Fungicide Seed Treatment Effects on Emergence of Deeply Planted Winter Wheat

Abstract

Winter wheat ( Triticum aestivum L.) seed is often planted deeply (>3 in.) into stubble‐mulch fallow in the semi‐arid Pacific Northwest (PNW). Stand establishment is affected by factors that influence the rate of seed germination and coleop‐tile growth. Systemically translocated triazole and imidazole fungicide seed treatments were compared for effects on seedling emergence under conditions of deep seed placement and variable seed‐zone moisture in stubble‐mulch fallow systems. Fungicides included RTU Vitavax Thiram (carboxin [2,3‐dihydro‐5‐carboxanilido‐6‐methyl‐l,4‐oxathiin] + thiram [tetramethylthiuram disulfide]), Dividend (difenoconazole [l‐ (2‐[4‐(4‐chlorophenoxy)‐2‐chlorophenyl]‐4‐methyl‐1,3‐diox‐olan‐2‐yl‐methyl)‐1H‐1,2,4‐triazole]), Raxil Thiram (tebuconazole [α[2‐(4‐chlorophenyl)ethyl]‐α‐(l,l‐dimethylethyl)‐lH‐1,2,4‐triazole‐l‐ethanol] + thiram), NuZone and Flo‐Pro IMZ (imazalil [1‐(2‐(2,4‐dichlorophenyl)‐2‐(2‐propenyloxy)ethyl)‐1H‐imidazole]), and Apron (metalaxyl [N‐(2,6—dimethylphenyl)‐N‐(methoxyacety1)‐alanine methyl ester)]). Gaucho (imidacloprid [1[(6‐chloro‐3‐pyridinyl)methyl]‐N‐nitro‐2‐imidazoli‐dinimine]), a seed‐treatment insecticide, was applied in some experiments. Seven field and three greenhouse experiments were performed from 1993 to 1995. Emergence in the field tended to be superior for seed treated with Dividend, alone or mixed with Apron, in plantings made from 1‐ to 5‐in. deep into soil with 5 to 17% seed‐zone water content and at 71 to 80°F. RTU Vitavax Thiram had variable effects on seedling emergence. NuZone and Raxil Thiram, alone or mixed with Gaucho, generally delayed emergence when treated seed was planted at depths more than 2 in. In the greenhouse, treated seed was planted at 1‐in. depth into moist (7, lO or 15% w/w), warm (75°F) silt loam overlain by 4‐in. of dry silt loam, simulating a planting at 5‐in. depth into stubble‐mulch fallow during September. Emergence of Dividend‐treated seed at each soil moisture content was usually more rapid than for other treatments, including seed planted without fungicide treatment. This research demonstrated that emergence of winter wheat planted deeply into stubble‐mulch fallow is often improved when seed is treated with Dividend compared with RTU Vitavax Thiram and, among the systemically translocated triazole fungicides, Dividend is a better choice than Raxil when planting depths exceed 2 in. Research Question Fungicide seed treatments are required to protect winter wheat from infection by smut pathogens on about 5 million acres in the Pacific Northwest (PNW). Dividend and Raxil Thiram seed treatments were recently registered to compete in a market dominated by RTU Vitavax Thiram. NuZone, FloPro IMZ or Apron are sometimes applied as a co‐fungicide with the smut‐controlling fungicide, to broaden the spectrum of diseases controlled. Gaucho is a new insecticide seed treatment with potential for controlling damage from aphids and aphid‐transmitted viruses. This research determined comparative effects of these seed treatments on seedling emergence in stubble‐mulch tillage systems in the field and as simulated in the greenhouse. Literature Summary Dividend‐treated wheat seed appeared to emerge from a stubble‐mulch fallow more quickly than seed treated with Vitavax Thiram during 1992. This effect was not quantified. Dividend belongs to the triazole class of fungicides that have potential for growth regulatory activity. Growth can be improved at very low rates of application and severely curtailed at high rates. Another recently registered triazole, Raxil, is not recommended for planting at depths below 2 in. Imidazole fbngicides (FloPro IMZ and NuZone) have also reduced emergence under certain circumstances. The triazole and imidazole fungicides have potential for shortening the