Garlic is a minor crop in Italy and is produced on more than 3400 ha (Istituto Nazionale di Statistica, 2017). Many regions of Italy have production areas characterized by typical cultivars. Some of these areas are mountainous and the agricultural activities are carried out on small terraces, as in the case of Vessalico village in the province of Imperia. The long tradition of garlic production in this area makes it a high-quality niche product and, in most cases, following organic production practices which improves the market value. However, traditional production and cultivation practices are inefficient, laborious, time-consuming, and costly. The mechanization of the production chain (sowing, weed control and harvesting) would make garlic production more profitable by reducing the losses and costs above all of weed control, thus, releasing time and labor to be used elsewhere in production (Melander et al., 2005).
Weeds limit crop yield and quality, and weed control is one of the most challenging crop management practices (Liebman and Davis, 2009). In organic farming, synthetic herbicides are not permitted and weed control is often limited to physical means where hand weeding and cultivation are the most popular (Datta and Knezevic, 2013). Garlic is a slow-growing bulb crop that is small in size, has shallow roots, and a thin canopy, which make it a poor competitor with weeds (Duranti and Cuocolo, 1989). Hand weeding requires labor availability and is expensive. Repeated cultivations increase soil disturbance and contribute to the destruction of the soil structure, loss of organic matter, and increase the chances of soil erosion (Wszelaki et al., 2007).
Flame weeding has been proposed as an alternative to these traditional techniques both in organic and conventional agricultural systems. In the latter, herbicide-resistant weeds and the pollution of surface and groundwater by herbicide residues have also raised public concerns and restrictions on herbicide use, which demand other alternatives (Datta and Knezevic, 2013; Rifai et al., 2002; Wszelaki et al., 2007).
Flame weeding exposes weeds to heat stress, which causes the denaturation of membrane proteins resulting in a loss of cell function and dehydration and leading to their death or a reduction in their competitive ability (Carrubba and Militello, 2013; Datta and Knezevic, 2013; Laguë et al., 2001). Plant response to flaming depends on their heat tolerance (Ascard, 1995). The susceptibility to heat stress is primarily species-specific, depending also on the position of the growing point and the growth stage of the plant (Cisneros and Zandstra, 2008; Ulloa et al., 2010).
Flaming does not disrupt the soil surface and does not contribute to germination of new flushes of weed seeds brought to the surface (Wszelaki et al., 2007). This weed control strategy can be used when fields are too wet or stony for cultivation. It has been used primarily before the crop emergence, especially in slow-growing vegetables (Ascard, 1995; Datta and Knezevic, 2013). Postemergence treatments have been found to be promising in heat-tolerant crops, such as maize (Zea mays), soybean (Glycine max), sorghum (Sorghum bicolor), and onion (Allium cepa) (Knezevic et al., 2013; Martelloni et al., 2016a; Sivesind et al., 2012; Ulloa et al., 2011a, 2011b).
However, using flaming as the only weed control tool can be costly and somewhat ineffective when weeds and the crop are not at the optimal growth stage for interventions. Integrated physical weed control based on the use of mechanical and thermal methods can lower intrarow weed control requirements and give satisfactory results, provided that they are used in combination (Melander et al., 2005; Stepanovic et al., 2016; Taylor et al., 2012). Our preliminary research on flame weeding in garlic cultivation showed positive results when postemergence broadcast flaming was performed, ensuring a lower weed biomass and higher garlic yields compared with weedy controls (Fontanelli et al., 2015a).
For an optimal weed control, the response of garlic production systems to flaming must be studied thoroughly and the optimal dose of liquefied petroleum gas, the growth stage and number of weeding interventions need to be defined. Our study was carried out during 2011–12 to develop a physical weed control strategy for garlic cultivation in complex landscapes. The aim was to test the effects of flaming intensities (i.e., different LPG doses obtained using a single LPG pressure of 0.2 MPa applied at different tractor driving speeds) and time of intervention (defined by the growth stage of the garlic plant) on the weeds and yield attributes of garlic. The study addressed the specific geographical and growing conditions of ‘Vessalico’ garlic, a typical product of the province of Imperia in Liguria, northwest Italy, managed under organic farming practices.
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