Evaluation of Methods to Quantify Populations of Rhizoctonia in Soil.

The best method to quantitatively determine populations of Rhizoctonia in soil from soybean fields undergoing rice and soybean rotations was determined for use in a large-scale spatial study to be done over multiple fields and years. The methods evaluated were the toothpick-baiting method, the multiple-pellet soil sampler, and the pour-plate method using elutriated organic matter from soil or surface residue. The toothpick-baiting method was calibrated using the multiple-pellet soil sampler and determined to assay an approximate soil volume of 15.43 cm3. The radius of isolation with the toothpick-baiting technique was approximately 1 cm. In 2009 and 2010, the toothpick method was determined to be the most reliable method for assaying soils, with the most isolates across space and greater recovery of Rhizoctonia solani AG1-IA, R. solani AG11, and R. oryzae, the major Rhizoctonia spp. in these fields, when quantified as propagules per volume of soil or organic matter. In 2011, the recovery of these three groups of Rhizoctonia did not differ statistically when the toothpick-baiting method was compared with the multiple-pellet soil sampler after the volume of soil assayed by the pellet sampler was increased to be similar to that of the toothpick method. However, the labor involved in assaying a similar volume of soil with the multiple-pellet soil sampler was limiting for a large-scale spatial study. The toothpick-baiting method was preferred over the other methods because it was determined to be thorough, inexpensive, nondestructive, and rapid. Additionally, the use of the toothpick-baiting method allows for the determination of the depth of inoculum of isolated fungi for intact soil cores. The mean depth of activity of R. solani AG1-IA, R. solani AG11, and R. oryzae was 1.15, 1.55, and 1.47 cm respectively.

Soil Texture Influence on Meloidogyne incognita and Thielaviopsis basicola and Their Interaction on Cotton.

Microplots were used to evaluate the impact of soil texture on Meloidogyne incognita, Thielaviopsis basicola, and their interaction on cotton. A native silt loam soil (48% sand) and four different artificial soil textures produced by mixing native soil with sand (53, 70, 74, and 87% sand) were studied. Each soil texture was infested with 0, 4, or 8 M. incognita eggs and 0 or 20 T. basicola chlamydospore chains per gram of soil in a factorial treatment arrangement. Plots were watered when soil moisture fell below -10 joules/kg for the first 21 days and -30 joules/kg from 22 days to harvest. Plant growth was suppressed early in the season and midseason by T. basicola. M. incognita suppressed plant growth and delayed plant development late in the season across all soil textures. Cotton yield was lower in the presence of either T. basicola or M. incognita. An interaction between M. incognita and T. basicola, which decreased plant growth and yield, occurred in 2006 when neither pathogen caused substantial plant damage. Plant growth, development, and yield were lowest in soils with >74% sand. Root colonization by T. basicola and fungal reproduction and survival decreased in soil having 87% sand. M. incognita generally caused more galling and reproduction in soils as sand content increased. Root galling severity and M. incognita reproduction were suppressed by the presence of T. basicola in soil at sand contents lower than 87%. Soil texture had a greater impact on T. basicola than on M. incognita in this study.

Effect of Fungicide Seed Treatments, Cultivars, and Soils on Soybean Stand Establishment.

Seedling diseases are one of the major production problems for soybean. The primary control of soybean seedling diseases is by fungicide seed treatments but comparisons of seed treatments are difficult because stand responses are often erratic in the field. The objective of this study was to determine the efficacy of soybean fungicide seed treatments in naturally infested soils under controlled environments at three temperatures. Seed of three soybean cultivars were treated with six fungicide treatments or not treated and planted in two soil types collected from two fields in April, May, and June 2008 and 2009. Tests were conducted in growth chambers at 21°C (April planting), 25°C (May planting), or 28°C (June planting). Stands were determined when at least 25% of the seedlings reached the V4 growth stage. Seed treatments improved stands at all three temperatures, in both soils, and in both years. In general, the broad-spectrum fungicides trifloxystrobin + metalaxyl and mefenoxam + fludioxonil + azoxystrobin resulted in the highest stands. The selective fungicide treatments metalaxyl and pentachloronitrobenzene + carboxin were effective at all three temperatures, implying that Pythium spp. and Rhizoctonia solani, respectively, were involved in seedling disease and active over a range of temperatures. The efficacy of fungicides was not consistently associated with the cultivars used in this study.

