Management of Striga hermonthica on Sorghum (Sorghum bicolor) Using Arbuscular Mycorrhizal Fungi (Glomus mosae) and NPK Fertilizer Levels

Trials were conducted in the screen house of Niger State College of Agriculture, Mokwa (09°18’N; 05°04’E) in the Southern Guinea Savannah agro-ecological zone of Nigeria during October-December, 2008 and January-March, 2009. The objective was to evaluate the effect of management of Striga hermonthica on sorghum (Sorghum bicolor) using Arbuscular mycorrhizal fungi and NPK fertilizer levels. The trials were laid out in split-split plot arrangement in a randomized complete block design. The main-plot treatments consisted of three sorghum varieties; SAMSORG 3, ICSVIII and SAMSORG 14 while the sub-plot treatments consisted of inoculations; Striga mixed with Glomus, Striga only and Glomus only as well as no inoculation control. The sub-sub-plot treatments were made up of NPK fertilizer levels; (100 kg N, 50 kg P₂O₅, 50 kg K₂O ha^-1), (50 kg N, 50 kg P₂O₅, 50 kg K₂O ha^-1) and (0 kg N, 0 kg P₂O₅, 0 kg K₂O ha^-1). The result obtained showed that sorghum variety SAMSORG 3 were taller, having more vigour and lower reaction to Striga parasitism which resulted in the crop producing higher dry matter compared to the other two varieties. The plots inoculated with Striga only supported shorter plants of sorghum varieties, higher vigour and lower reaction score to Striga compared to Striga mixed with Glomus. It is obvious in this study that the crop performance increases with increase in the rates of NPK fertilizer applied.

INTRODUCTION

Striga hermonthica [Del.] Benth. is a hemi-parasitic weed and major obstacle to crop production particularly cereals in the dry savannas of sub-Saharan Africa where producers lose half of their produce (Berner et al., 1995; Kanampiu et al., 2002). This weed causes significant yield losses in West Africa that range from 10 to 100% depending on cultivar and crop (Lagoke et al., 1991; Obilana and Ramaiah, 1992; Gressel et al., 2004). Sorghum (Sorghum bicolor [L.] Moench) is cultivated at large area which is up to 34% of total cereal production in Nigeria (Akintayo and Sedgo, 2001). However, it has been reported that Striga infestation in sorghum is higher in Nigeria than in other West African countries (Gressel et al., 2004).

The subterranean developmental stage of Striga is the most critical and damaging stage to its host which is approximately 75% of the overall Striga damage (Parker and Riches, 1993). Therefore, this hemi-parasite causes damage to host before its emergence. At present, there is no single method effective enough to put the problem of Striga at abeyance. In view of the physiology of Striga seed emergence, there is the need to concentrate on the immediate soil biotic environment for a management practice that will complement the existing control methods. Therefore integrated approach to Striga management may be crucial for successful Striga management. Recently, it has been reported that the use of certain soil microorganisms can be effective in inhibiting or suppressing the germination of Striga seed. (Parker and Riches, 1993; Berner et al., 1995; Mathimaran et al., 2005). Indeed, some studies (Giovannetti and Mosse, 1980; Mathimaran et al., 2005) have shown that certain soil-borne saprophytic bacteria have a potential to control S. hermonthica by inhibiting seed germination. However, the survival of inoculated bacteria was usually not sustained in subsequent crops and thus implying the need for repeated application every cropping season (Mathimaran et al., 2005). Low levels of applied nitrogen increased Striga emergence on soils of low fertility and decreased on fertile ones (Patterson, 1990; Gworgwor and Webber, 1991; Lagoke and Isah, 2010).

However, there are certain fungi in the soil known as Arbuscular Mycorrhizal Fungi (AMF) which maintain symbiotic association with root of plants. More than 90% of plant species are known to be involved in this association. Benefits derivable include improved water and nutrient uptakes in poor and dry soils (Osonubi et al., 1991), enhanced growth (Mathimaran et al., 2005) and tolerance to diseases (Vierheilig et al., 2000) among others. These fungi are able to confer these advantages in mycorrhizal plants due to their ability to extend the area of nutrient and water exploitation arising from hyphal extension, competing for infection sites with other microorganisms and possibly stimulation of certain antagonistic substances. Vierheilig et al. (2000) reported that plants colonized by an AM fungus suppress subsequent colonization by other pathogens through altered root exudates but did not discuss about the fungus being the secondary symbiotic. This study was therefore aimed at managing S. hermonthica using Arbuscular Mycorrhizal fungi (Glomus mosae) and NPK fertilizer levels on sorghum.

