author(s)
Najla Sayari, Tasnim Gueddes and Rym Jaouadi

abstract
Weed flora that competes with a particular crop species depends upon environmental factors and cropping systems. In this paper we present taxonomic analysis of the weed flora in potato grown conventionally under a semi-arid Tunisian climate, and its shift according to cultural practices particularly hand weeding and chemical treatments. A total of 29 weed species belonging to 16 families and 27 genus have been reported in potato crop. In the total floristic spectrum of potato crops weed flora, Poaceae family predominates with 17%, followed by Amarantaceae (14%), Fabaceae (10%) and Asteraceae (10%). In term of biological spectrum, therophytes were the most frequent with 72% followed by geophytes with 14%. Urtica urens, Chenopodium murale, Chenopodium album and Raphanus raphanistrum were the most dominant species. Chemical treatment is critical to provide an optimal growing environment for potatoes earlier in the season Results showed that weed biomass decreased when chemical treatments were applied between 2 and 3 weeks after planting. However, beyond 6 weeks after planting, chemical treatments were not effective in controlling weeds. These findings revealed that, under the study site conditions, the critical period for potato crop-weed competition was 4-6 weeks after planting and the crop should be well protected at this stage by creating weed free environment. Early postemergence application of Metribuzin at 0.7 kg a.i./ha was more efficient to suppress broadleaf weed species and grasses than pre-emergence application of Linuron at 0.2 kg a.i./ha. Furthermore, hand weeding practiced twice: the first at the planting and the second 3 weeks after was more efficient to decreases weed density than that practiced only one time 6 weeks after planting. However, the highest weed control efficiency was recorded when hand weeding was combined with herbicides application.