Detection of Suppressiveness against Rotylenchulus reniformis in Soil from Cotton (Gossypium hirsutum) Fields in Texas and Louisiana

Abstract

Rotylenchulus reniformis is a major problem confronting cotton production in the central part of the cotton belt of the United States of America. In this study, the hypothesis that natural antagonists in some cases are responsible for unusually low densities of the nematode in certain fields was tested by assaying soils from 22 selected fields for the presence of transferable agents in pots containing cotton plants. In one field, soil from four different depth ranges was tested. In the first of two types of assays, 1 part nematode infested soil was added to 9 parts test soil that was left untreated or autoclaved before mixing; this mixture was used to fill pots. In the second type of assay, 1 part test soil was added to 9 or 19 parts pasteurized fine sand, and nematodes were introduced in aqueous suspension. In three experiments representing both types of assay, transferable or autoclavable agent(s) from four fields in South Texas suppressed nematode populations by 48, 78, 90 and 95%. In one experiment, transferable agents in five fields in Louisiana suppressed populations from 37 to 66%. Identification and evaluation of these agents for biological control of R. reniformis merits further study.

The reniform nematode, Rotylenchulus reniformis Linford & Oliveira, is considered the most important nematode of cotton within the central cotton-producing states of Louisiana, Mississippi and Alabama and causes total annual losses to USA cotton production estimated to exceed US$100M (Blasingame, 2006). Nematicides and crop rotation are the primary approaches currently available for management. In many cases, growers recover only a fraction of the profit lost to this nematode (Starr and Page, 1990; Robinson, 2007, 2008).

Little is known about natural antagonists of R. reniformis. However, R. reniformis has been detected in 3,213 fields in Louisiana, and, in about 15 of these fields, the population density of the nematode was observed to remain inexplicably low despite conducive cropping history and soil characteristics, suggesting the presence of potent biological control agents (C. Overstreet, pers. comm.). In other fields, most commonly in the Lower Rio Grande Valley of Texas, population densities in the upper 30 cm of the soil profile are markedly less than in deeper soil, in striking contrast to the more common situation where most nematodes occur within the top 20 cm of soil where root density is greatest.

We hypothesize that in at least some cases, inexplicably low population densities of R. reniformis that have been consistently observed in some cotton fields are the result of suppression by transferable agents in the soil (Stirling, 1991; Westphal and Becker, 2000, 2001; Westphal, 2005). Our specific objective in this study was to test this hypothesis by assaying soil from 22 selected fields in Louisiana, the Mississippi Delta, the Texas High Plains, and the Lower Rio Grande Valley of Texas for the presence of transferable agents suppressing population buildup by R. reniformis in pots.

Materials and Methods

Soil collection: Soil was collected from 22 cotton fields in Louisiana, Mississippi and Texas. In 18 fields, the reniform nematode (Rotylenchulus reniformis) was present in lower population densities than expected based on soil texture, cropping history and infestation levels in other local fields. In some cases, soil samples from different depths in the same field were tested. Unless otherwise stated, soil was collected 15 or more cm deep, or as deep as necessary to be moist and friable. For shipment and storage, the soil was kept at 15 or 20°C within plastic bags to prevent drying.

Experiment 1: In the first of two types of assays, conducted in a greenhouse at Weslaco, TX, in 1999, 1 part nematode-infested soil was added to 9 parts test soil. The latter was either autoclaved or left untreated before mixing. This soil mixture was used to fill 1.2-liter pots. There were four replicate pots/soil origin with autoclaved and four with untreated soil. Autoclaved soil was autoclaved for 30 min on each of two consecutive d. Pots were planted with cotton cv. Suregrow 125 and maintained in a greenhouse for 14 wk, at which time soil was removed, nematodes were extracted by Baermann funnel, and roots were gently washed and weighed. All soil was from fields in the Lower Rio Grande Valley of Texas and included: cotton fields (#1, #2 and #4), an area near a cotton field but uncultivated for 10 yr (#3) and a cotton field 0- to 30-cm deep (#5-A) and 45- to 105-cm deep (#5-B). Soil textures for these fields are presented in Table 1. Nematode densities are expressed per gram soil, and means for autoclaved vs. untreated soil were separated by LSD to test for nematode suppressiveness.

Table 1.

Effects of autoclaving soil from cotton fields in Hidalgo County in the Lower Rio Grande Valley of Texas, on suppressiveness against Rotylenchullus reniformis. [Experiment 1].

Experiments 2 and 3: In a second type of assay, conducted in an environmentally controlled chamber at College Station, TX, in 2006, 1 part test soil was added to 19 (Experiment 2) or 9 (Experiment 3) parts steam-pasteurized fine sand supplemented with vermiculite and balanced nutrients (Robinson et al., 2004, 2007). This mixture was used to fill 0.5-liter pots that were planted with susceptible cotton cv. Fibermax 832.

