Evaluation of Onion (Allium cepa L.) Lines and Hybrids Possessing Bs1 for Resistance against Multiple Isolates of Botrytis squamosa

Abstract

Onion (Allium cepa L.) lines resistant to Botrytis leaf blight (BLB) were produced by backcrossing the Bs1 BLB resistance of Allium roylei Stearn into cultivated onion. This study evaluated important issues for the commercial use of BLB‐resistant onions: the relative degree of BLB control possible when Bs1 is homozygous rather than heterozygous, the control achieved against multiple pathogen isolates, and the potential for negative traits associated with transfer of Bs1. Homozygous BLB‐resistant onions tested in inoculated growth chamber screens with five distinct Botrytis squamosa Walker isolates and in naturally infested regional field trials showed significantly lower BLB symptoms than susceptible commercial hybrid controls. F1 hybrids heterozygous for the Bs1 gene showed levels of BLB symptoms between those of the homozygous resistant and susceptible controls, confirming previous reports of partial dominance. The difference in BLB control in plants heterozygous for Bs1 compared with homozygous plants was more noticeable under heavier disease pressure. Hybrids heterozygous for Bs1 had similar level of BLB control in most of the field trials as their parental homozygous BLB‐resistant lines, indicating that either heterozygous or homozygous BLB resistant hybrids could be used commercially for BLB control. There were no differences in the degree of pathogenicity across the five B. squamosa isolates, which were genetically different based on polymorphism for DNA markers, or among the naturally occurring pathogens present at the six different field locations used. No obvious association was noted between presence of the Bs1 resistance gene with unfavorable characteristics such as reduced bulb size or yield.

SNP-markers in Allium species to facilitate introgression breeding in onion

“…Our results also demonstrated that resistance to B. squamosa in A. roylei is based on a high level of partial resistance, indicating that plants still can be infected by the pathogen. This is in line with other studies in which Botrytis leaf blight symptoms were also observed in onion plants homozygous for the Bs1 gene [ 10 ]. These plants were obtained after two generations of back-crossing to onion of an interspecific F1 hybrid between onion and A. roylei , followed by three to four generations of selfing.…”

“…These plants were obtained after two generations of back-crossing to onion of an interspecific F1 hybrid between onion and A. roylei , followed by three to four generations of selfing. Under heavy disease pressure, heterozygous plants had levels of BLB symptoms between those of homozygous resistant inbred plants and susceptible controls [ 10 ]. After crossing the interspecific RF hybrid with onion, we found only one CCxRF plant with an infection level similar to A. roylei and RF, whereas all other plants, that we also considered resistant, had higher levels of infection.…”

“…Another approach to zoom in on the location of the Bs1 gene is genotyping the four available BLB-resistant onion lines containing the same source of resistance, CUBLB-R1, -R2, -R3 and -R4 (generation F1BC2S3 or S4) [ 10 ]. With the use of RF-SNP markers of Chr.…”

“…6 that segregate between onion and A. roylei we may identify the presence of A. roylei fragments still present in these homozygous resistant progeny plants obtained after five or six generations of selection. Indications for applicability are favourable, as heterozygous F1 hybrids obtained after crosses between the CUBLB-R1 to -R4 lines and parental onion lines had similar levels of resistance to B. squamosa in most experimental field trials as the homozygous lines, without observing any drawbacks in terms of reduced bulb size or yield [ 10 ].…”

“…In A. roylei resistance to Botrytis squamosa [7, 8], and Colletotrichum gloeosporioides [9] was discovered as well. Onion lines with resistance to B. squamosa from A. roylei are currently being developed [10]. In another relative of onion, A. fistulosum , accessions with resistance to B. squamosa [8, 11, 12], Fusarium oxysporum [13], Phoma terrestris [14, 15] and C. gloeosporioides [9] were identified.…”