effect of biocontrol agents and biofertilizers on root rot, yield, harvest index and nutrient uptake...

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Effect of biocontrol agents andbiofertilizers on root rot, yield, harvestindex and nutrient uptake of cassava(Manihot esculanta Crantz)A.C. Hridya a , G. Byju a & Raj Sekhar Misra ba Central Tuber Crops Research Institute, Thiruvananthapuram,Indiab Regional Centre of Central Tuber Crops Research Institute,Bhubaneshwar, India

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Effect of biocontrol agents and biofertilizers on root rot, yield, harvest

index and nutrient uptake of cassava (Manihot esculanta Crantz)

A.C. Hridyaa, G. Byjua* and Raj Sekhar Misrab

aCentral Tuber Crops Research Institute, Thiruvananthapuram, India; bRegional Centre ofCentral Tuber Crops Research Institute, Bhubaneshwar, India

(Received 23 April 2012; final version received 5 June 2012)

Cassava is an important subsidiary food and industrial raw material in thetropics. Root rot disease, caused by Phytophthora palmivora, poses a seriousthreat to cassava cultivation in Tamil Nadu, India. Field experiments (2008–09)were conducted to study the effect of biocontrol agents (Trichoderma spp. andPseudomonas fluorescens) and biofertilizers (Azospirillum, vesicular–arbuscularmycorrhizal fungi and phosphorus-solubilizing bacteria) on root rot, yield,harvest index and nutrient uptake of cassava at two NPK rates. The design of theexperiment was a split plot with two NPK rates, recommended and 50%recommended rate, as the main plot treatments and five biocontrol agents andbiofertilizers as subplot treatments. The results clearly indicated that use of abioinoculants consortium significantly reduced root rot infection/disease in-cidence over uninoculated controls. Azospirillum significantly improved the yieldof cassava at 50% of the recommended rate of NPK. NPK rates had nosignificant impact on harvest index of cassava and Trichoderma and vesicular–arbuscular mycorrhizal fungi resulted in a higher harvest index even at 50% of therecommended NPK rate. Nitrogen, phosphorus and potassium uptake wassignificantly improved when treated with biofertilizers and/or a consortium.

Keywords: cassava; Vertisols; Trichoderma; Pseudomonas fluorescens; VAM fungi;Azospirillum; phosphorus-solubilizing bacteria

Introduction

Biocontrol agents may be a viable alternative to chemicals in the management offungal crop diseases. A biocontrol mechanism involves the use of beneficialmicroorganisms, such as specialized fungi or bacteria to reduce/minimize andcontrol plant pathogens and the diseases they cause (Monte 2001). Trichoderma spp.,with effective antagonistic activities, are potential candidates for the biologicalcontrol of plant diseases (Papavizas 1985) and can control soil- or seed-borne fungaldiseases in several crops (Kubicek et al. 2001). Various modes of action have beenassociated with the ability of Trichoderma spp. to control plant pathogens (Jeffriesand Young 1994) including substrate competition, the ability to colonize theecological niche favoured by the pathogen (Grondona et al. 1997), antagonism byantibiotics (Ghisalberti and Rowland 1993) and the production of cell-wall-degrading enzymes (Haran et al. 1996). Trichoderma also has the ability to increase

*Corresponding author. Email: [email protected]

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root growth and development, crop productivity, resistance to abiotic stressesand the uptake and use of nutrients (Singh RN et al. 2006). Trichoderma strainssolubilize phosphates and micronutrients. Trichoderma helped to increase thenumber of deep roots, and thereby induced drought resistance in plants, in addition,certain compounds produced by Trichoderma induced resistance in plants (Singh RNet al. 2006).

Pseudomonas strains have been shown to suppress the activities of plantpathogens via the production of secondary metabolites such as siderophore, HCNand protease which show antagonistic activity against Phytophthora sp., Pythium sp.,Fusarium sp., Rhizoctonia solani, Macrophomina, etc. (Champbell 1989; O’Sullivanand O’Gara 1992; Ahmadzadeh et al. 2006). In an iron-deficient medium, Pseudo-monas strains have been shown to produce antibiotics including pyrolnitrin,pyoluteorin and phenezine-1-carboxylate, which are closely related to plant diseases(Dowling and O’Gara 1994). Pseudomonas strains have been characterized asphosphorus solubilizers with the ability to produce organic acids (such as oxalic acid,fumaric acid and citric acid) and phosphatases that facilitate the solubilization ofphosphorus and other nutrients (Rodriguez et al. 2006).