Importance of Fungicide Seed Treatment and Environment on Seedling Diseases of Cotton.

The importance of fungicide seed treatments on cotton was examined using a series of standardized fungicide trials from 1993 to 2004. Fungicide seed treatments increased stands over those from seed not treated with fungicides in 119 of 211 trials. Metalaxyl increased stands compared to nontreated seed in 40 of 119 trials having significant fungicide responses, demonstrating the importance of Pythium spp. on stand establishment. Similarly, PCNB seed treatment increased stands compared to nontreated seed for 44 of 119 trials with a significant response, indicating the importance of Rhizoctonia solani in stand losses. Benefits from the use of newer seed treatment chemistries, azoxystrobin and triazoles, were demonstrated by comparison with a historic standard seed treatment, carboxin + PCNB + metalaxyl. Little to no stand improvement was found when minimal soil temperatures averaged 25°C the first 3 days after planting. Stand losses due to seedling pathogens increased dramatically as minimal soil temperatures decreased to 12°C and rainfall increased. The importance of Pythium increased dramatically as minimal soil temperature decreased and rainfall increased, while the importance of R. solani was not affected greatly by planting environment. These multi-year data support the widespread use of seed treatment fungicides for the control of the seedling disease complex on cotton.

First Report of Black Root Rot Caused by Thielaviopsis basicola on Soybean (Glycine max) in Arkansas.

Thielaviopsis basicola (Berk. & Broome) Ferraris (synonym Chalara elegans Nag Raj & Kendrick) is a soilborne plant-pathogenic fungus reported in many parts of the world. In Arkansas, T. basicola is found commonly in cotton fields (4). This fungus colonizes cortical tissue of seedlings under cool wet conditions, causing a dark brown or black discoloration of the roots and hypocotyls, resulting in stunted, slow-developing plants (4). In 2008, large areas of stunted soybean plants with shortened internodes were reported in a field in Phillips County, AR, where cotton had previously been produced. Soybean was planted in this field in early April when cool soil temperatures (~21 to 24°C) prevailed. Soybean plants at the v3 to v5 growth stages were observed to have extensive areas of black cortical root necrosis. Plant samples were collected and roots were excised, washed, and surface disinfested in a 10% NaOCl solution. Root segments were incubated on the carrot-based selective medium TB-CEN (3). T. basicola was isolated from incubated segments after 2 weeks at 21°C in the dark. Chlamydospore chains (44.8 to 56.0 × 8.4 to 11.2 µm) consisting of an average of six spores and endoconidia (8 to 30 × 3 to 5 µm) were observed with a compound microscope. In addition to plant tissue, soil was assayed and confirmed to be positive for T. basicola by the pour plate technique (3) with the medium TB-CEN. Greenhouse trials were conducted to confirm field observations. Soil from the Phillips County field was sterilized and reinfested with 100 CFU of chlaymdospore suspension per gram (dry weight) of soil. Fifty soybean seeds (cv. Schillinger 457) were planted in infested and sterilized soil and grown for 29 days. Results showed that 38% of plants germinated and survived in the T. basicola-infested soil compared with 71% in the sterile soil treatment. Fifteen of the nineteen plants that survived in the infested soil were positive for T. basicola, while all plants in the sterilized soil were negative for the fungus. Soybean has previously been reported to be a host of T. basicola worldwide, but North American reports have been confined to Canada and Michigan, where cool soil temperatures persist for longer periods during the early part of the growing season (1,2). To our knowledge, this is the first report of T. basicola being important in the growth of soybean in warmer latitudes where the pathogen has been observed frequently on cotton and tobacco. In areas where cotton has historically suffered seedling damage from T. basicola, black root rot may become important on soybean as production of the latter crop increases. Since the initial field observation and confirmation in 2008, multiple soybean fields in 10 Arkansas counties have been documented with black root rot, with an estimated 5 to 30% of plants in each field infected. References: (1) T. R. Anderson. Can. J. Plant Pathol. 6:71, 1984. (2) J. L. Lockwood et al. Plant Dis. Rep. 54:849, 1970. (3) L. P. Specht and G. J. Griffin. Can. J. Plant Pathol. 7:438, 1985. (4) N. R. Walker et al. Phytopathology 89:613, 1999.

