Distribution and quantitative abundance of weedy sunflower Helianthus annuus L. in Serbia

The Helianthus annuus L. species which appears as a weedy sunflower differs from the sunflower crop in height, pronounced branching, phenotypic variability, presence of anthocyan pigment, numerous small heads, relatively small and morphologically variable seeds (shape, color, motlling), dormancy and seed brittleness, etc. Weedy sunflover (WS) has been spreading for more than two decades on crop and non-cropland in the area with intensive sunflower crop production in Serbia, where it causes large yield losses in many crops (row crops and cereals). Bearing this in mind, as well as the fact that there have been confirmed cases of reduced herbicide efficacy on WS, the aim of this study was to determine the distribution and define the population size of WS in the whole territory of Serbia. During a three year period, in the region with intensive sunflower crop production (Banat, Bačka, Srem, Stig), the present WS populations were monitored and the quantitative and qualitative traits of their populations were assessed. A scale from 1-4 was used to estimate the number of plants in the population (1 - species cover up to 5% of the surveyed area, 2 - species cover 10-15% of the surveyed area, 3 - species cover 25-50% of the surveyed area, and 4 - species cover over 50% of the surveyed area). Over 200 populations of WS have been recorded in the whole territory of Serbia, and this is shown on a UTM map of 10x10 km scale. The largest WS populations (5 to 15 ha), with the highest number of plants (> 100,000) and average density (about 5 m-2 plants) were recorded in the site of Padinska Skela (in crop and non-croplands), Zbeg (predominantly on non-cropland) and Surčin-Galovica (predominantly in cropland). Based of the phenotypic diversity/discrimination traits, the population from Stari Žednik was the most similar to sunflower crop [with the central head and apical branching, the lowest number of heads compared with other WS populations (18.5 plants-1, while some populations had up to 59.4 heads plant-1) and the largest head size (8.1 cm in diameter)]. These results should be a guide for the farmers when planning crop planting, to undertake the best measures to control WS within the concept of antiresistance strategy, given the confirmed cases of reduced susceptibility of this weed to herbicides, as well as the obligation to eliminate it on non-cropland, as its main corridors of expansion are areas along roads and canals, field crop margines, lost cropland, etc.