MATERIALS AND METHODS

The experiment was conducted in the screen house of Niger State College of Agriculture, Mokwa green house during October to December, 2008 and January to March, 2009. The design was a split-split plot arrangement in a randomized complete block design. The main plot treatments consisted of three sorghum varieties; SAMSORG 3, ICSVIII and SAMSORG 14 while sub-plot treatments consisted of inoculants; Striga mixed with Glomus, Glomus only, Striga only and no inoculation control. The sub-sub plot treatments consisted of compound fertilizer levels; 100 kg P₂O₅-50 kg K₂O/ha, 50 kg N-25 kg P₂O₅-25 kg K₂O/ha and 0 kg N-0 kg P₂O₅-0 kg K₂O/ha. Isah et al. (2009) had earlier reported the ability of SAMSORG 3 and ICSV III to be tolerant or resistant varieties to S. hermonthica parasitism while SAMSORG 14 was susceptible.

The soil which was collected from the crop farm was sieved to remove stone and debris and filled into the pots. The pots inoculated with Striga were filled to two-third depth while the uninoculated pots were filled to the brim. The remaining one third of the pots inoculated with Striga was filled with soil-Striga seed mixture for the inoculated pots. The soil were inoculated with approximately 3000 Striga seeds per pot using the procedure described by Kim (1994) and Berner et al. (1997). Fifty grams of the AM fungal inoculums collected from International Institute of Tropical Agriculture which was multiplied in the University of Agriculture, containing spores of Glomus mosae, hyphae and root fragments of maize as host was introduced into the planting hole of inoculated pots while attenuated inoculum of the same quantity was introduced into the planting hole of uninoculated plants. Three sorghum seeds (after surface sterilization in 1% NaOCl for 15 min and subsequent rinsing with demineralised water were sown per pot. The plants were watered to field capacity for the first weeks and thereafter with 10 mL of water in 48 h. The seedlings were later thinned down to two per pot at two weeks after planting. Mycorrhizal root colonization was determined by observing the stained root samples using grid-line intersect method of (Phillips and Torrey, 1970; Giovannetti and Mosse, 1980).

The data collected are sorghum height, vigour and reaction scores and Striga shoot count all at 6, 9 and 12 Weeks after Planting (WAP), days to first Striga emergence, sorghum dry weight and Striga shoot dry weight at harvest.

Data collected was subjected to Analysis of Variance (ANOVA) according to Steel and Torrie (1960) and Fishers Least Significant Difference (LSD) was used to compare treatment means using SAS (2009) statistical package.

RESULTS AND DISCUSSION

Sorghum varieties: Sorghum varieties differed significantly in plant height; crop vigour score, crop reaction score and Striga shoot count all at 6, 9 and 12 WAP as well as days to emergence of Striga, Striga and crop shoot dry weights at harvest and in the combined analyses. SAMSORG 14 had shorter plants than SAMSORG 3 in all cases and ICS VIII at 6 and 9 WAP only (Table 1). SAMSORG 3 also had significantly higher vigour score (Table 2) but lower reaction score (Table 3) at 6, 9 and 12 WAP than the other two sorghum varieties evaluated in this trial. Striga shoot emergence count at 6, 9 and 12 WAP followed the order SAMSORG 3<SAMSORG 14<ICSV III in the trial (Table 4). Days to Striga shoot emergence was significantly more on Samsorg 14 compared with the other two varieties (Table 5). SAMSORG 3 also had significantly higher crop shoot dry weight; however, Striga shoot dry weight was lower at 6, 9 and 12 WAP than the other two varieties evaluated. (Table 5). Isah et al. (2009) had earlier reported that SAMSORG 3 is resistant to S. hermonthica parasitism while SAMSORG 14 is susceptible.