Biofertilizers add nutrients through the natural processes of nitrogen fixation,solubilizing phosphorus and stimulating plant growth via the synthesis of growth-promoting substances. The stimulatory effect exerted by Azospirillum has beenattributed to several mechanisms including biological nitrogen fixation, enhance-ment of mineral uptake and excretion of phytohormones by plants (Okon andItzigsohn 1995; James 2000). Azospirillum-inoculated plants are able to absorbnutrients from soil at faster rates than uninoculated plants. Azospirillum producesphytohormones, such as auxins, that can stimulate root growth and induce changesin root morphology which, in turn, could affect the assimilation of nutrients andabsorption of water. Under iron-starved conditions, Azospirillum lipoferum iscapable of producing a catechol-type siderophore that exhibits antimicrobial activityagainst various bacterial and fungal isolates (Shah et al. 1992).

A mycorrhiza is a mutualistic association between plant roots and fungus.Vesicular–arbuscular mycorrhizal (VAM) fungi are a type of mycorrhiza in which anincrease in the uptake of nutrients is achieved via increased surface area of soilcontact, increased movement of nutrients into the mycorrhizae, modification of theroot environment and increased storage. The rate of inflow of phosphorus intomycorrhizae can be up to six times that of the root hairs (Bolan 1991). VAM fungican reduce damage caused by soil-borne pathogens (Sharma et al. 1992), biologicallycontrol pathogenic species such as Phytophthora, Fusarium, Pythium and Rhizoctonia(Kjoller and Rosendahl 1997) and alter the host plant susceptibility to insectherbivores by affecting the growth and reproduction of insects feeding on the hostplant (Gange and Bower 1997).

Phosphate-solubilizing bacteria (PSB) increase the uptake of nitrogen, phos-phorus, potassium and iron (Biswas et al. 2000). A major mechanism of mineralphosphate solubilization is the action of organic acids synthesized by soil micro-organisms (Craven and Hayasaka 1982; Leyval and Berthelin 1989). They have theability to solubilize inorganic and/or organic phosphorus from soil after inoculationand improve the supply of phosphorus to plants (Lifshitz et al. 1987). Some PSBbehave as mycorrhizal helper bacteria (Garbaye 1994; Frey-Klett et al. 1997). Thissynergistic interaction allows better exploitation of poorly soluble phosphorussources (Azcon-Aguilar et al. 1986; Piccini and Azcon 1987).

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Cassava (Manihot esculenta Crantz), which belongs to the family Euphorbiaceae,is an important root crop and is widely cultivated in the tropics between 308N and308S of the Equator (Cock 1984). The adventitious roots of cassava storecarbohydrates, and being rich in starch form an important subsidiary food. Thiscash crop is also a raw material for the starch and sago industries, and is acomponent of animal, fish and poultry feeds. Cassava starch has also found a placein the manufacture of biodegradable plastics and bioethanol (Abraham et al. 2006).

In India, cassava is grown mainly in the southern state of Tamil Nadu, mostly inirrigated Vertisols. Root rot diseases caused by Phytophthora palmivora pose aserious threat to cassava yield because they affect the roots directly. Infected plantsdo not show any external symptoms, but infected roots show a brown discolourationof the internal tissue and exhibit a foul smell. Infected roots rot and become unfit forconsumption or marketing, causing up to 60–80% yield loss in certain endemicareas. Excessive irrigation, poor drainage and the development of hard pan favourthe occurrence of root rot disease (Byju et al. 2010). Trichoderma and Pseudomonasfluorescens are two biocontrol agents commonly used for the management of soil-borne diseases in root crops such as cassava (Saraswathi et al. 2003).

