Biocontrol of Damping-off Diseases Caused by Rhizoctonia solani and Pythium ultimum with Alginate Prills of Gliocladium virens, Trichoderma hamatum and Various Food Bases

Alginate prills were formulated with the biomass of isolates of Gliocladium virens and Trichoderma spp. and various food bases (wheat bran, corn cobs, peanut hulls, soy fiber, castor pomace, cocoa hulls and chitin). Alginate prills with G. virens (Gl-21) biomass and all food bases except cocoa hull meal significantly reduced the damping-off of zinnia in a soil-less mix caused by Rhizoctonia solani and Pythium ultimum. The prills with bran, soy fiber, castor pomace or chitin resulted in stands similar to those in the non-infested control. In soil, prills with all the food bases and Thrichoderma hamatum (TRI-4) biomass controlled the damping-off of cotton caused by R. solani and gave stands comparable to, or better than, those in the non-infested control soil. Prills with all the food bases resulted in a proliferation of Gl-21 in a soil-less mix and of Gl-21 and TRI-4 in soil. Prills with food bases and TRI-4 biomass reduced the survival of R. solani in infested beet seed to less than 30%, with bran and chitin being the most effective food bases; prills with Gl-21 biomass and all food bases also reduced the survival of R. solani in beet seed, but not as much as did prills with TRI-4 biomass. In prills containing wheat bran, soy fiber or chitin, the biocontrol isolate Th-58 (T. harzianum) was almost as effective as TRI-4, but isolate Gl-3 (G. virens) was less effective. There was no significant interaction between the biocontrol fungus and the food base. The results suggest that the intrinsic properties of a selected fungus isolate are more important than some formulation variables in biocontrol.     

A new formulation system for the application of biocontrol fungi to soil

A new biocontrol formulation system was devised that does not require sterile conditions during preparation. It involves mixing vermiculite and powdered wheat bran with wet or dry fermentor biomass of Trichoderma spp. or Gliocladium virens, moistening with 0.05 N HCl, and drying the mixture. Before application to soil, the preparation (VBA‐FB) is activated by re‐moistening with 0.05 N HCl and incubated at room temperature for 2–3 days to stimulate development of young hyphae of the biocontrol fungus. Populations of biocontrol fungi proliferated to greater than 10⁷ colony‐forming units (cfu) per g of soil when activated VBA‐FB was added to soil. In soil artificially infested with Rhizoctonia solani, seven isolates of the 14 studied added as VBA‐FB reduced survival and 12 reduced saprophytic growth of the pathogen. Of these, two isolates of T. hamatum (TRI‐4, Tm‐23) and one of T. harzianum (Th‐87) were the most effective. Preparations formulated with either wet or dry biomass effectively reduced pathogen survival, but activated VBA‐FB was more effective than non‐activated VBA‐FB. Storage of VBA‐FB at 25°C for 24 weeks before activation reduced viability of isolates considerably more than storage at 5°C for 24 weeks. In addition, VBA‐FB stored at 5°C before activation more effectively reduced survival of R. solani than VBA‐FB stored at 25° C. Survival of R. solani was reduced by activated VBA‐FB applied to several soil types (sandy loam, sandy clay loam, clay). Some nitrogen fertilizers increased the efficacy of VBA‐FB preparations of several isolates.     

Biological Control of Fusarium spp. in Cotton, Wheat and Muskmelon by Trichoderma harzianum

A new isolate of Trichoderma harzianum (T-35) was isolated from the rhizosphere of cotton plants from a field infested with Fusarium. Under glasshouse conditions, the antagonist was applied to soil growing in a bran/peat mixture (1:1, v/v) or as a conidial suspension or used as a seed coating. When T. harzianum was tested against Fusarium oxysporum f. sp. vasinfectum, F. oxysporum f. sp. melonis or F. roseum‘Culmorum”, a significant disease reduction, was obtained in cotton, melon and wheat, respectively. Biological control of Fusarium wilt of cotton was achieved when tested at two inoculum levels of the pathogen (2 × 10⁷ and 2 × 10⁸ microconidia/kg soil), decreasing the Fusarium spp. soil population. The long term effect of T. harzianum on Fusarium wilt of cotton was studied using successive plantings. The antagonist persisted in soil throughout three consecutive plantings, reducing the Fusarium, wilt incidence in each growth cycle. At the first planting the largest amount of preparation was found superior, whereas at the third planting, no significant difference could be observed between the four rates of Trichoderma preparation. T. harzianum (T-35) controlled Fusarium wilt in cotton and muskmelon when applied in both naturally or artificially infested alluvial vertisol and sandy-loam soils, respectively. Soil or seed treatments with the antagonist provided a similar disease control of F. roseum‘Culmorum’ and of F. oxysporum f. sp. melonis.     