Resistance of the Soybean Cultivar Archer to Pythium Damping-Off and Root Rot Caused by Several Pythium spp.

Archer, a maturity group I soybean cultivar with demonstrated flood tolerance and resistance to Pythium ultimum, was compared with Hutcheson, a widely planted maturity group V cultivar in Arkansas, for resistance to P. ultimum, P. irregulare, P. aphanidermatum, P. vexans, and group HS. Emergence and establishment assays demonstrated that Archer had greater emergence and fewer disease symptoms after 10 days for all pathogenic species of Pythium than did Hutcheson. Archer also demonstrated higher root weights and fewer disease symptoms compared with Hutcheson in assays conducted for 6 weeks. Similar results were found using two different seed lots of the cultivars, suggesting that the results were not due to seed quality differences. This study indicates that resistance in Archer to Pythium damping-off and root rot compared with Hutcheson is robust, with efficacy over a number of Pythium spp. and a range of plant developmental stages.

Potential for Site-specific Management of Meloidogyne incognita in Cotton Using Soil Textural Zones.

The effect of various edaphic factors on Meloidogyne incognita population densities and cotton yield were evaluated from 2001 to 2003 in a commercial cotton field in southeastern Arkansas. The 6.07-ha field was subdivided into 512 plots (30.5 m x 3.9 m), and each plot was sampled for M. incognita prior to fumigation (Ppre), at planting (Pi), at peak bloom (Pm) and at harvest (Pf) each year. Soil texture (percent sand fraction) and the pre-plant soil fertility levels each year were determined from each plot. To ensure that a range of nematode population densities was available for study, 1,3-dichloropropene was applied in strips (3.9-m wide) at rates of 14.1, 29.2 and 42.2 liter/ha (128 plots each) each year 2 wk prior to planting. Data were evaluated using both stepwise and multiple regression analyses to determine relationships among edaphic factors, nematode population densities and yield. Although Pi and the percent sand fraction of the soil were the most important factors in explaining the variation in cotton yield, regression models only accounted for <26% of the variation in yield. When the same data were evaluated on a more homogeneous large-scale platform based on similar geographic locations, soil types and nematicide treatments, regression models that included both Pi and sand content explained 65%, 86% and 83% of the variability in yield for 2001, 2002 and 2003, respectively. Prediction profiles of the combined effects also demonstrated that damage potential for M. incognita on cotton in this study varied by soil texture.

Soybean Response to Flooded Soil Conditions and the Association with Soilborne Plant Pathogenic Genera.

The role of soilborne pathogens in flood damage on soybeans, Glycine max, was examined using six genotypes representing a reputed range of flood tolerances. Genotypes were planted in single-row plots from 1996 to 1998 with flood treatments of no flood, flood at emergence (3-day duration), or flood at the fourth leaf node growth stage (7-day duration). Three or four days after removing each flood treatment, plant stands were estimated and 15 plants were collected from each plot, weighed, and rated for root discoloration. Roots were assayed for the presence of fungi and other filamentous eukaryotic organisms. Plant stands were reduced by flooding at emergence compared with the nonflooded control. Flooding at both growth stages caused significant increases in root discoloration compared with nonflooded soybeans. Plant weights were reduced in 2 of 3 years for flooding at emergence. Pythium was the only genus of filamentous organisms whose isolation frequency increased with flooding. Of the 60 Pythium isolates evaluated representing the different cultural groups based on appearance and growth rate on potato dextrose agar, cornmeal agar, and V8 agar, 47% were moderately to highly virulent on soybean. Pythium spp. isolated from soybean included the pathogenic species P. ultimum, P. aphanidermatum, P. irregulare, and P. vexans and Group HS. Nonpathogenic P. oligandrum also was isolated from soybean.

The Effect of Pythium ultimum and Soil Flooding on Two Soybean Cultivars.