Considering the importance of cassava in India and the significant yield loss inthe farms of Tamil Nadu, field studies were conducted to evaluate the most suitablebioinoculants and/or their combinations with two different doses of chemicalfertilizers (NPK) to minimize the root rot infection and increase the root yield andnutrients uptake.

Materials and methods

Study site

In order to study the influence of different biocontrol agents and biofertilizers on rootrot infection, yield, harvest index and nutrients uptake of cassava, farm experimentswere conducted at Panamarathupatty, Salem district, Tamil Nadu, India (11.398Nlatitude; 78.128E longitude; 180 m a.s.l.) during two consecutive cropping seasons of2008 and 2009. The climate of the region is described as semi-arid tropics, with two shortrainy seasons during June–September and October–December (average annual rainfallwas 858.3 mm; mean minimum and maximum temperatures were 19.5 and 41.58C, andrelative humidity ranged between 52.6 and 91.2%). The soil of the experimental locationis classified as clayey, smectitic, hyperthermic, udic Haplusterts. The soils contained0.18% total N, 94.22, 34.12 and 290.15 mg kg71 available N, P and exchangeable K,respectively, and 0.78% organic carbon.

Experimental details

The study used a split plot lay out, with main plot treatments and subplottreatments, and three replicates per treatment. The main plots were two levels ofchemical fertilizers (NPK) application, namely 100 and 50% of the recommendedrate (Nair, Ramanathan et al. 2004). The NPK fertilizers were applied at rates of100:50:100 kg ha71 for the 100% rate and 50:25:50 kg ha71 for the 50% rate. Thesubplots were eight different combinations of biofertilizers and/or biocontrol agentsapplied at a rate of 5 kg ha71, as given in Table 1. Trichoderma was applied to alltreatments except the uninoculated control. The bioinoculants were prepared in theLaboratory of Microbiology, Central Tuber Crops Research Institute, India.

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NPK application and all other agronomic practices were as per standardrecommendations (Nair, Ramanathan et al. 2004). Cultivar H-226 released fromCentral Tuber Crops Research Institute, India was used for planting. H-226 is the mostpopular industrial cassava variety in India, with a 30% starch content, 200 mg kg71

HCN content and good cooking quality (Abraham et al. 2006). Crops were plantedon 26 February 2008 and on 25 February 2009 and harvested on 25 December 2008and on 24 December 2009, respectively, during the first and second year of fieldexperimentation. The crop was irrigated at the rate of 4 mm day71.

Field measurements

The biometric characteristics recorded were number of fallen and standing leavesand total weight of leaf, stem and roots. Fifty grams each of leaf, stem and rootsample from selected plants were collected separately. Total root weights were alsomeasured from all plants in each plot, excluding the border row at the time ofharvesting. Root rot infection was calculated as the percentage of infected roots perplant. Harvest index (total root yield/total biological yield) was measured on a freshweight basis (Putthacharoen et al. 1998).

Laboratory studies

Leaf, stem and root samples were dried in a hot air oven at 658C for 48 h or untilconstant weight was attained, and the dry weights of the samples were recorded todetermine the dry matter content. Dried samples were ground in a stainless steel WileyMill. The nitrogen content was determined by digesting the samples in H2SO4, followedby Kjeldahl analysis of the total nitrogen (Bremner andMulvaney 1982) using a Kjeltecautomatic nitrogen digestion and distillation system (Tecator Model No. 1023). Tissuephosphorus was determined by the vanado-molybdo phosphoric yellow colour methodafter digestion with triple acid (HNO3/HClO4/H2SO4 10:4:1), and tissue potassiumwas determined using a flame photometer and the same digest (Jackson ML 1972).

Statistical analysis

The results of field measurements and plant analysis were statistically analysed as asplit plot design, using SAS (SAS Institute 2002). The least significant difference

Table 1. Details of the different subplot treatments used in the on-farm experiment.

Treatment no. Details

1 Control2 Trichoderma soil application3 Pseudomonas fluorescens þ Trichoderma4 Azospirillum þ Trichoderma5 VAM fungi þ Trichoderma6 P-solubilizing bacteria (PSB) þ Trichodermaa

7 Soil application of VAM fungi, Pseudomonas fluorescens, Azospirillumand P-solubilizing bacteria and Trichoderma

8 Sett treatment of Trichoderma, Pseudomonas fluorescens, Azospirillum,VAM fungi and P-solubilizing bacteria

Note: aTrichoderma was applied to all treatments except the uninoculated control.