Chemical,physical and biological control of carrot seedling diseases

In naturally infested soil containingPythium ultimum, P. acanthicum andPhytophthora megasperma, onlyP. ultimum was associated with root rot and damped-off seedlings. Damping-off was promoted by low soil temperatures and by flooding. Seedling stands were markedly reduced when seed was pre-incubated in soil at 12°C but not at 25°C or 35°C. Dusting carrot seed with metalaxyl significantly increased seedling stands in the field at rates from 1.5–6 g kg⁻¹ seed and in both flooded and unflooded, naturally infested soil at 3.15 g kg⁻¹. In greenhouse experiments using artifically infested soil,P. ultimum andP. paroecandrum caused damping-off of carrot seedlings andRhizoctonia solani reduced root and shoot weights.R. solani caused damping-off in nutrient-enriched soil.P. acanthicum andP. megasperma were not pathogenic to seedlings, although both fungi colonized roots. Soil populations of allPythium spp., particularlyP. ultimum, increased during growth of seedlings and population growth ofP. megasperma was promoted by periodic flooding. Infestation of soil withP. acanthicum did not reduce damping-off of carrot seedlings byP. ultimum orP. paroecandrum, but significantly increased root and shoot weights and decreased root colonization byR. solani P. acanthicum has potential as a biocontrol agent againstR. solani.     

Formulation of the Biocontrol FungusCladorrhinum foecundissimumto Reduce Damping-Off Diseases Caused byRhizoctonia solaniandPythium ultimum

Five isolates ofCladorrhinum foecundissimum, added to soilless mix as 10-day-old fresh bran preparations (1.0% w/w), significantly reduced (P≤ 0.05) damping-off of eggplant and pepper caused byRhizoctonia solanistrain R-23. After 4 weeks of growth, plant stands in the biocontrol-amended, pathogen-infested treatments (>80%) were comparable to those in the noninfested controls. Since plant stands were similar at 2 and 4 weeks, most of the disease was preemergence damping-off. The bran preparations also reduced saprophytic growth of the pathogen, and there was an inverse correlation (r²= −0.94) between saprophytic growth and eggplant stand. Added to soilless mix at a rate of 2.0% (w/w), alginate prill containing 20% fermentor-produced biomass of six biocontrol isolates ofC. foecundissimumreduced (P≤ 0.05) damping-off of eggplant caused byR. solani, but only the prill with biomass of isolates Cf-1 or Cf-2 yielded plant stands (>80%) comparable to that in the noninfested control. As with the bran preparations, there was also an inverse correlation (r²= −0.80) between saprophytic growth of R-23 and eggplant stand with the alginate prills. Alginate prill with biomass of Cf-1 or Cf-2 also reduced (P≤ 0.05) damping-off of eggplant and pepper caused by other isolates (195, NG-2, DPR-1) ofR. solani, but only the stands (>80%) of pepper were similar to that in the noninfested control. Alginate prill formulations ofC. foecundissimum(Cf-1, Cf-2, and Cf-3) also reduced (P≤ 0.05) populations of the pathogen and damping-off of eggplant and pepper caused byPythium ultimum(PuZ3). However, although the plant stands in the treatments were not as high as those in the noninfested controls, they were higher than those in the pathogen-infested controls. The treatments also reduced populations ofP. ultimumin the soilless mix so that there were inverse correlations between the pathogen population and eggplant stand (r²= −0.81) and pepper stand (r²= −0.78). Extruded flour/clay granules containing 5.0% biomass of Cf-1 and Cf-2, added toR. solani-infested soilless mix (2.0%), reduced (P≤ 0.05) damping-off of eggplant and pepper. However, only the Cf-2 treatments resulted in stands (>80%) equal to those in the noninfested controls for the crops after 4 weeks of growth. The influence of bran and alginate prill of Cf-1 or Cf-2 on the spatial spread ofR. solaniand its ability to incite damping-off of eggplant showed that prill with Cf-1 or Cf-2 and bran with Cf-2 were equally effective in reducing the spread of the pathogen from the point source of the inoculum to the center of the flats.     