The effect of flooding and Pythium ultimum on soybean, Glycine max, was determined in a series of greenhouse experiments using the cultivars Hutcheson and Archer. Seeds were planted into pasteurized soil either not infested or infested with sand-cornmeal inoculum of P. ultimum and either flooded at emergence for 2 days or at the four leaf node stage (V4) for 5 days. A nonflooded control was included in each experiment. Seeds placed directly into infested soil resulted in little or no stand for Hutcheson regardless of flood treatment, whereas stand was reduced for Archer only in the flooded infested soil treatment. Additional experiments were conducted by placing seed onto a 2- to 5-mm layer of pathogen-free soil on top of the infested soil. Flooding at emergence reduced plant height, growth stage, and top dry weight for Hutcheson and root fresh weight for both cultivars. Greater reductions for Hutcheson in root weight, and top dry weight in P. ultimum-infested soil in the soil layer experiments, also indicated that Hutcheson was more susceptible than Archer. Flooding alone decreased root weights, and infestation with P. ultimum reduced weights further resulting in an additive effect. This also was the case for plant height, growth stage, and top dry weight for Hutcheson for flooding at emergence. Root discoloration was greatly increased for both cultivars in infested soil flooded at emergence. Similar results were found when plants were flooded at V4; however, the effect was not as great as with flooding at emergence. These studies indicate that Pythium damping-off and root rot may account for a portion of the negative response of soybean to flooding. The results also indicate that Archer has some resistance to P. ultimum.

Effects of Nocturnal Soil Temperatures and Meloidogyne incognita Densities on Cotton Seedling Growth and the Interaction with Thielaviopsis basicola.

Controlled studies were conducted to evaluate the effects of soil temperatures typical of field conditions during the first 6 weeks of the growing season in Arkansas and different population densities of Meloidogyne incognita on damage to cotton (Gossypium hirsutum) seedlings associated with the interaction between M. incognita and Thielaviopsis basicola. Treatments consisted of varying nocturnal temperatures that approximated the temperatures that occurred during the 2001, 2002, and 2003 growing seasons in southeastern Arkansas. Nocturnal temperatures in the study were as follows: high, the first week at 15°C, followed by 3 weeks at 17°C, 1 week at 21°C, and 1 week at 17°C (approximating the 2002 season); medium, 3 weeks at 15°C and 3 weeks at 19°C (approximating the 2003 season); and low, 1 week at 15°C, 1 week at 13°C, 2 weeks at 17°C, 1 week at 15°C, and 1 week at 17°C (approximating the 2001 season). Pathogen population densities were either 0 or 100 chlamydospores of T. basicola per gram of soil and 0, 2,000, 4,000, or 8,000 eggs of M. incognita per 500 cm3of soil. Plant height and fresh top weight increased with an increase in nocturnal temperature across treatments. There were significant reductions in plant growth and development with T. basicola, but not with M. incognita, at these nocturnal temperatures, but decreased plant height and weight were seen where both pathogens were present in comparison with either pathogen alone. Trends of increased disease associated with T. basicola were observed with increasing inoculum rates of M. incognita, indicating that the interaction between T. basicola and M. incognita occurs even at soil temperatures below the minimum temperature reported as necessary for damage from M. incognita.

Viability and stability of biological control agents on cotton and snap bean seeds.

Cotton and snap bean were selected for a multi-year, multi-state regional (south-eastern USA) research project to evaluate the efficacy of both commercial and experimental bacterial and fungal biological control agents for the management of damping-off diseases. The goal for this portion of the project was to determine the viability and stability of biological agents after application to seed. The biological seed treatments used included: (1) Bacillaceae bacteria, (2) non-Bacillaceae bacteria, (3) the fungus Trichoderma and (4) the fungus Beauveria bassiana. Seed assays were conducted to evaluate the following application factors: short-term (< or = 3 months) stability after seed treatment; quality (i.e. isolate purity); compatibility with chemical pesticides and other biocontrol agents; application uniformity between years and plant species. For the bacterial treatments, the Bacillaceae genera (Bacillus and Paenibacillus) maintained the greatest population of bacteria per seed, the best viability over time and the best application uniformity across years and seed type. The non-Bacillaceae genera Burkholderia and Pseudomonas had the least viability and uniformity. Although Beauveria bassiana was only evaluated one year, the seed fungal populations were high and uniform. The seed fungal populations and uniformity for the Trichoderma isolates were more variable, except for the commercial product T-22. However, this product was contaminated with a Streptomyces isolate in both the years that it was evaluated. The study demonstrated that Bacillaceae can be mixed with Trichoderma isolates or with numerous pesticides to provide an integrated pest control/growth enhancement package.

Influence of Meloidogyne incognita and Thielaviopsis basicola Populations on Early-Season Disease Development and Cotton Growth.