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(LSD) was used at the p 4 0.05 level of probability to test difference betweentreatment means. Analysis of variance (ANOVA) was performed to determine theeffects of NPK rates, biocontrol agents and/or biofertilizers and their interaction.

Results and discussion

Root rot infection

The interaction effects of year 6 NPK rate 6 biocontrol agents and biofertilizerswere not found to be significant and the data are therefore presented as an averageover both years. The effects of NPK rates and biocontrol agents and biofertilizers onroot rot infection of cassava at harvest are given in Table 2. Root rot infection wasanalysed at the main and interaction levels to assess the mean and interaction effectsof NPK rates and biocontrol agents and biofertilizers. Root rot infection was notaffected by the rate of NPK application alone. Field observations showed that rootrot infection was significantly higher (18.15%) in uninoculated controls and variedfrom 0 to 1.15% in microbial-culture-inoculated plots. Root rot infection of cassavawas due to Phytophthora palmivora which poses a serious threat to cassava rootproduction in Tamil Nadu, India (Byju et al. 2010). While analysing the mean, nosignificant differences were observed among the biocontrol agents, biofertilizers ortheir combined application. The complementary effects of NPK rates and biocontrolagents and biofertilizers showed no significant effect in all inoculations at two NPKrates. The rate of NPK application did not influence the biocontrol agents andbiofertilizers to have a significant impact on the reduction in root rot infection. Nosymptoms of root rot infection were observed in Trichoderma, PSB and soil and settapplication for all biocontrol agents and biofertilizers at both NPK fertilizer rates.Cassava root rot was significantly reduced in treatments in which bioinoculants wereapplied. Trichoderma soil application alone was able to control root rot in cassava(Alvarez et al. 2000), and is widely accepted as a potential biocontrol agent againstpathogenic fungi (Woo et al. 2006). Biocontrol agents, biofertilizers or theircombined inoculation was able to reduce the incidence of root rot by 98.85–100%.Trichoderma reduced the incidence root rot in cassava to 7.3%. Combinedapplication of Trichoderma and Pseudomonas fluorescens also reduced the root

Table 2. Effects of NPK rates, and biocontrol agents and biofertilizers on root rot infection(%) in cassava (mean of 2 years).

Biofertilizers and biocontrol agentsRecommended

rate50% Recommended

rate Mean

Control 16.30 20.00 18.15Trichoderma 0.00 0.00 0.00Pseudomonas þ Trichoderma 0.80 1.55 1.15Azospirillum þ Trichoderma 0.90 1.00 0.95VAM fungi þ Trichoderma 0.16 0.20 0.18P-solubilizing bacteria þ Trichoderma 0.00 0.00 0.00All – soil 0.00 0.00 0.00All – sett treatment 0.00 0.00 0.00Mean 2.10 1.85

Note: LSD (NPK rate) (0.05), not significant; LSD (biocontrol agents and biofertilizers) (0.05), 3.14; LSD(interaction) (0.05), 1.68.


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incidence of cassava (Saraswathi et al. 2003). An effective management mechanismof inoculating with Trichoderma could significantly reduce root rot in cassava inTamil Nadu (Nair, Jeeva et al. 2004). In the present study, treatment withAzospirillum, VAM fungi and PSB, along with Trichoderma could effectively reducethe root incidence of cassava. The synergistic effect of microbial combinationsreduced the incidence of root rot in cassava.

