ON THE SELECTIVE RESISTANCE OF CARNATION CUTTINGS TO ISOLATES OF PHYTOPHTHORA PARASITICA

In order to investigate the mechanisms of host-parasite interaction in this system, 15 isolates from as many different host-plants were inoculated on Carnation cuttings in three ways: (I) fully rooted cuttings (RC) were planted separately in soil bags and 10 000 zoospores in suspension were poured at the foot; time-course of mortalities was followed over one month; (II) freshly prepared non rooted cuttings (0-C) were placed standing on mycelial mats grown on filter paper, in perlite pots under a mist-system; inoculum was removed after two days and symptoms recorded after 7 days; (III) the same inoculation technique was applied to cuttings previously incubated for 7 days in rooting conditions (7-C).

In type I inoculation, which closely resembles natural infection, 4 isolates only (including the one from Carnation) reproduced the typical evolution of the disease: complete to nearly complete mortality in 2–3 weeks. Among the other strains, most gave no or scarce attacks and a few ones had an intermediary behaviour with delayed and reduced mortalities. Bonnet and Maia have demonstrated various degrees of parasitic specialization in P. parasitica towards Carnation, Tomato and Citrus. While high pathogenicity on Carnation is mostly found in isolates from Carnation, this work indicates that the same behaviour may also occur in isolates from other hosts; all these isolates will be referred to as "Carnation-type strains" (CTS). On the host side, this means that rooted cuttings exhibit a selective resistance to all strains but CTS.

In contrast, nearly all isolates were more or less pathogenic to 0-C in type II inoculation. On a disease intensity index scale (integrating both percent of infection and necrosis extent) the four CTS were high, but not very different from some isolates which had given low attacks in type I inoculation. Therefore 0-C express some quantitative differences in receptivity, but selective resistance must become established only at some time during the three weeks rooting.

However, the ranking of the isolates (on the same pathogenicity scale) in type III inoculation was very similar to that on RC, the CTS being far ahead from all other isolates. For these, pathogenicity was always much lower on 7-C than on 0-C: 7-C thus exhibit most of the

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