CNR, Centro di Studio dei Fitofarmaci, via Filippo Re 8, 40126 Bologna, Italy

Background and objectives
Epiphytic pathogen infection may be largely influenced by both surface chemistry and morphology of the plant organs. Leaf cuticle is usually considered as structural barrier which permeability is greatly affected by wax components [1]. The amount of cuticular wax deposition is proportional to light intensity, which, therefore may be considered an important factor in host-pathogen interaction. Moreover, the performance of pesticides could be affected by this interaction.

The present study concerns the influence of two different values of photon fluence rate on the interaction among courgette cotyledonous leaf surface, Sphaeroteca fuliginea infection occurrence and the performance of bitertanol as locally systemic fungicide.

Materials and methods
Potted courgette (Cucurbita pepo) plants cv. Storr's green, were grown into climatic chamber for 2 ;weeks. Either a series of 120 ;cm fluorescent tubes or a high-pressure mercury-vapour powerstar lamp provided a photosynthetic active radiation (PAR) of 200 and 1000 ;Ámol/m2.s, respectively. At the end of the light exposure period, the leaf cuticle was examined by SEM in order to assess the epicuticular waxes. The proportion of the surface covered by waxes was estimated using a scanner and Macintosh Image software. Plants were inoculated spraying an aqueous conidia suspension to the point of run-off. The fungicide (bitertanol) was used at the recommended application rate both before and after inoculation. The infection was visually estimated from the percentage of foliar surface covered by the whitish mycelium. GC analysis were carried out to assess the a.i. amount in leaf at different times from the fungicide application. Eight replicates per treatment were set up, each consisting of one potted plant.

Results and conclusions
The proportion of the upper foliar surface covered by epicuticuiar waxes was estimated ranging from 30 to 40% under high light intensity compared with 10-20% assessed at 200 ;Ámol/m2.s. The cotiledonous leaf surface of plants grown at low light conditions was completely covered by the mycelium, whereas only 50-60% of foliar surface was affected by the powdery mildew when courgettes were submitted at 1000 ;Ámol/m2.s. The fungicide post-infection activity was notable in both light condition: a reduction of 70% in white spot appearance was achieved at 200 ;Ámol/m2.s. Under high light intensity no infection could be observed at both pre- and post-infection treatment. Conversely, with plants grown at low PAR, pre-infectional applications showed 40% in reduction of powdery mildew infection severity. The amount of chemical detected by GC analysis on leaves cultivated at 200 ;Ámol/m2.s significantly decreased with time, compared with the results assessed at 1000 ;Ámol/m2.s.

Light intensity influenced the development of leaf cuticle and, in turn, the severity of the colonization of epiphitic fungus S. ;fuliginea, as well as the bitertanol performance. At first, surface waxiness affected the deposition of aqueous inoculum and later the cuticle could act as a mechanical barrier against the haustoria penetration. In particular, at low-light growing conditions, the interaction among host, pathogen and fungicide, showed less waxed leaves, higher infection severity and a decrease in chemical perfomance (especially because of the amount of a.i. lost with time) compared with the results obtained at 1000 ;Ámol/m2.s. These low light values are typical of controlled growing conditions and may have a great influence in host-pathogen interaction and, consequently, in the effectiveness of the control, compared with the field conditions [2].

1. Cutter EG, 1976. In: Dickinson CH, Preece TF, ed. Microbiology of Aerial Plant Surfaces. London: Academic Press, pp. 1-40.
2. Hewitt HG, Caseley J, Copping LG, Grayson BT, Tyson D, 1994. Monograph No. 59, Brighton Crop Protection Council: Comparing glasshouse and field pesticide performance, p. 11.