Alternatives to Benomyl for Management of Cercospora Leaf Spot on Turnip Greens

Benomyl was compared with copper hydroxide, azoxystrobin, tebuconazole, acibenzolar-S-methyl, and basic copper sulfate for effi cacy of cercospora leaf spot [incited by Cercospora brassicicola P. Henn] management on turnip greens [Brassica rapa L. var. (DC.) Metzg. utilis]. Treatments included various application times and were evaluated in three fi eld experiments over 2 years. The few yield effects that occurred were not consistent between years. Copper hydroxide and basic copper sulfate were not useful alternatives to benomyl due to a combination of phytotoxicity symptoms and ineffective disease control. Inconsistent results were observed with acibenzolar-S-methyl. A single, early application of tebuconazole greatly reduced cercospora leaf spot severity relative to the control in both years. Tebuconazole may be a good alternative to benomyl if a label can be obtained. Multiple (at least two) applications of azoxystrobin may be needed to achieve the same degree of cercospora leaf spot control as would result from a single properly timed application of benomyl. Although azoxystrobin is now labeled for turnip greens, grower costs will likely increase as a result of benomyl being discontinued. Chemical names used: methyl-1[(butylamino)carbonyl]-H-benzimidazol-2-ylcarbamate (benomyl); methyl (E)-2-{2-[6-(2cyanophenoxy)pyrimidin-4-yloxy]phenyl}-3-methoxyacrylate (azoxystrobin); alpha-[2-(4chlorophenyl)ethyl]-alpha-(1,1-dimethylethyl)-1H-1,2,4-triazole-1-ethanol (tebuconazole); 1,2,3-benzothiadiazole-7-thiocarboxylic acid-S-methyl-ester (acibenzolar-S-methyl). Turnip greens grown for processing are mechanically harvested in bulk. All foliage is removed above the cutting bar, and hand separation of unacceptable leaves is diffi cult and expensive (Sumner et al., 1991). Numerous fungi are pathogens of turnip foliage, and cercospora leaf spot (hereafter abbreviated CLS) is one of the more widespread foliar diseases (Sumner et al., 1978). Leaf spots cause discoloration and decay of the leaf petioles and blades, reducing product grade and value. Fields with severely diseased plants may be discounted or rejected by processors. Therefore, growers of turnip greens require tools for management of foliar diseases. Some early studies on fungicidal control of leaf spotting diseases on turnip greens were cited by Sumner (1971). Sumner et al. (1991) later published a study including data that supported the effi cacy of benomyl for 2.5-m centers to minimize spray drift between treatments. Each bed contained four (Research Station) or fi ve (Genoff’s) rows. ‘Alltop’ turnip greens were seeded in plots on 30 Aug. 1999 and on 9 Sept. 2000. Weeds were controlled with preplant-incorporated trifl uralin [2,6-dinitro-N,N-dipropyl-4-(trifl uoromethyl) benzenamine] at 280 g·ha (Research Station) or 420 g·ha (Genoff’s) and by hoeing. Nitrogen fertilizer (urea) was applied at 34 kg·ha preplant-incorporated and again at 34 kg·ha as a topdressing on 30 Sept. 1999, and at 56 kg·ha as a topdressing on 22 Sept. 2000, at the Research Station. Adequate P and K were available from fertilization of previous trials at the Research Station. A preplant-incorporated application of 43N–19P–35K (kg·ha) was made at the Genoff’s in 1999. Nineteen treatments (plus a nonsprayed control) were evaluated as shown in Table 1. One rate was used for each chemical material as appropriate to each given material. The adjuvant Latron CS-7 (60% blend of alkyl aryl polyethoxylate and sodium salt of alkylsulfonatedalkylate, 40% ingredients ineffective as spray adjuvants; BFR Products, Five Points, Calif.) was added at 2.4 mL per 2.8 L of spray mix for benomyl, azoxystrobin, acibenzolar-Smethyl, and tebuconazole. In 1999, treatments were broadcast through fl at-fan nozzles (8003 VK) spaced 46 cm apart on a wheelbarrow sprayer that applied 337 L·ha at 275 kPa (36 gpa at 40 psi). In 2000, the nozzles were changed to 8002 VK, so the sprayer applied 309 L·ha at 275 kPa (33 gpa at 40 psi). Two to three applications were expected to be enough to establish a point of diminishing returns for most materials. Turnip greens are a short season crop, and the focus material (benomyl) could not be applied more than twice per season. Natural infection was allowed to occur; the plots were not inoculated. Plots were irrigated by sprinklers to supplement rainfall based on subjective observations of plants and soils. Insects were controlled with approved pesticides. Yield and quality were determined by harvesting 2.5 m from two middle rows per plot. Plants were cut by hand with knives at about 3 cm above soil level. Harvests occurred on 20 Oct. 1999 and 3 Nov. 2000 (Research Station) and 25 Oct. 1999 (Genoff’s). Harvested leaves were bulked, weighed, mixed, and blindly sampled. Leaf spot incidence (the percentage of leaves infected) and severity (the percentage of leaf area infected) were assessed on 30 leaves per plot. The experimental design was a randomized complete block with four replications. Data were evaluated with analysis of variance procedures of the Statistical Analysis System (SAS) (SAS Institute, 1999). An arcsin transformation was applied to percentage data containing frequent zeroes before analysis. Individual t tests were calculated using