Superiority of a soil debris isolation method over a beet seed colonization method for assay of Rhizoctonia solani at high soil inoculum densities.

A quantitative soil debris isolation method (all debris from known weight of soil plated) and a garden beet seed saprophytic colonization method were compared over a 1-year period for assaying Rhizoctonia solani population. Four fields of different soil textures were selected. Within each field four areas of healthy and four areas of diseases (rhizoctonia root and crown rot) sugarbeets were sampled bimonthly from August 1976 until June 1977. The maximum numbers of R. solani colonies obtained by the debris method were 2 per gram of soil in areas of healthy beets, and 11 per gram of soil in areas of diseased sugarbeets. At such high inoculum densities the beet seed colonization method underestimated R. solani populations, because the inoculum per unit of soil exceeded the numbers of beet seeds per unit of soil available for colonization. Modifications of the beet seed method did not significantly alter results of colonization assays. Ranked correlation comparisons of assay methods yielded r = 0.81 for all data.     

Inoculum dynamics ofGliocladium virens associated with roots of cotton seedlings

A system was developed to evaluate the effects of root growth of cotton seedlings on the inoculum dynamics ofGliocladium virens in nonsterile soil. In soil infested withG. virens, inoculum densities of the fungus increased when plants remained alive. After 30 days, shoots were excised and the roots allowed to deteriorate. During this portion of the experiment (30–60 days) soil inoculum densities ofG. virens declined. In infested soil without a seedling, inoculum densities remained constant throughout the duration of the experiments. Colonization of roots byG. virens was found to increase throughout the duration of the experiments. At 60 daysG. virens was recovered from approximately 60% of the root pieces (1-cm) sampled. The percentage of primary, secondary, or tertiary roots colonized was different (P = 0.01), but the total colonization of roots at three depths (0–10, 10–20, and 20–30 cm) was not different (P = 0.64). In noninfested soil, colonization of roots by indigenous propagules ofG. virens was never greater than 3%. Offprint requests to: C. M. Kenerley.     

Survival of Sclerotia of Rhizoctonia solani AG3PT and Effect of Soil‐Borne Inoculum Density on Disease Development on Potato

Sclerotia produced by a single isolate of Rhizoctonia solani AG3PT were buried in small plot experiments to investigate the effects of sclerotial production method, soil type and burial depth on sclerotial viability in field soil. The factor with the greatest effect on sclerotial viability, defined as the percentage of sclerotia germinating on agar following retrieval, in all experiments was the duration of burial. After 18 months, on average across all experiments, 20% of retrieved sclerotia were viable. A comparison between sclerotia produced in vitro on malt yeast extract agar and in vivo using micropropagated tubers in field soil found no significant differences between the two production methods on sclerotial viability. Burial in field soil at 20‐cm depth was found to significantly reduce sclerotial viability to 50% compared to 60% at 5 cm. In two pot experiments, amending the growing medium and soil with increasing inoculum densities of R. solani was found to increase stem number, stem canker and black scurf severity regardless of whether this soil‐borne inoculum was derived from mycelium or sclerotia. Black scurf incidence and severity were assessed 30–32 days posthaulm destruction and found to be similar for a range of sclerotial soil‐borne inoculum densities (1.0 × 10^?1 g/kg d.w. soil to 6 × 10^?3 g/kg d.w. soil). The significance of these findings in relation to pathogen survival, detection in soil and disease development is discussed.     

Influence of inoculum density of the antagonistic bacteria Pseudomonas fluorescens and Pseudomonas corrugata on sugar beet seedling colonisation and suppression of Pythium damping off

The effect of initial inoculum density of the antagonistic bacterial strains Pseudomonas fluorescens B5 and Pseudomonas corrugata 2140 (10³ to 10⁸ CFU per seed pellet) on sugar beet seedling colonisation, in situ bioluminescence and antagonistic activity towards Pythium ultimum was investigated. Populations of the bacteria colonising sugar beet root systems approached an apparent carrying capacity of 10⁵ to 10⁶ CFU per plant after 12 d growth, irrespective of inoculum density. This meant an up to 320-fold population increase at low inoculum densities and a decrease at high densities. Population densities of both bacteria and their corresponding in situ bioluminescence (resulting from luciferase enzyme expression from the inserted luxAB genes) reached highest levels in the hypocotyl region and in the upper root region 0–20 mm below seed level (10⁴–10⁶ CFU/cm section, 10¹–10³ RLU/cm section) and decreased with root depth. In situ bioluminescence, which indicates physiological activity, was measurable at lowest antagonist initial inoculum density (10³ CFU per seed pellet) and did not increase significantly with increasing inoculum density. Bioluminescence was also significantly correlated with population density. For Pseudomonas fluorescens B5, the total population size per plant and downward colonisation of the root (below 40 mm depth) increased significantly with antagonist inoculum density applied to the seeds. For Pseudomonas corrugata 2140, no significant influence of initial inoculum density on root colonisation was observable. Survival and dry weight of sugar beet seedlings in Pythium infested soil increased significantly with increasing inoculum density of Pseudomonas fluorescens B5, whereas for Pseudomonas corrugata 2140, initial densities of 10⁴ to 10⁶ CFU per seed resulted in maximal survival of plants.     