length or growth rate of subcrown internodes, thereby influencing emergence and stand development. Deep seed placement is routinely practiced in PNW stubble‐mulch fallow systems that are characterized by a deep dust mulch over moist soil. Emergence of seedlings from depths of 2‐ to 5‐in. is required, and stand failure often precludes the opportunity for replanting at the time when optimum yield potential can be achieved. Stand failures also occur when rain causes surface crusting on the dust mulch before seedlings emerge. Any fungicide that reduces the rate of emergence increases the risk of stand failure for wheat in semi‐arid nonirrigated regions of the PNW. Study Description Established and recently registered fungicide seed treatments were compared for effects on seedling emergence under conditions of deep placement in stubble‐mulch fallow systems. Three greenhouse and seven field experiments were performed from 1993 to 1995. Emphasis was on RTU Vitavax Thiram (carboxin + thiram), Dividend (difenoconazole), Raxil Thiram (tebuconazole + thiram), NuZone or Flo‐Pro IMZ (imazalil), and Apron (metalaxyl). The insecticide Gaucho (irnidacloprid) was mixed with Raxil Thiram in selected field tests. In the greenhouse, treated seed was planted at 1‐in. depth into moist (7, 10, or 15% water), warm (75°F) silt loam overlain by 4‐in. of dry silt loam, simulating a planting at 5‐in. depth into stubble‐mulch fallow during September. Experiments in the field were located in eastern Oregon and Washington where winter wheat‐summer fallow rotations are used in 10‐ to 16‐in. precipitation zones. Plantings in the field were made at 1‐ to 5‐in. depths, seed‐zone water contents of 5 to 17%, and seed‐zone soil temperatures of 71 to 80°F. Applied Questions Do modern seed treatment fungicides and insecticides have positive or negative effects on seedling emergence when winter wheat seed is planted deeply or at suboptimal soil moisture contents for rapid seedling emergence in PNW stubble‐mulch fallow systems? This research demonstrated that emergence of winter wheat treated with Dividend or Dividend + Apron is generally equal to or better than that treated with RTU Vitavax Thiram (Table ), that Dividend is a better choice of systemically translocated triazole chemistry than Raxil Thiram when planting depths are greater than 2 in., and that NuZone tended to delay emergence from seed planted deeply into stubble‐mulch fallow. Raxil Thiram delayed emergence only when treated seed was planted at depths exceeding the maximum recommended depth (2 in.) for this hngicide, and this effect was neither improved nor worsened by mixing Gaucho insecticide with Raxil Thiram. Emergence of Dividend‐treated seed at 7, 10, and 15% seed‐zone moisture contents in the greenhouse was usually more rapid than for treatments containing RTU Vitavax Thiram, NuZone, Raxil Thiram, or seed planted without fungicide treatment. Emergence of winter wheat seedlings at weekly intervals after planting at 5‐in. depth near Benton City, WA, during 1993. Emergence ratings on four dates Final stand Treatment (fl oz/cwt) 14 21 28 34 counts Control 1.5 2.1 2.1 2.0 4.7 Dividend (0.5) 2.2 2.6 2.5 2.6 7.0 Dividend + NuZone 0.9 1.3 1.5 1.5 5.2 (0.5+1.5) RTU Wtavax Thiram (5) 0.9 1.5 1.8 1.7 2.3 RTU Vitavax Thiram 0.4 0.8 1.4 1.3 4.3 + NuZone (5+l.5) LSD ( P = 0.05) 0.5 0.5 0.4 0.4 1.1 Signif. of F: <0.01 <0.01 <0.01 <0.01 <0.01 Data within each column differ ( P < 0.05) from emergence of untreated seed. Emergence was rated visually 14, 21, 28, and 34 d after planting. Ratings were as follows: 0 = < 10% of seedlings emerged, 1 = 10–50%, 2 = 50–85%, and 3 = >85%. Data were combined for two cultivars; a soft‐white winter wheat (cv. Rohde) and a hard‐red winter wheat (cv. Weston). Stand counts (seedlings per foot of row) for Weston wheat 34 d after planting.

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