 2-/4-leaves stages and a resistance phenotype for plants inoculated at a more advanced growing stage. Moreover, analysis of infected plants highlighted (i) a poor quality of infected cv. Architect as a source of virus for transmission and (ii) an extended latency period for infected plants. Thus, dynamics of virus spread in the field should to be slower for Architect compared to susceptible rapeseed cultivars, which should lead to the maintenance of a higher proportion of healthy plants in the field.

Keywords: aphid; rapeseed; resistance; transmission; turnip yellows virus; viral disease.

Figure 1

Infection rates of rapeseed genotypes (A) and virus load in infected plants (B) according to the development stage of plants at inoculation. Series of 20 plants at different development stages were inoculated with viruliferous (Turnip yellows virus, TuYV) aphids (2 aphids per plant) for 2 h inoculation access period. Three weeks after inoculation, the presence of TuYV in plants was tested by ELISA in the presence of standards corresponding to serially diluted fractions of purified TuYV-PS isolate. The positive threshold of the test was twice the OD₄₀₅ value of healthy plant controls with a minimum value of OD₄₀₅ = 0.1. The average infection rates (A) and the viral load in 100 µL of crude sap from infected plants (B) are presented. The experimental procedure was repeated at least twice for each genotype/developmental stage combination. The vertical bars represent the standard deviations associated with the data. At 4-leaves stage, only one out of the inoculated Architect plants was infected.

Figure 2

Number of aphids produced from a single L₁ larvae of Mp34. Aphid larva L₁ obtained from synchronized Mp34 population were placed on rapeseed plants in order to measure the size of the population produced in a 9 days period. The average population sizes obtained are presented. The experimental procedure, carried out on a series of 20 plants per genotype, was repeated 3 times. The vertical bars represent the standard deviations associated with the data.

Figure 3

Transmission efficiency associated to source plants infected by TuYV for 3 to 21 days. Series of 8 plants (cvs. Architect, Quizz, and DK Exception) infected for 3, 7, 10, 14, and 21 days were used as sources for aphid-mediated transmission of TuYV-PS to susceptible cv. DK Exception test plants. Each source plant was used to inoculate ten test plants. Three weeks after inoculation, the presence of TuYV in test plants was tested by ELISA. Evolution of the quality of plants as a source for virus transmission are illustrated for each tested genotype. Linear regressions obtained from data from source plants infected for 3 to 14 days (cvs. Architect and Quizz) and for 3 to 21 days (cv. DK Exception) are drawn. The durations (expressed in days) of infection of source plants required to obtain 50% of the maximum infection rate are presented for each genotype by black triangles. The experimental procedure was repeated 3 times for each genotype. The vertical bars represent the standard deviations associated with the data.