Development of pythium ultimum in soil in relation to presence and germination of seeds of different crops

Summary A quantitative study was made to determine the effect of various crop seeds on the population ofPythium ultimum in naturally infested soil. Seeds of corn, peas, watermelon, wheat, barley, beet, and cabbage caused an increase inPythium population within 48 hours of planting. The extent of population rise varied with plant species sown, distance of soil from seed, and duration of stay of seeds in the soil. This rise in the population ofPythium in the soil has been attributed to non-specific stimulation provided by amino acids and sucrose in the seed exudates.The author wishes to express his grateful thanks to the United States Agency for International Development for giving him a traineeship during 1960 and to Dr. J. E.Mitchell of the Department of Plant Pathology, University of Wisconsin, U.S.A. for providing facilities, encouragement, and guidance.     

The Influence of the Inoculum Source of Rhizoctonia solani on Development of Black Scurf on Potato

Rhizoctonia solani, the causal agent of stem canker and black scurf on potato, survives as sclerotia on tubers, in soil and in plant residues. The objective of the present study was to evaluate the importance of inoculum source on disease development. Disease‐free minitubers and seed tubers contaminated with low levels of R. solani were planted in fumigated or artificially inoculated growth mixture in greenhouse experiments. Black scurf incidence and severity were significantly higher when the inoculum was present in both seed tubers and soil, compared with either of them separately. The severity of disease symptoms on the subterranean parts of the plant also were significantly higher in plots where both seed tubers and soil were contaminated, compared with plots where the inoculum source was either the seed tubers or the soil. Thus, both major sources of inoculum, seed tubers and soil, are important in disease development. However, when both sources are present, black scurf incidence and severity are increased, leading to economical damage to tuber yield and quality. Additional results from field trials support these findings. Disease incidence and severity on daughter tubers were correlated with levels of contamination in seed tubers and soil. When seed tubers and soil were heavily infested, the levels of black scurf incidence and severity on daughter tubers were very high; when seed tuber and soil infestation were very low, black scurf incidence and severity on progeny were also lower. Disease levels were reduced by in‐furrow fungicide treatment, but were less effective when the initial levels of the fungus on the seed tubers and in the soil were high.     

Induction of cytoplasmic leakage from Rhizoctonia solani hyphae by Gliocladium virens and partial characterization of a leakage factor

The biocontrol fungus Gliocladium virens (Gl‐21)was grown on various solid and liquid substrates. Aqueous extracts of wheat bran and peanut hull meal (PHM)as well as spent glucose tartrate broth (GTB), Czapek‐Dox broth (CDB) and potato dextrose broth (PDB) upon which Gl‐21 was grown caused leakage of carbohydrates and electrolytes from hyphae of the soilborne plant pathogen Rhizoctonia solani. In addition, the mycelial weight of R. solani was reduced. Most of the leakage factor was formed in the substrates within six days of incubation. Acidification of the bran and the PHM media before inoculation stimulated production of the leakage factor by G. virens. Size‐fractionation experiments indicated that a combination of factors produced by G. virens induced leakage from R. solani. Although the < 1 k Da fraction from the water‐extracted bran culture contained significant leakage activity, it was less than in the non‐fractionated bran culture. The > 1 k Da fraction from bran culture did not induce leakage activity. When the < 1 k Da and the > 1 k Da fractions were recombined, leakage activity was restored to a level similar to that of the non‐fractionated preparation. Gliotoxin was detected in culture filtrates from G. virens grown on bran and PHM media. Gliotoxin preparations induced leakage of carbohydrates and electrolytes from R. solani and caused a concomitant reduction in mycelial weight, which suggests that it is a leakage factor.     