Root yield

The effects of NPK rates and biocontrol agents and biofertilizers on root yield ofcassava at harvest are given in Table 3. The mean effects of NPK rates on cassavaroot yield did not differ significantly. All microbial inoculations significantlyimproved the yield of cassava roots at harvest over uninoculated control. Onaverage, Azospirillum þ Trichoderma inoculation produced *33.79% higher yieldthan uninoculated controls and was not significantly different from soil applicationof all biocontrol agents and biofertilizers and VAM fungi inoculation. The com-plementary effects of biocontrol agents and biofertilizers showed that the magnitudeof the increase in yield was much greater with Azospirillum þ Trichoderma at therecommended rate and no significant differences were observed with VAM fungi andsoil application of all inoculants at the recommended rate and Azospirillum þTrichoderma at 50% of the recommended rate. Azospirillum þ Trichodermainoculation resulted in higher root yield at half the recommended fertilizerapplication rate.

The effect of biocontrol agents and biofertilizers revealed the importance ofmicrobial inoculations on cassava root yield. Azospirillum and VAM fungi þTrichoderma effectively increased the yield of cassava, even at the 50% NPKapplication rate. The effectiveness of the Azospirillum þ Trichoderma inoculationwas generally enhanced when nitrogen fertilizer at one third of the normal rate wasgiven during the initial stage of sweet potato growth (Mortley and Hill 1990; Byjuand Ravindran 2009). Azospirillum was able to increase sweet potato root yield by22% compared with plants given nitrogen fertilizer at the normal rate (Saad et al.1999). Suchetha et al. (1990) reported a significant effect of Azospirillum on cassava

Table 3. Effects of NPK fertilizer rates, and biocontrol agents and biofertilizers on root yield(t ha71) of cassava (mean of 2 years).

Biocontrol agents and biofertilizersRecommended

rate50% Recommended

rate Mean


Note: LSD (NPK rate) (0.05), not significant; LSD (biocontrol agents and biofertilizers) (0.05), 2.05; LSD(interaction) (0.05), 2.44.


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growth. The response of cassava to biofertilizers has been studied by Suja et al.(2005) who reported that the rate of application of nitrogen and phosphorusfertilizers can be reduced to half by the integrated use of Azospirillum andPhosphobacterium þ Trichoderma. VAM fungi inoculation increased the root yieldof crops compared with uninoculated controls (Howeler et al. 1982; Sailo andBagyaraj, 2005).

Harvest index

On average, NPK application at the recommended rate and at 50% of therecommended rate did not significantly change the harvest index of cassava(Table 4). Biocontrol agents and biofertilizers or their combined applicationsignificantly improved the harvest index of cassava over uninoculated controls.When averaged over biocontrol agents and biofertilizers, the harvest index washighest (0.47) with Trichoderma and VAM fungi þ Trichoderma inoculation and wassignificantly higher than with other bioinoculants. The interaction effects of NPKrates and biocontrol agents and biofertilizers were found to be statistically significantfor harvest index and a higher harvest index was observed with Trichoderma orVAM fungi þ Trichoderma inoculation at 50% of the recommended rate. Allbiocontrol agents and biofertilizers, including the combined application of allbiocontrol agents and biofertilizers, did not lead to any significant difference,regardless of the NPK fertilizer rates. Biocontrol agents and biofertilizers, includingthe combined application at the recommended rate, resulted in no significantdifference compared with the uninoculated control. The harvest index at therecommended NPK rate was significantly higher than with the 50% NPK rate inuninoculated controls.

Distribution of dry matter to the economically useful parts of the plant ismeasured by the harvest index. In cassava, harvest index represents the efficiency ofstorage root production and is usually determined by the ratio of storage root weightto total plant weight (Alves 2002). Trichoderma and VAM fungi þ Trichodermaplayed an important role in minimizing chemical input and improving the harvestindex of cassava.

Table 4. Effects of NPK rates, and biocontrol agents and biofertilizers on harvest index ofcassava (mean of 2 years).

Biocontrol agents and biofertilizersRecommended

rate50% Recommended

rate Mean


Note: LSD (NPK rate) (0.05), not significant; LSD (biofertilizers and biocontrol agents) (0.05), 0.02; LSD(interaction) (0.05), 0.03.


