Evaluation of the resistance of 36 Hevea clones to Microcyclus ulei and relation to their capacity to accumulate scopoletin and lignins

Thirty-six Hevea clones were inoculated with conidia of Microcyclus ulei, t he causal agent of South American Leaf Blight in an air-conditioned chamber . Components of disease resistance such as latent period, infectious period , lesion size, percentage of damaged leaf area, number of lesions, spore pr oduction and stromatic generation period were studied during a single infec tion cycle. The clones were easily separated on the basis of these characte rs that were evaluated under controlled conditions. The main components tha t were highly correlated were the spore production, lesion size and number of lesions with the percentage of leaf area. Others, such the stromatic gen eration period, were slightly correlated with all other components; latent period and infectious period was slightly correlated with the lesion size; and lesion size was slightly correlated with lesion density. For screening of varieties, damaged leaf area and spore production were the two component s that should be considered the most important. Furthermore, the clones wer e tested for their capacity to produce scopoletin, a phytoalexin of Hevea, and to produce lignins in their infection sites. A strong correlation was o bserved between scopoletin accumulation and clone resistance. Moreover, str ong lignin accumulation was often associated with a longer stromatic genera tion period. These two physiological reactions could interfere by limiting fungal development in several clones. However, neither scopoletin nor ligni n accumulation could individually explain the behaviour of all clones.