Biocontrol of Rhizoctonia Damping-off of Tomato by Bacillus subtilis Combined with Burkholderia cepacia

Bacillus subtilis strain RB14‐C and Burkholderia cepacia strain BY were used in combination to control damping‐off of tomato plants caused by Rhizoctonia solani. Microcosm tests showed complete inhibition of R. solani growth on filter disks buried in soil added with the mixture of both bacteria. Single BY inhibited the fungus, but not completely, and RB14‐C had only slight inhibitory effect on pathogen growth. The efficacy of this combining treatment was checked in pot experiments, where bacteria were applied to the soil in several combinations: RB14‐C and BY together 4 days before seed planting, RB14‐C 4 days and BY 2 days before seed planting, RB14‐C 4 days and BY immediately before seeds. The effect of these treatments on population of R. solani in soil and infection of plants was compared with the activity of single application of each agent. All bacterial treatments significantly decreased damping‐off of tomato plants. The best control was obtained when BY was added 2 days after RB14‐C. In this treatment plant protection was significantly higher than that obtained in other combined applications and obtained by single strains, except BY added to the soil 4 days before seed planting. The lowest suppression indicated BY introduced to the soil before seed planting. RB14‐C only slightly decreased number of R. solani in the soil. In contrast, BY drastically reduced population of the pathogen. However, there was not a clear relation between decrease of pathogen density in soil and the rate of plant infection. The results show that combination of B. subtilis RB14‐C with B. cepacia BY can lead to greater damping‐off suppression than biocontrol exhibited by these strains used separately, but the effect of combining bacterial agents was clearly related to the order in which both agents were introduced.     

L-glutaminase production by <Emphasis Type="Italic">Trichoderma koningii</Emphasis> under solid-state fermentation

Solid state fermentation was conducted for the production of L-glutaminase by Trichoderma koningii Oud.aggr. using different agro-industrial byproducts inlcuding wheat bran, groundnut residues, rice hulls, soya bean meal, corn steep, sesamum oil cake, cotton seed residues and lentil industrial residues as solid substrates. Wheat bran was the best substrate for induction of L-glutaminase (12.1 U/mg protein) by T. koningii. The maximum productivity (23.2 U/mg protein) and yield (45.0 U/gds) of L-glutaminase by T. koningii occurred using wheat bran of 70% initial moisture content, initial pH 7.0, supplemented with D-glucose (1.0%) and L-glutamine (2.0% w/v), inoculated with 3 ml of 6 day old fungal culture and incubated at 30°C for 7 days. After optimization, the productivity of L-glutaminase by the solid cultures of T. koningii was increased by 2.2 fold regarding to the submerged culture.     

Geographic range, impact, and parasitism of lepidopteran species associated with the invasive weed Lantana camara in South Africa

Six isolates of Trichoderma were screened for antagonism to Armillaria in tea stem sections buried in the soil. The inability of Armillaria to invade Trichoderma-colonized stem sections and the reduction of its viability in the plant materials following invasion of these by Trichoderma were used as indicators of antagonism. Four isolates of the species Trichoderma harzianum significantly (P<0.001) reduced the incidence of the pathogen in the plant materials. Isolate T4 completely eliminated the pathogen from plant materials in sterile soil and also antagonized two different isolates of the pathogen in nonsterile soil. Application of this T. harzianum isolate to the soil as a wheat bran culture significantly (P<0.001) reduced viability of Armillaria in woody blocks of inoculum. Soil amendment with coffee pulp also reduced the inoculum viability but did not affect the incidence of Trichoderma in the blocks of inoculum. We conclude that the direct application of wheat bran-formulated T. harzianum into soil surrounding woody Armillaria inoculum sources can suppress the pathogen. Further, no organic amendment is needed to enhance development of the antagonist in the soil as a pre-requisite to suppressing the pathogen.     

Extracellular xylanase production by Fusarium species in solid state fermentation

Fusarium sp. has been shown to be a promising organism for enhanced production of xylanases. In the present study, xylanase production by 21 Fusarium sp. isolates (8 Fusarium culmorum, 4 Fusarium solani, 6 Fusarium verticillioides and 3 Fusarium equiseti) was evaluated under solid state fermentation (SSF). The fungal isolate Fusarium solani SYRN7 was the best xylanase producer among the tested isolates. The effects of some agriculture wastes (like wheat straw, wheat bran, beet pulp and cotton seed cake) and incubation period on xylanase production by F. solani were optimized. High xylanase production (1465.8 U/g) was observed in wheat bran after 96 h of incubation. Optimum pH and temperature for xylanase activity were found to be 5 and 50 degrees C, respectively.     