Influence of inoculation on growth of sorghum: Inoculation had significant effect on plant height, vigour and reaction scores and Striga shoot emergence count all at 6, 9 and 12 WAP as well as crop and Striga shoot dry weight at 12 WAP and days to Striga emergence in the combined analyses.

Table 1:	Effects of inoculation and fertilizer on plant height of Sorghum varieties at 3, 6, 9 and 12 WAP in 2008, 2009 and combined
Means with the same alphabets within same column are not significantly different from one another at p<0.05, WAP: Week after planting

Table 2:	Effects of inoculation and fertilizer on crop vigour score of sorghum at 6, 9 and 12 WAP in 2008, 2009 and combined
Means with the same alphabets within the same column are not significantly different from one another at p<0.05, WAP: Week after planting, Vigour score: Using scale (1-5), 1: Not vigorous, 2: Poorly vigorous, 3: Fairly vigorous, 4: Moderately vigorous, 5: Very vigorous

Table 3:	Effects of inoculation and fertilizer on crop reaction score of sorghum varieties at 6, 9 and 12 WAP in 2008, 2009 and combined
Means with the same alphabets within the same column are not significantly different from one another at p<0.05, WAP: Week after planting, Sorghum reaction score used was scale (1 to 9), where 1 was assigned to normal plant growth and 9 was completely dead plant

Table 4:	Effects of inoculation and fertilizer on Striga shoot count at 6, 9 and 12 WAP in 2008, 2009 and combined
Means with the same alphabets within the same column are not significantly different from one another at p<0.05, WAP: Week after planting

Table 5:	Effects of inoculation and fertilizer on days to Striga emergence and crop and Striga dry shoot weight of sorghum varieties in 2008, 2009 and combined
Means with the same alphabets within the same column are not significantly different from one another at p<0.05

Plants of pots inoculated with Striga alone were shorter than those of the pots inoculated with its mixture with Glomus, Glomus alone and the control (Table 1) as this is in agreement with the study of Mobasser et al. (2012) that reported a significant influence of mycorrhizal on plant height of maize. Also Samra et al. (1997) reported that the general application of mycorrhiza increased plant heights compared to the control treatments without mycorrhizal infection. The results of this trial are similar to those of other reports showing that infections of mycorrhizal fungi increased plant height compared to controls (Ahiabor and Hirata, 1994; HashemiDezfuli et al., 1999; Olatunji et al., 2008).The various forms of inoculation resulted in shorter plants than those of the control except with Glomus at 12 WAP which was not significant (Table 1). Plants inoculated with Striga alone had lower vigour scores than those of the other treatments while those of Striga plus Glomus also had lower scores compared with those of Glomus alone and control at 9 and 12 WAP (Table 2). The reaction scores of plants of pots with Striga inoculation was obviously higher than those of the other treatments which were similar at 9 and 12 WAP while Glomus alone caused higher reaction score than the control at 12 WAP (Table 3). Striga shoot emergence count and weight were significantly lower with Striga/Glomus mixture inoculation than Striga alone while emergence was delayed by the mixture. Striga inoculation alone resulted in significantly lower crop shoot dry matter production compared with the other treatments, although Striga/Glomus mixture also caused lower values than Glomus alone and the control (Table 5).The ability of AMF to reduce Striga count can be explained in three ways: (1) The formation of metabolites, especially strigolactones that are responsible for the induction of Striga germination is down regulated upon mycorrhizal colonization; (2) Plant metabolites, such as cyclohexenones which arise through carotenoid degradation, that are up-regulated upon mycorrhizal colonization inhibit Striga germination and (3) Mycorrhizal colonization induces mycorrhizosphere effects that negatively impact on Striga germination (Lendzemo et al., 2007).

