Abstract: ABSTRACT Root rot caused by the oomycete Phytophthora cinnamomi (Pc) is the main phytosanitary limitation in avocado production in many regions of the world. Management of this disease has been based on epidemiological studies and integrated crop management, where resistant rootstocks are considered one of the main options. This study aimed to evaluate the resistance levels of six avocado genotypes from the Colombian Germplasm Avocado Collection (CCGA) to Pc under nursery conditions, as these genotypes are known to be promising for use as rootstocks. Each genotype was propagated by cloning and evaluated with and without the pathogen in conformity with epidemiological, physiological, and histological parameters in a randomized complete block experimental design with three replicates per treatment, with each replicate being represented by three plants as an experimental unit. The area under the disease progress curve (AUDPC) and the apparent rate of infection (rAUDPC) discriminated against four levels of genetic resistance to the pathogen. The AUDPC in inoculated plants of each genotype was related to the degree of root necrosis (RN), with significant reductions in photosynthetic capacity, stomatal conductance (gs), and biomass loss in root and foliage in the most susceptible genotypes. Histological analysis of roots confirmed the ability of Pc to invade and damage roots in different avocado genotypes. At the same time the formation of tyloses was identified as a mechanism of histological response to infection. The selections CCGA0143, CCGA0080, CCGA0122, and Duke-7 are proposed as elite genotypes resistant to Pc, due to reduced root damage and etiological performance to infection under nursery conditions.
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