Potential of Allyl Isothiocyanate to Control Rhizoctonia solani Seedling Damping Off and Seedling Blight in Transplant Production

The synthetic mustered flavouring essential oil, allyl isothiocyanate (AITC), was evaluated for its effect on suppression of Rhizoctonia solani growth in vitro, and in field soils for reducing inoculum density, saprophytic substrate colonization and seedling damping off and blight using snap bean and cabbage as indicator plants. In vitro growth was completely inhibited at the concentration of 50 μl/l. Inoculum density and saprophytic substrate colonization by the fungus in soil were not affected by AITC concentrations of 50 or 75 μl/kg soil. The inoculum density estimation by the use of soil‐drop technique created an artefact leading to an erroneous conclusion that the fungus was eradicated from soil within 1–3 days after AITC treatment at 150 or 200 μl/kg soil. The saprophytic substrate colonization showed that although the activity of R. solani was greatly reduced, the fungus still colonized 45% of the substrate units at these concentrations, and up to 100% at lower concentrations within 1 day after treatment. At higher concentrations the recovery rate from the substrates gradually declined over time to <6%. Drenching R. solani infested sandy‐loam or silty‐clay‐loam soil with water containing the emulsified AITC to provide 150 or 200 μl/l soil, a few days prior to planting, gave over 90% disease control in snap bean and cabbage, with no apparent phytotoxic effect. The effect of AITC was not influenced by the physical soil texture. AITC appears to have a good potential to replace methyl bromide fumigation of the substrate used for transplant production.     

Biocontrol of Rhizoctonia solani damping-off of sugar beet with native Streptomyces strains under field conditions

A 3-year study was conducted to evaluate the effectiveness of two disease-suppressive Streptomyces spp. to control sugar beet Rhizoctonia solani damping off under field conditions. Streptomyces seed treatments reduced seedling damping off in naturally (2005) and artificially (2006 and 2007) infested soils. All biocontrol agents provided better efficacy than Vitavax to control seedling damping-off. There were no significant differences among Streptomyces isolates. Isolate C increased plant stand by 19.5, 50.5 and 53.75% in 2005, 2006 and 2007, respectively. Evaluation of final harvest revealed that the root yield of the biocontrol agents increased compared to untreated control in these years.     

Yield,development, and quality response of dual‐toxin Bt cotton to Helicoverpa spp. infestations in Australia

To verify current thresholds for Bollgard II^® cotton in Australia, the impact of Helicoverpa spp. (Lepidoptera: Noctuidae) larvae on yield, development, and quality under various infestation intensities and durations, and stages of growth, was tested using small plot field experiments over two seasons. Infestation with up to 80 eggs m^?1 of Helicoverpa armigera (Hübner) and Helicoverpa punctigera Wallengren showed that species, infestation level, and stage of growth had no significant effect on yields of seed‐cotton or lint and on maturity and fibre quality. The duration of infestation of white flowers with H. punctigera neonates (maximum of every day for up to 4 weeks) had no impact on the yield of seed‐cotton or lint, maturity, and fibre quality, but when 100% of flowers were infested (compared with 0 or 50%), seed‐cotton and lint yields were significantly reduced and maturity was delayed. Infestation with up to 18 medium H. armigera larvae m^?1 at several plant stages did not significantly affect yields of seed‐cotton and lint, maturity, and fibre quality. A heliocide spray applied on a commercial farm at the current threshold resulted in a significantly higher lint yield, compared with a farm where no spray was applied. In conclusion, Bollgard II^® cotton is highly resistant to Helicoverpa spp. infestation.     

Build-up and decline ofRhizoctonia solani inoculum under field conditions

Summary Inoculum potential ofRhizoctonia solani Kühn was studied in an infested carnation field during two successive growth seasons. This inoculum potential was expressed as diseased carnation plants in the field and diseased bean seedlings planted in soil samples. Disease incidence in the field soil samples increased during the first season, up to 60% and 100%, respectively. Removing the carnation plants and keeping the soil wet for 45 days, resulted in a sharp decline in inoculum potential. Both inoculum potential and disease incidence in carnations were lower after plant removal. The use of either methyl bromide or vapam resulted in complete control of the disease and reduced inoculum potential. Results suggest possible reduction ofR. solani inoculum by maintaining the soil moist between growth periods.