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Nutrients uptake

The effects of NPK fertilizer rates and biocontrol agents and biofertilizers onnutrients uptake of cassava at harvest are given in Table 5 and Figure 1. Nutrientuptake was assessed at the main plot level, allowing us to determine the separateeffects of NPK rates and biocontrol agents and biofertilizers at harvest of cassava(Table 5). There was no significant difference between the recommended and 50%recommended NPK rate in terms of nitrogen uptake by cassava. Azospirillum þTrichoderma inoculation significantly increased the nitrogen uptake by cassava overuninoculated control by 35.62%. No significant difference was observed with VAMfungi and soil application of all biocontrol agents and biofertilizers. On average,Trichoderma inoculation showed no positive impact on the N uptake with similarresults to those obtained with the uninoculated control, having the least N uptakeamong the subplot treatments. Owing to the complementary effect of NPK fertilizerrates and biocontrol agents and biofertilizers (Figure 1), nitrogen uptake was highest(208.62 kg ha71) in Azospirillum þ Trichoderma inoculation at the recommendedNPK fertilizer rate, followed by VAM fungi þ Trichoderma and soil application ofall biocontrol agents and biofertilizers at the recommended fertilizer rate. The resultswere also not significantly different from the soil application of all biocontrol agentsand biofertilizers at 50% of the recommended rate. Regardless of the NPK rates,VAM fungi and soil application of all biocontrol agents and biofertilizers showedsimilar results.

The black soil in which cassava is grown in Tamil Nadu, India is poor in organicmatter, nitrogen and phosphorus, but fairly rich in potassium. Cassava has beenrecognized for its ability to grow and produce reasonably well on poor soils, andextracts large quantities of plant food elements. The high-yielding hybrids extractedmore nutrients and were capable of removing 180–200 kg N, 15–22 kg P and 140–160 kg K ha71 (CTCRI 1983).

A significant increase in yield due to Azospirillum inoculation was observed insweet potato cultivar (Farzana and Radizah 2005). Bashan and Levanony (1990)

Table 5. Effects of NPK rates, and biocontrol agents and biofertilizers on nutrient uptake ofcassava at harvest (mean of 2 years).

N uptake P uptake K uptake

Treatment Kg ha71

NPK ratesRecommended rate 181.03 23.92 166.6750% recommended rate 163.23 22.92 159.83LSD (0.05)* NS NS 3.73

Biocontrol agents and biofertilizers (averaged over both NPK rates)Control 155.20 18.93 158.56Trichoderma 169.07 23.26 154.34Pseudomonas þ Trichoderma 177.89 23.07 155.43Azospirillum þ Trichoderma 210.48 21.77 189.06VAM fungi þ Trichoderma 203.43 26.76 192.21P-solubilizing bacteria þ Trichoderma 181.99 26.76 166.64All – soil 205.41 26.48 191.23All – sett treatment 185.27 24.84 172.63LSD (0.05)* 14.13 2.05 NS

*Significant at 0.05 probability level; NS, not significant.


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reported that enhanced mineral uptake in the plant as a result of Azospirilluminoculation was a popular explanation for the inoculation effects. Azospirillum playsa vital role in the nitrogen economy in taro (Colocacia esculenta L. Schott.)rhizosphere (Jolly et al. 2010). The response of cassava to biofertilizers was studiedby Suja et al. (2005) who reported that the rate of application of nitrogen andphosphorus fertilizers can be reduced to half by the integrated use of Azospirillumand Phosphobacterium. In the present study, even though Azospirillum þ Tricho-derma helped to improve the nitrogen uptake, it did not help to decrease the amountof fertilizer needed. Integration of Azospirillum with nitrogen fertilizer at differentlevels indicated that the nitrogen fertilizer amount could be reduced to one-third insweet potato and have a beneficial effect on the fertility status of the soil in terms ofavailable phosphorus and potassium (Byju and Ravindran 2009). VAM fungifacilitate inorganic nitrogen uptake by plants and the transport of nitrogen fromsoils to roots (Jackson LE et al. 2008).

