Identification of Sources of Resistance to Aphanomyces Root Rot in Pisum

Abstract

Aphanomyces root rot (ARR), caused by Aphanomyces euteiches, is one of the most devastating diseases that affect the production of peas. Several control strategies such as crop rotation, biocontrol, and fungicides have been proposed, but none provides a complete solution. Therefore, the deployment of resistant cultivars is fundamental. ARR resistance breeding is hampered by the moderate levels of resistance identified so far. The available screening protocols require post-inoculation root assessment, which is destructive, time-consuming, and tedious. In an attempt to address these limitations, we developed a non-destructive screening protocol based on foliar symptoms and used it to identify new sources of resistance in a Pisum spp. germplasm collection. Accessions were root inoculated separately with two A. euteiches isolates, and leaf symptoms were assessed at 5, 10, 14, 17, and 20 days after inoculation (DAI). Although the majority of accessions exhibited high levels of susceptibility, thirty of them exhibited moderate resistance. These thirty accessions were selected for a second experiment, in which they were inoculated with both A. euteiches isolates at two inoculum doses. The objective of this second trial was to confirm the resistance of these accessions by evaluating root and biomass loss, as well as foliar symptoms, and to compare root and foliar evaluations. As a result, a high correlation (R² = 0.75) between foliar and root evaluations was observed, validating the foliar evaluation method. Notably, accessions from P.s. subsp. humile exhibited the lowest symptomatology across all evaluation methods, representing valuable genetic resources for breeding programs aimed at developing pea varieties resistant to ARR.

Keywords: Aphanomyces euteiches; aerial evaluation; pea; root disease.

Figure 1

Average Foliar Symptoms Index (FSI) and Standard Error (SE) of a 322 Pisum collection at 5, 10, 14, 17, and 20 days after inoculation (DAI). The genotypes were inoculated with a 10³ zoospores/mL solution of two A. euteiches isolates: Aph1 (blue) and Aph2 (orange). FSI was assessed on a scale of 0 (healthy plant) to 5 (dead plant). For each time, bars with different letters (a or b) show significant differences between the two isolates according to the LSD test (α ≤ 0.05).

Figure 2

Distribution of genotypes according to Foliar Symptoms Index (FSI) at 5, 10, 14, 17, and 20 days after inoculation (DAI) of a 322 Pisum collection with a 10³ zoospores/mL solution of two A. euteiches isolates: Aph1 and Aph2. FSI was assessed on a scale of 0 (healthy plant) to 5 (dead plant). Colors represent FSI values for each genotype: dark green for FSI ≤ 1, light green for 1 < FSI ≤ 2, yellow for 2 < FSI ≤ 3, orange for 3 < FSI ≤ 4, and red for FSI > 4.

Figure 3

Average Foliar Symptoms Index 20 days after inoculation (FSI₂₀) and Standard Error (SE) for the Pisum collection with 10³ zoospore/mL solutions of two A. euteiches isolates. The x-axis represents Aph1, and the y-axis represents the Aph2 isolate. FSI was assessed on a scale of 0 (healthy plant) to 5 (dead plant). Colors represent the mean FSI values for both isolates: dark green for FSI ≤ 1, light green for 1 < FSI ≤ 2, yellow for 2 < FSI ≤ 3, orange for 3 < FSI ≤ 4, and red for FSI > 4.

Figure 4

Average Foliar Symptoms Index 20 days after inoculation (FSI₂₀) and Standard Error (SE) of 322 genotypes from the following taxa: P.s. subsp. humile (n = 28), P.s. subsp. elatius (n = 18), P.s. ssp. jomardii (n = 83), P.s. subsp. arvense (n = 81), P. fulvum (n = 13), P.s. subsp. sativum (n = 69), P. abyssinicum (n = 7), and P. sativum “Indian ecotype” (n = 23). The genotypes were inoculated with 10³ zoospores/mL of two A. euteiches isolates: Aph1 (blue) and Aph2 (orange), and FSI₂₀ was assessed using a scale of 0 (healthy plant) to 5 (dead plant).

Figure 5

Average Root Rot Index and Foliar Index 20 days after the inoculation (RRI₂₀) and Standard Error (SE) for the 40 selected accessions inoculated with both A. euteiches isolates: (a,b) inoculation with Aph1 at doses of 10³ zoospores/mL and 10⁴ zoospores/mL, respectively, and (c,d) Aph2 inoculations at doses of 10³ zoospores/mL and 10⁴ zoospores/mL, respectively. RRI₂₀ was assessed on a scale of 0 (not root symptoms) to 9 (dead roots). Asterisks indicate significant differences (* p ≤ 0.1, ** p ≤ 0.05, and *** p ≤ 0.01) for each genotype with respect to the susceptible control Messire (Mes.).

Figure 6

Average percentage of wet biomass loss with respect to the non-inoculated control of each accession 20 days after the inoculation, and Standard Error (SE), for the 40 selected accessions inoculated with both A. euteiches isolates: (a,b) inoculation with Aph1 at doses of 10³ zoospores/mL and 10⁴ zoospores/mL, respectively, and (c,d) Aph2 inoculations at doses of 10³ zoospores/mL and 10⁴ zoospores/mL, respectively. Asterisks indicate significant differences (* p ≤ 0.1, ** p ≤ 0.05, and *** p ≤ 0.01) for each genotype with respect to the susceptible control Messire (Mes.).

Figure 7

Taxonomic composition of the evaluated Pisum collection. The collection comprises 322 accessions from eight different Pisum taxa, including P.s. subsp. jomardii (86 accessions), P.s. subsp. arvense (81 accessions), P.s. subsp. sativum (69 accessions), P.s. subsp. humile (28 accessions), P. sativum “Indian ecotype” (23 accessions), P.s. subsp. elatius (18 accessions), P. fulvum (13 accessions), and P. abyssinicum (7 accessions).