Florida beggarweed [Desmodium tortuosum (Sw.) DC] is listed among the four most common and troublesome weeds in Alabama, Florida, and Georgia (Webster 2005). A single Florida beggarweed plant reduced peanut yield 19% within 60 cm of the weed (Cardina and Brecke 1991), while a maximum yield loss from a density of 8 weeds/m² approached 36% (Hooper 1978; Webster and Cardina 2004). Chlorimuron is a postemergence, broadleaf herbicide labeled for use in the Southeast U.S. to control Florida beggarweed (Anonymous, 2008). It was registered for use on peanut in 1989 (Hammes et al., 1990). Currently, chlorimuron is used on approximately 10% of the peanut hectares grown in Georgia (N. L. Smith, unpubl. data, 2006). Chlorimuron applications will not recover crop yield losses associated with early-season Florida beggarweed interference, but will improve harvest efficiency (Webster and Cardina, 2004).

Previous research indicated that the tolerance of peanut to chlorimuron was acceptable when applied no earlier than 60 days after emergence (DAE) (Brown et al., 1993; Colvin and Brecke, 1988; Hammes et al., 1990). The tolerance of peanut at this time is based upon a reduction in absorption, translocation, and more extensive metabolism of the herbicide (Wilcut et al., 1989). Despite the peanut tolerance observed at 60 DAE, chlorimuron can suppress growth by reducing the lengths of the cotyledonary lateral branches and main stem (Mitchem et al., 1995). Newer research has shown that chlorimuron poses a yield-reducing risk only when the crop has been stressed by other factors (Grey and Wehtje 2004).

Spotted wilt disease, caused by tomato spotted wilt tospovirus, was first observed in peanut in Brazil in 1941 (Sherwood and Melouk, 1995). Since the early 1990's, spotted wilt has become the most damaging disease in the southeastern U.S. peanut production region (Brown et al. 2005, Todd et al., 1998). In response to this problem, a risk index of management strategies, including cultivar, planting date, seeding rate, and others was developed to help producers minimize the potential impact of this disease (Brown et al. 2008). Although the risk index has been very successful and widely adopted, there is some concern that other management factors not presently accounted for in the index, such as herbicide use, may also affect the incidence of spotted wilt disease.

Observations from commercial peanut fields since 1999 suggested that chlorimuron may influence the incidence of spotted wilt. Earlier studies conducted on the interaction of chlorimuron with certain plant diseases indicated that a correlation does not exist (Johnson and Brown, 1990; Johnson et al., 1994). However, the cultivars used in these studies are no longer commercially available. Thus, the objective of this research was to evaluate the effects of chlorimuron, applied at various timings, on the development of spotted wilt disease on currently used peanut cultivars.

Field trials were conducted at fifteen locations (15 site-years) in Georgia from 2000 through 2007. A complete description of the cultivars, planting, and harvesting dates can be found in Table 1. Common production practices and Cooperative Extension recommendations were used at all locations.

Chlorimuron at 9 g ai/ha was applied at various intervals ranging from 60 to 105 DAE and were grouped into the following six categories: 1) non-treated control; 2) 60–69 DAE; 3) 70–79 DAE; 4) 80–89 DAE; 5) 90–99 DAE; and 6) 100+ DAE. A non-ionic surfactant (Nonionic surfactant, 80/20, United Agri Products, Inc., P.O. Box 1286, Greeley, CO 80632-1286) at 0.25% v/v was included with all treatments. At all locations, treatments were arranged in a randomized complete block design with 3 or 4 replications. The treatments were applied with a CO₂ pressurized backpack sprayer calibrated to deliver 140 to 187 L/ha at 220 to 275 kPa. The plot areas were maintained weed-free using a combination of mechanical cultivation, hand-weeding, and commonly used preemergence and postemergence peanut herbicides on an as needed basis.

Incidence of spotted wilt was measured just prior to peanut inverting by counting the number of disease loci per linear row in 31 cm sections and transforming the data to percentage infection based upon total row length. Peanut yield data were obtained using commercial inverting and harvesting equipment. Peanut yields are adjusted to 10% moisture. Data were subjected to analysis of variance (Proc GLM) with site-year as a random effect. Prior to analysis, spotted wilt incidence data were arcsine transformed. However, there were no differences in the conclusions of the analysis when the non-transformed data were analyzed, therefore, original means are presented.

There were no significant peanut variety by chlorimuron timing interactions, but main effects for variety and chlorimuron were significant. Due to the differences in replication among site-years, a Dunnett's Test for each treatment mean was calculated for comparison to a standard treatment (P  =  0.05) (Lentner and Bishop, 1993; Steel and Torie, 1980). Treatment means for the significant main effects were compared to Georgia Green (Branch, 1996) for the variety main effect and to the nontreated control for main effect of chlorimuron application timing. Georgia Green is currently considered to be the industry standard and is grown on approximately 60% of the hectarage in the southeast (J.P. Beasley, pers. commun., 2008).

The results of this research confirms on-farm observations that chlorimuron has the potential to increase the incidence of spotted wilt disease of peanut. However, peanut yields appear to be unaffected by this increase. Thus, the effects of chlorimuron on spotted wilt are minimal in comparison to the many other production practices that influence this disease such as cultivar, planting date, plant population, tillage, soil insecticide, and row spacing (Brown et al. 2005, 2008). Consequently, late-season Florida beggarweed populations that have the potential to reduce peanut harvest efficiency and fungicide spray deposition should continue to be managed with chlorimuron. Additionally, the peanut cultivars evaluated in these tests did not exhibit differential tolerance to chlorimuron.