Influence of fertilizer rates on growth of sorghum: Fertilizer application had significant effect on height, vigour score, reaction score of sorghum as well as shoot count, days to emergence, dry matter production and dry weight of Striga. Sorghum plant height at 6 WAP (Table 1) and vigour score at 6, 9 and 12 WAP (Table 2) crop shoot dry weight at 12 WAP (Table 5) increased with the rates of fertilizer applied, while both rates of fertilizer similarly increased plant height of sorghum at 9 and 12 WAP. Conversely, crop reaction score (Table 3) and emergence shoot count (Table 4) and dry weight of Striga (Table 5) decreased with the rates of applied fertilizer. The two rates of fertilizer applied significantly delayed Striga emergence on sorghum compared to no fertilizer control. Gworgwor and Webber, 1991) observed that application of high nitrogen increases the performance of cereal crops under Striga infestation. This is due to the fact that nitrogen fertilizer reduced the severity of Striga attack while simultaneously increasing the host performance (Lagoke and Isah, 2010).

CONCLUSION

Sorghum variety SAMSORG 3 was resistant to Striga hermonthica in this study. There was an improvement in the performance of sorghum varieties when Striga was mixed with Glomus indicating that Arbuscular Mycorrhizal fungi is effective in S. hermonthica management. It was further enhanced when higher levels of fertilizer are used especially with those sorghum varieties found to be susceptible to S. hermonthica parasitism.

INTRODUCTION

Striga hermonthica [Del.] Benth. is a hemi-parasitic weed and major obstacle to crop production particularly cereals in the dry savannas of sub-Saharan Africa where producers lose half of their produce (Berner et al., 1995; Kanampiu et al., 2002). This weed causes significant yield losses in West Africa that range from 10 to 100% depending on cultivar and crop (Lagoke et al., 1991; Obilana and Ramaiah, 1992; Gressel et al., 2004). Sorghum (Sorghum bicolor [L.] Moench) is cultivated at large area which is up to 34% of total cereal production in Nigeria (Akintayo and Sedgo, 2001). However, it has been reported that Striga infestation in sorghum is higher in Nigeria than in other West African countries (Gressel et al., 2004).

The subterranean developmental stage of Striga is the most critical and damaging stage to its host which is approximately 75% of the overall Striga damage (Parker and Riches, 1993). Therefore, this hemi-parasite causes damage to host before its emergence. At present, there is no single method effective enough to put the problem of Striga at abeyance. In view of the physiology of Striga seed emergence, there is the need to concentrate on the immediate soil biotic environment for a management practice that will complement the existing control methods. Therefore integrated approach to Striga management may be crucial for successful Striga management. Recently, it has been reported that the use of certain soil microorganisms can be effective in inhibiting or suppressing the germination of Striga seed. (Parker and Riches, 1993; Berner et al., 1995; Mathimaran et al., 2005). Indeed, some studies (Giovannetti and Mosse, 1980; Mathimaran et al., 2005) have shown that certain soil-borne saprophytic bacteria have a potential to control S. hermonthica by inhibiting seed germination. However, the survival of inoculated bacteria was usually not sustained in subsequent crops and thus implying the need for repeated application every cropping season (Mathimaran et al., 2005). Low levels of applied nitrogen increased Striga emergence on soils of low fertility and decreased on fertile ones (Patterson, 1990; Gworgwor and Webber, 1991; Lagoke and Isah, 2010).

However, there are certain fungi in the soil known as Arbuscular Mycorrhizal Fungi (AMF) which maintain symbiotic association with root of plants. More than 90% of plant species are known to be involved in this association. Benefits derivable include improved water and nutrient uptakes in poor and dry soils (Osonubi et al., 1991), enhanced growth (Mathimaran et al., 2005) and tolerance to diseases (Vierheilig et al., 2000) among others. These fungi are able to confer these advantages in mycorrhizal plants due to their ability to extend the area of nutrient and water exploitation arising from hyphal extension, competing for infection sites with other microorganisms and possibly stimulation of certain antagonistic substances. Vierheilig et al. (2000) reported that plants colonized by an AM fungus suppress subsequent colonization by other pathogens through altered root exudates but did not discuss about the fungus being the secondary symbiotic. This study was therefore aimed at managing S. hermonthica using Arbuscular Mycorrhizal fungi (Glomus mosae) and NPK fertilizer levels on sorghum.

