Interactions in the pepper‐ Phytophthora capsici system

To analyse interactions in the Phytophthora capsici ‐pepper ( Capsicum annuum L.) system, three pepper lines (‘Morrón IN1A 224’, ‘PI201232’ and ‘SCM 334’) with various degrees of resistance to P. capsici and three P. capsici isolates with increasing aggressiveness were used. Inoculations were made when plants had 4–6 leaves, either by irrigating the culture substrate with P. capsici zoospore suspensions at increasing concentrations of up to 3 × 10 5 /ml, or by adding mycelial suspension. P. capsici isolate ‘Ca’ was the least aggressive, followd by ‘8303’ and ‘Bl’. The logistic function was appropriate to describing cumulative mortalities of pepper seedlings inoculated with different P. capsici zoo‐spore concentrations. The inoculum concentration required for 50% mortality classified pepper lines as susceptible (‘Morrón INIA224’) and resistant (‘PI201232’ and ‘SCM334’), the former being less resistant than the latter. Isolates ‘Ca’ and ‘8303’ caused very different mortalities both with zoospore and mycelial inoculations, which allowed them to be classified as belonging to two different vertical pathotypes (PO and PI). Isolate ‘Bl’ acted similarly to isolate ‘8303’ when inoculated on ‘Morrón’ and ‘SCM334’, but its behaviour was dependent on zoospore concentration when inoculated on ‘PI201232’. It also proved to be similar to ‘Ca’ using mycelial inoculation. As the case of ‘Bl’ would not be an exception among P. capsici isolates, the two vertical pathotypes PI and PO would be the higher (isolate ‘8303’) and lower (isolate ‘Ca’) limits of a range of aggressiveness. Although there are cases of host‐parasite interaction, absence of interaction seems to be the rule.