Microplot studies were used to examine the effect of various population densities of Meloidogyne incognita and Thielaviopsis basicola on cotton-plant development and disease severity. Plots were infested with 0, 20, or 100 T. basicola chlamydospores/g and 0, 5, or 10 M. incognita eggs and juveniles/cm3 of soil in a factorial arrangement in 1997 and 1998. Combinations of M. incognita and T. basicola reduced plant survival in both years compared to the noninfested control, except in 1998 for the high rate of T. basicola over all nematode rates. Plant height-to-node ratios were reduced by pathogen combinations compared to the noninfested control or to either pathogen alone. Plant dry weight was reduced by M. incognita in 1998 and the high rate of T. basicola in 1997. Root necrosis was increased by increasing rates of T. basicola in 1997 and by M. incognita over all rates of T. basicola in both years. Colonization of root tissue by T. basicola was increased by the low inoculum density of the nematode at 20 CFU/g soil in 1997 and 100 CFU/g in 1998. Nematode reproduction with the high M. incognita treatment rate was reduced in both years of the study by the high T. basicola rate. This study suggests the importance of population level of each pathogen to the severity of disease and confirms the potential of this disease interaction to impact cotton production.

Effect of Temperature on and Histopathology of the Interaction Between Meloidogyne incognita and Thielaviopsis basicola on Cotton.

ABSTRACT Controlled environments were used to study the relationship between the root-knot nematode (Meloidogyne incognita) and Thielaviopsis basicola on cotton. Temperature treatments were continuous 20, 24, and 28 degrees C or two cyclic linear regimes with ranges of 14 to 32 or 18 to 28 degrees C over 24 h. Cotton seeds were planted in fumigated soil infested with T. basicola, M. incognita, or both. After 42 days, pathogen effects on plant growth and pathogen development were evaluated. Histology was conducted on roots collected 14, 28, and 42 days after planting in the continuous 24 degrees C treatment. Reductions in plant height-to-node ratio and total fresh weight were observed for soils infested with both pathogens compared with the control or with soils infested with either pathogen, except for M. incognita-infested soil at 28 degrees C. T. basicola reduced root galling and reproduction of the nematode at all temperatures. Vascular discoloration caused by T. basicola was greater in the presence of M. incognita compared with that by T. basicola alone. At 2 and 4 weeks, histological studies showed that plants grown in all T. basicola-infested soils contained chlamydospore chains on the root surface and in cortical cells. The fungus was not observed inside the vascular cylinder. Roots from 4-week-old plants from soils infested with T. basicola and M. incognita showed fungal sporulation in vascular tissue and localized necrosis of vascular tissue adjacent to the nematodes. At 6 weeks, plants grown in soil infested with T. basicola alone exhibited no remaining cortical tissue and no evidence of vascular colonization by the fungus. Six-week-old plants grown in T. basicola + M. incognita-infested soils exhibited extensive vascular necrosis and sporulation within vascular tissue. These studies suggest that coinfection expands the temperature ranges at which the pathogens are able to cause plant damage. Further, M. incognita greatly increases the access of T. basicola to vascular tissue.

Interaction Between Meloidogyne incognita and Thielaviopsis basicola on Cotton (Gossypium hirsutum).

The effects of Meloidogyne incognita and Thielaviopsis basicola on the growth of cotton (Gossypium hirsutum) and the effects of T. basicola on M. incognita populations were evaluated in a 2-year study. Microplots were infested with M. incognita, T. basicola, or a combination of M. incognita and T. basicola. Uninfested plots served as controls both years. Seedling survival was decreased by the M. incognita + T. basicola treatment compared to the control. Meloidogyne incognita alone and M. incognita + T. basicola reduced plant height-to-node ratio for seedlings in both years. Seed cotton yield was reduced, and the length of time required for boll maturation was lengthened by M. incognita + T. basicola in 1994 and M. incognita both alone and with T. basicola in 1995. Position of the first sympodial node on the main stem was increased by M. incognita in both years and was higher for plants treated with M. incognita + T. basicola in 1995 in comparison to the control. The number of sympodial branches with bolls in the first and second fruiting position and the percentage of bolls retained in the second position were reduced both years by M. incognita + T. basicola compared to either the control or T. basicola alone. Orthogonal contrasts indicated that effects on height-to-node ratio, number of days to first cracked boll, and yield were significantly different for combined pathogen inoculations than with either pathogen alone. Meloidogyne incognita eggs at harvest were reduced by T. basicola in 1994 and 1995 compared to M. incognita alone. The study demonstrated a significant interaction between M. incognita and T. basicola on cotton that impacted the survival and development of cotton and the reproduction of M. incognita on cotton.