MATERIALS AND METHODS

The experiment was conducted in the screen house of Niger State College of Agriculture, Mokwa green house during October to December, 2008 and January to March, 2009. The design was a split-split plot arrangement in a randomized complete block design. The main plot treatments consisted of three sorghum varieties; SAMSORG 3, ICSVIII and SAMSORG 14 while sub-plot treatments consisted of inoculants; Striga mixed with Glomus, Glomus only, Striga only and no inoculation control. The sub-sub plot treatments consisted of compound fertilizer levels; 100 kg P₂O₅-50 kg K₂O/ha, 50 kg N-25 kg P₂O₅-25 kg K₂O/ha and 0 kg N-0 kg P₂O₅-0 kg K₂O/ha. Isah et al. (2009) had earlier reported the ability of SAMSORG 3 and ICSV III to be tolerant or resistant varieties to S. hermonthica parasitism while SAMSORG 14 was susceptible.

The soil which was collected from the crop farm was sieved to remove stone and debris and filled into the pots. The pots inoculated with Striga were filled to two-third depth while the uninoculated pots were filled to the brim. The remaining one third of the pots inoculated with Striga was filled with soil-Striga seed mixture for the inoculated pots. The soil were inoculated with approximately 3000 Striga seeds per pot using the procedure described by Kim (1994) and Berner et al. (1997). Fifty grams of the AM fungal inoculums collected from International Institute of Tropical Agriculture which was multiplied in the University of Agriculture, containing spores of Glomus mosae, hyphae and root fragments of maize as host was introduced into the planting hole of inoculated pots while attenuated inoculum of the same quantity was introduced into the planting hole of uninoculated plants. Three sorghum seeds (after surface sterilization in 1% NaOCl for 15 min and subsequent rinsing with demineralised water were sown per pot. The plants were watered to field capacity for the first weeks and thereafter with 10 mL of water in 48 h. The seedlings were later thinned down to two per pot at two weeks after planting. Mycorrhizal root colonization was determined by observing the stained root samples using grid-line intersect method of (Phillips and Torrey, 1970; Giovannetti and Mosse, 1980).

The data collected are sorghum height, vigour and reaction scores and Striga shoot count all at 6, 9 and 12 Weeks after Planting (WAP), days to first Striga emergence, sorghum dry weight and Striga shoot dry weight at harvest.

Data collected was subjected to Analysis of Variance (ANOVA) according to Steel and Torrie (1960) and Fishers Least Significant Difference (LSD) was used to compare treatment means using SAS (2009) statistical package.

RESULTS AND DISCUSSION

Sorghum varieties: Sorghum varieties differed significantly in plant height; crop vigour score, crop reaction score and Striga shoot count all at 6, 9 and 12 WAP as well as days to emergence of Striga, Striga and crop shoot dry weights at harvest and in the combined analyses. SAMSORG 14 had shorter plants than SAMSORG 3 in all cases and ICS VIII at 6 and 9 WAP only (Table 1). SAMSORG 3 also had significantly higher vigour score (Table 2) but lower reaction score (Table 3) at 6, 9 and 12 WAP than the other two sorghum varieties evaluated in this trial. Striga shoot emergence count at 6, 9 and 12 WAP followed the order SAMSORG 3<SAMSORG 14<ICSV III in the trial (Table 4). Days to Striga shoot emergence was significantly more on Samsorg 14 compared with the other two varieties (Table 5). SAMSORG 3 also had significantly higher crop shoot dry weight; however, Striga shoot dry weight was lower at 6, 9 and 12 WAP than the other two varieties evaluated. (Table 5). Isah et al. (2009) had earlier reported that SAMSORG 3 is resistant to S. hermonthica parasitism while SAMSORG 14 is susceptible.

Influence of inoculation on growth of sorghum: Inoculation had significant effect on plant height, vigour and reaction scores and Striga shoot emergence count all at 6, 9 and 12 WAP as well as crop and Striga shoot dry weight at 12 WAP and days to Striga emergence in the combined analyses.

