Abstract
An indirect phenotyping method was developed in order to estimate the susceptibility of rubber tree clonal varieties to Corynespora Leaf Fall (CLF) disease caused by the ascomycete Corynespora cassiicola. This method consists in quantifying the impact of fungal exudates on detached leaves by measuring the induced electrolyte leakage (EL%). The tested exudates were either crude culture filtrates from diverse C. cassiicola isolates or the purified cassiicolin (Cas1), a small secreted effector protein produced by the aggressive isolate CCP. The test was found to be quantitative, with the EL% response proportional to toxin concentration. For eight clones tested with two aggressive isolates, the EL% response to the filtrates positively correlated to the response induced by conidial inoculation. The toxicity test applied to 18 clones using 13 toxinic treatments evidenced an important variability among clones and treatments, with a significant additional clone x treatment interaction effect. A genetic linkage map was built using 306 microsatellite markers, from the F1 population of the PB260 x RRIM600 family. Phenotyping of the population for sensitivity to the purified Cas1 effector and to culture filtrates from seven C. cassiicola isolates revealed a polygenic determinism, with six QTL detected on five chromosomes and percentages of explained phenotypic variance varying from 11 to 17%. Two common QTL were identified for the CCP filtrate and the purified cassiicolin, suggesting that Cas1 may be the main effector of CCP filtrate toxicity. The CCP filtrate clearly contrasted with all other filtrates. The toxicity test based on Electrolyte Leakage Measurement offers the opportunity to assess the sensitivity of rubber genotypes to C. cassiicola exudates or purified effectors for genetic investigations and early selection, without risk of spreading the fungus in plantations. However, the power of this test for predicting field susceptibility of rubber clones to CLF will have to be further investigated.
Highlights
The rubber tree (Hevea brasiliensis, 2n = 36) is the only crop developed for the production of natural rubber
We have previously proposed a classification of C. cassiicola isolates based on phylogenetic clusters and toxin classes defined from the cassiicolin isoform(s) detected by PCR (Cas1 to Cas6), with the toxin class Cas0 corresponding to isolates without cassiicolin gene [10]
The indirect toxicity test based on induced electrolyte leakage measurements seems a practical and sensitive method for monitoring the early events of H. brasiliensis x C. cassiicola interaction, taking into account the genetic diversity of C. cassiicola without the risk of spreading new virulent strains in the environment
Summary
In Latin America, from which Hevea originated, rubber cropping is severely hampered by the South American Leaf Blight, caused by Microcyclus ulei, alias Pseudocercospora ulei [1]. This disease remains so far confined to that continent. This ascomycete fungus was first isolated from rubber tree in Sierra Leone [2], in India [3] and Malaysia [4] It was really considered dangerous after the epidemic outbreak in Sri Lanka in 1985 [5]. Detection and exclusion of highly susceptible clones is essential to avoid inoculum proliferation and epidemic outbreaks
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