Differential responses of citrus calli and protoplasts to culture filtrate and toxin of Phoma tracheiphila.

Nucellar calli from two citrus cultivars with known tolerance to mal secco disease were chosen as experimental material, to test the pathogen's response to culture filtrate (CF) and partially purified toxin (PPT). The response of the two calli to the CF was in reverse order to the known response of the two cultivars to natural and artificial inoculations with Phoma tracheiphila. HPLC analysis of P. tracheiphila CF indicated the presence of a relatively high level of indole acetic acid (IAA). The response of the two calli and protoplasts derived from these calli to increasing amounts of IAA in the culture media was in the same order as that of calli and protoplasts to CF. In contrast, the responses to PPT of calli and protoplasts from these two types confirmed the relative tolerance of 'Femminello' lemon and 'Tarocco' orange trees to mal secco disease.