Long-term resistance in promising rubber tree genotypes as a breeding source for improving South American leaf blight management under high disease incidence in the Colombian Amazon

Abstract

South American Leaf Blight (SALB), caused by Pseudocercospora ulei, is the most important disease in Hevea brasiliensis grown in South America. This study aimed to evaluate long-term resistance in the field for 11 years in 10 rubber tree genotypes (nine hybrid clones and one primary clone) in a large-scale clonal trial (LSCT) and under controlled and nursery conditions. Resistance was evaluated using two resistance components: type of reaction (TR, ranging from 1 to 6) related to asexual sporulation, and stroma density (ST, ranging from 0 to 4), which is linked to sexual sporulation. Resistance status ranking (RSR) based on the maximum scores of the variables was adapted to classify the genotypes (completely resistant, partially resistant or highly susceptible) over time, and against four indigenous isolates of P. ulei. Eight of the ten clones exhibited partial resistance in the field, and clones FDR 5597, GU 198 and FDR 5788 were more resistant with TR ≤ 4 and ST ≤ 2. These clones were completely resistant in the laboratory/nursery with TR < 3 and ST = 0. Only clone MDF 180 did not exhibit the sexual phase of the pathogen (i.e., ST = 0) in the field or in the nursery. Clones CDC 56 and IAN 873 (control) were highly susceptible, with TR > 5 and ST > 2. In this study, sources of partial resistance were evidenced for the effective management of SALB under conditions of high P. ulei incidence in the Amazon.