Peanut stunting at the Ty Ty location did not exceed 5% from any rate or timing of pyroxasulfone (Figure 1). At the Plains location, there was a sigmoidal relationship between peanut stunting and pyroxasulfone rate when applied PRE (p<0.0001). Pyroxasulfone applied PRE at 240 g/ha or less caused less than 5% crop stunting. However, significant crop stunting occurred at 360 g/ha (18% injury) and 480 g/ha (48% injury). This contrast in crop response between locations may be explained due to the differences in soil type, environment, and cultivars. At Plains, soil silt and clay contents were 11 to 14% higher than at Ty Ty. Additionally, combined May + June rainfall totals were greater at the Plains location (Table 1). Crop injury symptoms may be enhanced when residual herbicides are applied to heavier, more adsorptive soils that receive higher rainfall amounts. When compared to AP-3, Georgia Green is considered to be a small-seeded peanut cultivar (Branch 1996; Gorbett 2007). Previous research concluded that smaller-seeded market classes of dry bean were more sensitive to PRE applications of pyroxasulfone than dry beans with larger seeds (Sikkema et al. 2007). In soybean, it has been speculated that plants developing from larger seeds could survive lethal herbicide doses better than plants developing from smaller seeds because they could survive on more cotyledonary food reserves until the herbicide dosage reaches a non-lethal level (Anderson 1970). Additional pyroxasulfone/peanut cultivar tolerance research is needed due to the fact that the cultivars used in these studies have been replaced by larger-seeded, superior disease resistant, and higher yielding cultivars such as Georgia-06G (Branch 2007).

Peanut injury from POST applications of pyroxasulfone was less than 10% in these studies. Pyroxasulfone provides no POST weed control activity. However, in other crops, the inclusion of pyroxasulfone in combination with POST herbicides is a strategy to enhance the residual control of susceptible weeds (King et al. 2007; King and Garcia 2008). In peanut, this POST use pattern could be an effective means for controlling weeds with season-long emergence patterns, especially species that are difficult to control with other management tools such as Benghal dayflower and ALS-resistant Palmer amaranth.

Due to lack of significant interactions, data were combined over years and pyroxasulfone rates. There was a significant effect of pyroxasulfone application timing on the incidence of TSWV. Pyroxasulfone applied PRE resulted in higher incidences of TSWV than POST applications (Table 2). This was unexpected since AP-3 is considered to have excellent resistance to TSWV (Gorbet 2007). In previous research, only POST applications of chlorimuron had an influence on the incidence of TSWV in peanut (Prostko et al. 2009). In other peanut research, applications of diclosulam (PPI), flumioxazin (PRE), imazapic (POST), paraquat (POST), and 2,4-DB (POST) did not affect the incidence of TSWV (Bailey et al. 2000; Faircloth and Prostko 2010; Tubbs et al. 2010; Wilcut et al. 2001). The mechanism of how certain herbicides influence TSWV in peanut has not yet been discovered.

There were no significant effects of pyroxasulfone rates or application timings on peanut yield (Figure 2). In other legume crops, PPI or PRE applications of pyroxasulfone at rates ≥ 209 g ai/ha caused significant yield reductions (Soltani et al. 2008; Sikkema et al. 2007). While there was significant crop injury (stunting) from pyroxasulfone applied PRE at Plains, there was no correlation between this injury and peanut yield for application timing at either location (Figure 3).