There was no significant difference in the phosphorus uptake of cassava atharvest with different NPK rates. Averaged over biocontrol agents and biofertilizers,VAM fungi and PSB þ Trichoderma resulted in a 41.36% increase in phosphorusuptake (26.76 kg ha71) over the uninoculated control (18.93 kg ha71), which wasnot significantly different from the combined application of all biocontrol agentsand biofertilizers as soil and sett treatment. All inoculations resulted in an increasein phosphorus uptake compared with the uninoculated control. The highestphosphorus uptake was observed in PSB þ Trichoderma-treated plots at therecommended NPK rate followed by VAM fungi þ Trichoderma at 50% of therecommended NPK rate. The effects of NPK fertilizer rates and biocontrol agents

Figure 1. Interaction effects on nutrient uptake as influenced by NPK fertilizer rates, andbiocontrol agents and biofertilizers at harvest of cassava (mean of 2 years). *Always +Trichoderma.


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and biofertilizers showed no significant difference from the VAM fungi þ Tricho-derma-treated plots at two fertilizer levels. VAM fungi were shown to be not sensitiveto fertilizer application and improved phosphorus uptake at lower fertilizer levels(Potty, 1990). Among the different inoculations at two NPK rates, Azospirillum þTrichoderma resulted in the lowest phosphorus uptake.

The phosphorus requirement of cassava is smaller than the requirement fornitrogen and potassium. Nair et al. (1988) investigated the response of cassava tograded levels of phosphorus in acid laterite soil and determined optimum dose to be50 kg P ha71. The amount of phosphorus liberated by naturally occurring VAMfungi and PSB was not sufficient for a substantial increase in plant growth.Inoculation of plants by microorganisms at a much higher concentration thannormal is essential. In the present study, PSB increased the uptake of phosphorusbecause they have the ability to solubilize inorganic and/or organic phosphorus fromthe soil after inoculation (Kloepper et al. 1980) and improve the supply ofphosphorus to plants (Lifshitz et al. 1987).

Cassava responded faster to mycorrhizal inoculation than other tuber crops suchas coleus, sweet potato and yams. Cassava is highly dependent on mycorrhizalassociation for phosphorus uptake from low phosphorus soils (Howeler 2002). VAMfungi increased the phosphorus uptake at 50% of the recommended NPK rate andwere not significantly different from that at the recommended rate. Cassava hadmuch lower critical level for available phosphorus compared with less mycorrhizal-dependent crops like maize and beans (Potty 1990). It has also been reported thatcassava responded significantly to inoculation when 100 kg P ha71 was applied, butthe response was not significant at low levels or at higher levels of 200 kg P ha71

(Howeler and Sieverding 1983). By contrast to the above results VAM fungi couldgive good results even at 25 kg P ha71.

The NPK rate had a significant effect on potassium uptake with the highestpotassium uptake being found at the recommended rate. The average effect ofbiocontrol agents and biofertilizers on potassium uptake at harvest of cassava wasnot significant. Potassium uptake by the whole cassava plant was significantly higherwith soil application of all biocontrol agents and biofertilizers at the recommendedNPK rate and was not significantly different from that obtained with VAM fungiand Azospirillum þ Trichoderma at the recommended NPK rate. The result was notsignificantly different from that found with VAM fungi inoculation at 50%recommended rate. So VAM fungi þ Trichoderma can enhance potassium uptake atlower levels of NPK application. Potassium uptake in Trichoderma inoculation wassignificantly decreased (20.58%) at 50% of the recommended NPK rate comparedwith the recommended NPK rate.

Potassium analysis of inoculated plants in the field indicated that there was asignificant accumulation of potassium in sorghum (Saring et al. 1984). Corn seedsinoculated with Azospirillum significantly enhanced the uptake of Kþ in 3–4-day-oldand 2-week-old root segments (Lin et al. 1983). Application of VAM fungi improvedthe uptake of potassium in coleus (Singh R et al. 2009). Similar results were observedin the present study.

Conclusion

In general, microbial inoculations significantly improved the mineral nutrientuptake, yield, harvest index and repression of root rot infection of cassava over


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uninoculated controls. This indicates the importance of microbial inoculations tocontrol root rot infection and increased growth due to reduction in disease incidenceand improved growth-enhancing property of biocontrol agents and biofertilizers.Application of Trichoderma in combination with PSB effectively controls root rot incassava.

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effect of biocontrol agents and biofertilizers on root rot, yield, harvest index and nutrient uptake...

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