Treatments in Experiment 2 included soil from two fields (#6 and #7) on the Texas High Plains, one field (#9) in the Mississippi Delta, and two fields (#5 and #8) from the Rio Grande Valley. Field #5, which was the same Field #5 tested in 1999, was represented by newly collected soil from the 0 to 15 cm (#5-C) and 23 to 38 cm (#5-D) depths. Field #5 is also referred to as North farm. Field #8 was on a different farm about 10 km away. Experiment 3 also included new collections of soil from Field #8 and the two depths of Field #5 tested in Experiment 2 (referred to as #5-E and #5-F in Experiment 3), plus soil from 13 fields in six parishes of Louisiana.

In both experiments there were two controls: Fibermax 832 and the resistant accession G. barbadense GB713 planted in sand with no test soil added. In Experiment 3, there were 12 instead of six replicates of the Fibermax 832 control.

Two weeks after planting, each pot was inoculated with 4,000 vermiform R. reniformis previously propagated in the greenhouse on cotton and tomato, and 7 wk after inoculation, three 15-cm³ cores were removed from each pot to evaluate nematode populations in pots (Robinson et al., 2007). Nematode population densities were measured by counting vermiform stages collected by Baermann funnel extraction and compared by Dunnett's test with the Fibermax 832 sand-only control. Roots were not weighed, but plants were confirmed to be comparable in size, and plant heights were measured at the end of Experiment 3.

Results and Discussion

Experiment 1: Autoclaving increased final nematode populations by four- to 18-fold for all soils except #3, which had not been cultivated for 10 years. For other soils, the calculated suppressiveness was 51 to 93% (P ≤ 0.05) (Table 1). Consequently, even though there were appreciable differences in physical and, most likely, chemical characteristics of the soils in which plants were grown, these differences did not influence the effect of autoclaving on nematode population increase.

Experiment 2: The resistant GB713 control in sand with no added test soil suppressed the nematode population 77% (Table 2). Soil #5-C (North farm, 0- to 15-cm deep), which in this experiment was present as 5% of the soil of the pot rather than as 90% in Experiment 1, suppressed the population 80% compared to the Fibermax 832 control (Table 2).

Table 2.

Testing various soils for suppressiveness against Rotylenchulus reniformis, by testing for suppressiveness transferability [Experiment 2].

Experiment 3: Uniform plant heights indicated that adding 10% test soil to sand had a negligible effect on plant growth (Table 3). Soil #5-E, which in this experiment was present at twice the concentration as the comparable soil (#5-C) in Experiment 2 (Table 2), suppressed the nematode population by 95% compared to the Fibermax control (P = 0.01), suggesting a dosage effect. Soil from five fields in Louisiana suppressed populations 37 to 66% (P ≤ 0.05) (Table 3).

Table 3.

Testing various soils for suppressiveness against Rotylenchulus reniformis, by testing for suppressiveness transferability [Experiment 3].

Altogether, suppressiveness was detected in soil from five fields in the Lower Rio Grande Valley of Texas and five fields in Louisiana. The strongest suppression detected (95%) was for soil from field #5 at the North farm in the Lower Rio Grande Valley (LRGV), collected from the upper 15 cm. In Experiments 2 and 3, suppression by soil from the upper 15 cm of Field #5 was comparable to or better than that achieved with the highest level of resistance to R. reniformis known in G. barbadense (Robinson et al., 2004). Also in Experiments 1 and 2, the suppression (51 and 36%) measured for deeper soil in Field #5 was significantly less than that for the 0- to 15-cm layer.

Vertical distributions of R. reniformis in more than 200 graduated vertical samples taken to a depth of 122 cm in 17 fields in Texas, Louisiana, Arkansas, Louisiana, Mississippi, Alabama and Georgia indicate that the mean depth of R. reniformis in cotton tends to be several centimeters deeper than the mean root depth. In most cases, nematode density, like root density, was greatest within the top 30 cm of soil and diminished with depth (Robinson et al., 2005a, 2005b, 2006). However, in at least four fields of the Texas Lower Rio Grande Valley, vertical distribution patterns for R. reniformis were atypical compared to other areas (Robinson and Cook, 2001; Westphal et al., 2004; Robinson et al., 2006), suggesting that the nematodes in the upper profiles were suppressed in the four fields.

Further research is needed to determine the organism (s) involved in population suppression of R. reniformis in some of the more suppressive fields identified in this study. We predict that the antagonist (s) responsible for suppression in the North farm and other Lower Rio Grande Valley fields will be at their greatest densities near the surface.