Characterization of the Suppressiveness of Hairy Vetch-Amended Soils to Thielaviopsis basicola.

ABSTRACT Factor(s) involved in soil suppressiveness to Thielaviopsis basicola when hairy vetch was used as a green manure were studied in a cotton production system. Soil suppressiveness was assessed in vitro at hairy vetch amendment levels of 0, 0.25, and 0.75% (wt/wt) by observing chlamydospores, using a nylon fabric technique. Chlamydospore germination in all soils was below 5%, and microscopic examination showed no germ tube lysis or visible propagule destruction. Viability (chlamydospore germination on T. basicola-carrot-etridiazol-nystatin [TB-CEN] medium) was reduced by 29% within 48 h after hairy vetch amendment. Viability also was reduced in atmospheres of amended soils, suggesting that the suppressiveness was due to a volatile factor. In a field study, chlamydospore viability in amended soils was reduced by 16%. T. basicola hyphal growth was more sensitive to ammonia than Rhizoctonia solani or Pythium ultimum, and chlamydospore mortality of T. basicola was 100% in petri dish atmospheres with 0.4 ppm of ammonia (50% lethal dose = 0.15 ppm). Soil atmospheric ammonia was 0.08 and 0.10 ppm for 0.25 and 0.75% amendment levels, respectively, both at 3 and 7 days after incorporation. In the field, 0.11 and 0.14 ppm of ammonia were detected in soil atmospheres 3 and 7 days after incorporation, respectively. The levels of ammonia detected were sufficient to account for the loss in T. basicola chlamydospore viability, indicating that ammonia is responsible for the suppressiveness observed.

Efficacy of Bacterial Seed Treatments for Controlling Pythium Root Rot of Winter Wheat.

Pythium root rot, caused by various Pythium spp., is a widespread disease of wheat. The objective of this study was to identify bacterial strains from wheat roots in Arkansas that suppressed Pythium root rot and to compare their efficacy with that of bacterial strains from other areas. Bacterial strains (applied as seed treatments) that suppressed Pythium root rot in growth chamber assays were evaluated further for in vitro antibiosis against three Pythium spp. and for efficacy under field conditions. Pseudomonas fluorescens strain 2-79R, Burkholderia cepacia strain 1-23, and Pseudomonas sp. strain 1-30 were the most effective for suppressing Pythium root rot under field conditions and significantly (P = 0.10) increased yield in one experiment. Strains that were effective in the field also expressed in vitro antibiosis to at least two of three Pythium spp.; however, strains expressing the highest levels of antibiosis were not effective in the field. In the field, root rot suppression and yield enhancement were inconsistent across experiments and generally small in magnitude. Therefore, these strains have little potential for commercial use under the conditions in which they were tested.

Importance of antibiotic production in antagonism of selected Streptomyces species to two soil-borne plant pathogens.

Antagonism of ten Streptomyces spp., five of which produce antibiotics, to the plant pathogens Rhizoctonia solani and Phytophthora megasperma var. sojae was studied. Antibiotic activity was detected in culture for the five antibiotic producers. S. griseus, S. hygroscopicus var. geldanus, and S. noursei produced wide zones of inhibition to R. solani and P. megasperma var. sojae. Similar activity was found for S. reticuli var. protomycicus to P. megasperma var. sojae. S. cellulosae reduced Rhizoctonia root rot on pea when sterile soil was infested simultaneously with the antagonist and R. solani. S. hygroscopicus gave almost complete disease control when the streptomycete was added 7 days prior to infesting the soil with R. solani. Several of the Streptomyces spp. reduced Phytophthora root rot on soybean when the streptomycetes were added to soil at the same time as P. megasperma var. sojae or 7 days prior to adding the pathogen. S. herbaricolor and S. coeruleofuscus gave the most consistent control. No relationship was found between reported antibiotic activity or antagonism on agar media and reduction in disease severity. Only S. hygroscopicus var. geldanus gave both control of Rhizoctonia root rot and large zones of inhibition on agar media when the streptomycetes were preincubated in soil for 7 days.