least squares means with a PDIFF option to compare chemical treatments to the control. The same procedure was used to provide dose/response information among the various application strategies within each tested product. leaf spot control on turnip greens. Benomyl was labeled for this use and became popular among growers. However, when the present study was initiated (1999), there was a high probability of cancellation of benomyl. At that time benomyl, copper hydroxide, and basic copper sulfate were the only fungicides labeled for foliar disease management on turnip greens in the United States. Subsequently, benomyl was discontinued by its manufacturer. The objective of the present study was to identify possible alternatives to benomyl for management of CLS of turnip greens. We chose to examine the two labeled copper-based products and three different potential alternative materials: azoxystrobin, a strobilurin fungicide with a wide range of activity against various fungal species, low application rates, and a benign environmental profi le (Dacol et al., 1998); tebuconazole, a systemic, sterol-inhibiting fungicide (Labrinos and Nutter, 1993); and acibenzolar-S-methyl, an activator of systemic acquired resistance to various plant pathogens (Oostendorp et al., 2001). Materials and Methods Two fi eld experiments were conducted in 1999, one at the Vegetable Research Station, Bixby, Okla. (hereafter called the Research Station), and the second at a cooperator’s farm (Thomas Genoff) about 1.5 km to the south of the Research Station. A third experiment was conducted at the Research Station in 2000. The soil was a Severn very fi ne sandy loam [coarsesilty, mixed (calcareous), thermic Typic Udifl uvent]. Plots consisted of beds 6 m in length on HORTSCIENCE 40(5):1324–1326. 2005. Received for publication 16 Dec. 2004. Accepted for publication 29 Jan. 2005. Approved for publication by the Director, Oklahoma Agricultural Experiment Station. This research was supported in part under project H-2026, and in part by a grant from the Southern Region Pesticide Impact Assessment Program. We thank Thomas Genoff and his family for their cooperation. The information given in this publication is for educational purposes only. Mention of a trademark, proprietary product, or vendor does not constitute a guarantee or warranty of the product, nor does it imply approval or disapproval to the exclusion of other products or vendors that may also be suitable. Professor, Dept. of Horticulture and Landscape Architecture, and corresponding author. Professor, Dept. of Entomology and Plant Pathology. Professor, Dept. of Agricultural Economics. AugustBook.indb 1324 6/14/05 12:16:53 PM 1325 HORTSCIENCE VOL. 40(5) AUGUST 2005 Results Genoff’s, 1999. Symptoms of CLS never developed at the Genoff farm, even though the test fi eld was <2 km from the Research Station plots where CLS was present. However, the Genoff’s experiment provided an opportunity to examine treatment effects on yield and phytotoxicity. None of the treatments decreased yield relative to the control (Table 2). A single application of either benomyl or copper hydroxide on 24 Sept. increased yield relative to the control. Yield responded inconsistently to increasing doses of copper hydroxide, but no dose/response patterns for yield were evident with other fungicide treatments (Table 2). Single applications of copper hydroxide or basic copper sulfate did not increase phytotoxicity symptoms relative to the control, but multiple applications did (Table 2). None of the other chemical materials produced symptoms of phytotoxicity. Research station, 1999. None of the treatments decreased yield relative to the control (Table 3). A single application of tebuconazole on 4 Oct. increased yield relative to the control. A delayed single application of tebuconazole (on 10 Oct.) was detrimental to yield compared to the earlier single application on 4 Oct. No dose/response patterns for yield were evident with other fungicide treatments (Table 3). Only one treatment increased phytotoxicity symptoms relative to the control: three applications of basic copper sulfate resulted in a 12.5% incidence of phytotoxicity (other data not presented). Incidence of CLS was reduced relative to the control by two of the treatments with benomyl, all of the treatments with azoxystrobin, one of the treatments with acibenzolar-S-methyl, and two of the treatments with tebuconazole (Table 3). Most of the treatments reduced CLS severity relative to the control (Table 3). Research station, 2000. Yield was decreased relative to the control by three or four applications of acibenzolar-S-methyl (Table 3). There was evidence of decreasing yield as the number of acibenzolar-S-methyl applications increased from one to three. No other treatments had signifi cant effects on yield. Phytotoxicity could not be evaluated in 2000 due to scattered leaf burn from an unusually early freeze before harvest (–5 °C on 9 Oct.). Incidence of CLS was reduced relative to the control by two of the treatments with benomyl, two of the treatments with azoxystrobin, the treatment involving four applications of acibenzolar-S-methyl, and two of the treatments with tebuconazole (Table 3). Each of these treatments, plus a few others, also reduced the severity of CLS relative to the control (Table 3).