© 2007 Plant Management Network.
Sweet Potato Yield Reduction Caused by Reniform Nematode in the Mississippi Delta
Craig A. Abel and Larry C. Adams, Southern Insect Management Research Unit, and Salliana R. Stetina, Crop Genetics and Production Research Unit, USDA-ARS, Stoneville, MS 38776
Abel, C. A., Adams, L. C., and Stetina, S. R. 2007. Sweet potato yield reduction caused by reniform nematode in the Mississippi Delta. Online. Plant Health Progress doi:10.1094/PHP-2007-1115-01-RS.
A study was conducted to determine the effect of variable reniform nematode, Rotylenchulus reniformis Linford and Oliveira, population densities on sweet potato production. In 2003, post-harvest soil samples revealed that nematicide-treated plots had 1019 ± 257.2 reniform nematodes per 500 cm³ sample of soil compared to 2255 ± 383.3 for untreated plots. There were 1057 kg/ha more marketable sweet potato harvested from the nematicide-treated plots compared to the control. In 2004, there were fewer nematodes in the nematicide-treated plots at pre-planting, mid-season, and pre-harvest, with the most pronounced difference occurring at mid-season with 927 nematodes per 500 cm³ sample of soil in the nematicide untreated plots compared to 140 for the treated plots. The difference in nematode levels resulted in a 996-kg/ha increase in US #1 sweet potatoes harvested from nematicide-treated plots compared to the untreated plots. In 2005, two nematicides, K-PAM and Temik, were not different in their level of nematode control during the growing season, however, there was an increase in US #1 class sweet potatoes produced from the Temik-treated plots when compared to K-PAM. Both nematicide treatments produced more US #1 sweet potatoes when compared to a Lorsban control and an untreated control.
Sweet potato, Ipomoea batatas, is an important food crop with approximately 38,700 ha grown in the United States. Sweet potato production is increasing in the Mississippi Delta region where the majority of the crop is grown by small-acreage, low-income farmers (2006 USDA-NASS). This region is a fertile, flat, alluvial plane that has primarily been used for intense cotton production since the early 19th century. The reniform nematode, Rotylenchulus reniformis, occurs in the southeastern US, primarily in areas of intense cotton production (4), with high population densities occurring in the Mississippi Delta (7). The most common method of controlling reniform nematodes is by the use of chemical nematicides.
Reniform nematodes have been associated with yield losses in sweet potato (1) and with cracking leading to lower storage root quality (3). Henn (5) demonstrated the benefits of using nematicides to improve the production of US #1 sweet potatoes. For this study, the first objective was to measure the impact that high populations of reniform nematodes have on sweet potato yield and quality in the Mississippi Delta. The second objective was to compare reniform nematode control in soil treated with two different nematicides.
Transplants of the sweet potato variety Beauregard line B-14 were planted in the field in a Bosket sandy loam soil at Stoneville, MS on 16 May 2003, 25 May 2004, and 2 June 2005. Individual plots were 6.1 m wide by 15.2 m long, containing six rows of plants spaced 3.3 m apart, and transplanted at a rate of 300 plants per plot. Each plot was surrounded by a 4.6-m fallow alleyway. All plots were previously in continuous cotton production. Plots were treated equally with insecticides for controlling foliar and soil insects, and herbicides for weed control. The sweet potatoes were harvested 100 days after planting.
To understand the impact of reniform nematode on sweet potato yield, a treatment consisting of one pre-planting application of 1,3-D (Telone II) applied within furrow at 56.0 liters/ha on 28 March 2003 and 6 April 2004 was compared to a control in which no nematicide was applied. The study was arranged as a randomized complete block with eight replications. To compare nematicides for reniform nematode control, four treatments consisting of one pre-plant application applied in furrow of: (i) aldicarb (Temik 15G) at 11.2 kg/ha combined with chlorpyrifos (Lorsban); (ii) metam sodium (K-Pam) at 74.8 liters/ha combined with chlorpyrifos; (iii) a control treatment of chlorpyrifos; and (iv) a control in which no nematicide or insecticide was applied. Chlorpyrifos was applied at 4.7 liters/ha. All nematicides were applied 30 to 36 cm below the soil surface using a "knife"-type applicator. The study, which was conducted in 2005, was arranged as a randomized complete block with four replications.
Soil samples to quantify reniform nematode densities were collected before planting (3 February) and after harvest (6 October) in 2003 and before planting (19 February) and mid-season (20 July) in 2004. In 2005, soil samples were collected at pre-plant (14 April), mid-season (28 July), pre-harvest (20 August), and post-harvest (7 September). Pre-plant sampling was done to quantify nematodes before nematicides suppressed population densities. Two soil cores (9-cm diameter × 30.5-cm length) were collected from each plot. The soil cores were combined and a 200-m³ sub-sample was used to extract reniform nematodes by elutriation (2) and sucrose centrifugation (6). Vermiform stages retained on a 38-µm-pore sieve were counted to determine population density.
After mechanical harvest, storage roots were examined for damage and classified as US #1 = roots 2 to 3½ inches in diameter, length of 3 to 9 inches, well shaped and free of defects; Canners = roots 1 to 2 inches in diameter and 2 to 7 inches in length; and, Jumbos = roots that exceed the above requirements but are of marketable quality. Nematode levels and yield data were analyzed using the restricted maximum likelihood analysis of variance method and means were separated using the least-squares means method.
Impact on Nematodes and Yield
Fewer reniform nematodes were found in plots treated with 1,3-D than in control plots in 2003 (P = 0.02). The number of nematodes per 500 cm³ of soil at mid-season were 1019 in the treated plots and 2255 in the untreated plots. In 2004, the mid-season samples had 927 nematodes/500 cm³ of soil in untreated plots compared to 140/500 cm³ in nematicide-treated plots. In 2003, there were 1057 kg/ha more (P < 0.01) marketable sweet potato harvested from the treated plots compared to the control. The treated plots produced 518 kg/ha more (P = 0.01) Canner class and 339 kg/ha more (P = 0.04) US #1 class (Fig. 1). In 2004, the sweet potato yields from the nematicide-treated plots were greater than the untreated plots (P < 0.01), but the only quality category that differed was the US #1s (P < 0.01) (Fig. 1).
For the nematicide comparison study, the number of mid-season reniform nematodes in the metam sodium + chlorpyrifos and aldicarb + chlorpyrifos-treated plots was lower than chlorpyrifos alone and the untreated control (Fig. 2). The greatest difference in yield among the quality classes occurred for the US #1 (Fig. 3). The aldicarb + chlorpyrifos treatment had the highest US #1 yields followed by the metam sodium + chlorpyrifos treatment. Total marketable yields in plots treated with aldicarb + chlorpyrifos and metam sodium + chlorpyrifos were the highest and these two treatments did not differ from each other.
In all three years, we detected no nematode physical damage (cracking) to the harvested storage roots when comparing the treated and untreated plots. Therefore, a potential concern for sweet potato production in soils with uniformly high populations of reniform nematodes is that the damage may go unnoticed. Our results indicated that, under high reniform nematode population densities, the application of nematicides before planting is critical to optimize yield of US #1 sweet potatoes. This finding is important because US #1 sweet potatoes provide an approximate 7.2-fold and a 4.6-fold increase in profit when compared to Canner and Jumbo classes, respectively.
This article reports the results of research only. Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the US Department of Agriculture.
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