Insensitivity to Metalaxyl Among Isolates of Phytophthora capsici Causing Root and Crown Rot of Pepper in Southern Italy
Authors: Pennisi A. M. , Agosteo G. E. , Cacciola S. O. , Pane A. , Faedda R. 
Published: May 2007

Pepper (Capsicum annuum L.) has become an economically important crop in the coastal provinces of Catanzaro and Vibo Valentia, in Calabria (southern Italy). An old local selection Riggitano, very susceptible to root and crown rot caused by Phytophthora capsici Leonian, is the prevalent cultivar in this area. Although repeated applications of metalaxyl are used as a soil drench, severe outbreaks occur each year on greenhouse crops. To examine metalaxyl resistance in P. capsici, 60 single-hypha isolates of P. capsici were tested in vitro for their level of sensitivity to metalaxyl. The isolates were collected from 1992 to 1997, during epidemic outbreaks of root and crown rot, from two commercial greenhouse pepper crops, near Vibo Valentia and Lametia Terme (Catanzaro). Fungicide sensitivity was determined by plating mycelial plugs onto potato dextrose agar (PDA) amended with metalaxyl. The fungicide was added to PDA after autoclaving, at final concentrations of 0.1, 1, 5, 10, 50, 100, and 200 μg/ml a.i. The percentage of inhibition of radial growth on metalaxyl-amended medium compared with the growth on unamended medium was determined after 6 days of incubation in the dark at 25°C. Three replicate petri dishes were used per treatment and each test was performed twice. The isolates were paired in culture on V8 agar with isolates of P. capsici of known mating type and all proved to be A2 mating type. Significant variation was observed among the isolates tested in responce to metalaxyl. The ED50 values for in vitro inhibition of mycelial growth by metalaxyl ranged from 1 to 11 μg/ml, whereas an ED 50 value of 0.1 μg/ml had been reported for a wild-type isolate of P. capsici obtained from pepper in northern Italy (3). The variation observed among the isolates from Calabria appeared unrelated to both the geographical origin and the year of isolation. The isolates from Calabria were inhibited by between 1 and 12% at 0.1 μg/ml and by between 7 and 27% at 1 μg/ml, proving to be less sensitive to metalaxyl than isolates from Capsicum spp. originating from Central America, tested by other authors (1). According to the criterion used in a recent screening for sensitivity to metalaxyl (2), 19% of the isolates from Calabria should be considered sensitive, as they were inhibited by more than 60% at 5 μg/ml, while all the others were intermediate, as they were inhibited less than 60% at 5 μg/ml but more than 60% at 100 μg/ml. On the basis of this preliminary screening, we report the presence of insensitivity to metalaxyl in field isolates of P. capsici in southern Italy. Although no isolate tested appeared highly resistant to metalaxyl, the presence of a high proportion of isolates with an intermediate level of resistance should be a reason for the growers to use metalaxyl more cautiously to control root and collar rot.

References: (1) M. D. Coffey and L. A. Bower. Phytopathology 74:502, 1984. (2) G. Parra and J. Ristaino. Plant Dis. 82:711, 1998. (3) M. L. Romano and A. Garibaldi. La difesa delle piante 3:153, 1984.