Table 1:	Effects of inoculation and fertilizer on plant height of Sorghum varieties at 3, 6, 9 and 12 WAP in 2008, 2009 and combined
Means with the same alphabets within same column are not significantly different from one another at p<0.05, WAP: Week after planting

Table 2:	Effects of inoculation and fertilizer on crop vigour score of sorghum at 6, 9 and 12 WAP in 2008, 2009 and combined
Means with the same alphabets within the same column are not significantly different from one another at p<0.05, WAP: Week after planting, Vigour score: Using scale (1-5), 1: Not vigorous, 2: Poorly vigorous, 3: Fairly vigorous, 4: Moderately vigorous, 5: Very vigorous

Table 3:	Effects of inoculation and fertilizer on crop reaction score of sorghum varieties at 6, 9 and 12 WAP in 2008, 2009 and combined
Means with the same alphabets within the same column are not significantly different from one another at p<0.05, WAP: Week after planting, Sorghum reaction score used was scale (1 to 9), where 1 was assigned to normal plant growth and 9 was completely dead plant

Table 4:	Effects of inoculation and fertilizer on Striga shoot count at 6, 9 and 12 WAP in 2008, 2009 and combined
Means with the same alphabets within the same column are not significantly different from one another at p<0.05, WAP: Week after planting

Table 5:	Effects of inoculation and fertilizer on days to Striga emergence and crop and Striga dry shoot weight of sorghum varieties in 2008, 2009 and combined
Means with the same alphabets within the same column are not significantly different from one another at p<0.05

Plants of pots inoculated with Striga alone were shorter than those of the pots inoculated with its mixture with Glomus, Glomus alone and the control (Table 1) as this is in agreement with the study of Mobasser et al. (2012) that reported a significant influence of mycorrhizal on plant height of maize. Also Samra et al. (1997) reported that the general application of mycorrhiza increased plant heights compared to the control treatments without mycorrhizal infection. The results of this trial are similar to those of other reports showing that infections of mycorrhizal fungi increased plant height compared to controls (Ahiabor and Hirata, 1994; HashemiDezfuli et al., 1999; Olatunji et al., 2008).The various forms of inoculation resulted in shorter plants than those of the control except with Glomus at 12 WAP which was not significant (Table 1). Plants inoculated with Striga alone had lower vigour scores than those of the other treatments while those of Striga plus Glomus also had lower scores compared with those of Glomus alone and control at 9 and 12 WAP (Table 2). The reaction scores of plants of pots with Striga inoculation was obviously higher than those of the other treatments which were similar at 9 and 12 WAP while Glomus alone caused higher reaction score than the control at 12 WAP (Table 3). Striga shoot emergence count and weight were significantly lower with Striga/Glomus mixture inoculation than Striga alone while emergence was delayed by the mixture. Striga inoculation alone resulted in significantly lower crop shoot dry matter production compared with the other treatments, although Striga/Glomus mixture also caused lower values than Glomus alone and the control (Table 5).The ability of AMF to reduce Striga count can be explained in three ways: (1) The formation of metabolites, especially strigolactones that are responsible for the induction of Striga germination is down regulated upon mycorrhizal colonization; (2) Plant metabolites, such as cyclohexenones which arise through carotenoid degradation, that are up-regulated upon mycorrhizal colonization inhibit Striga germination and (3) Mycorrhizal colonization induces mycorrhizosphere effects that negatively impact on Striga germination (Lendzemo et al., 2007).

Influence of fertilizer rates on growth of sorghum: Fertilizer application had significant effect on height, vigour score, reaction score of sorghum as well as shoot count, days to emergence, dry matter production and dry weight of Striga. Sorghum plant height at 6 WAP (Table 1) and vigour score at 6, 9 and 12 WAP (Table 2) crop shoot dry weight at 12 WAP (Table 5) increased with the rates of fertilizer applied, while both rates of fertilizer similarly increased plant height of sorghum at 9 and 12 WAP. Conversely, crop reaction score (Table 3) and emergence shoot count (Table 4) and dry weight of Striga (Table 5) decreased with the rates of applied fertilizer. The two rates of fertilizer applied significantly delayed Striga emergence on sorghum compared to no fertilizer control. Gworgwor and Webber, 1991) observed that application of high nitrogen increases the performance of cereal crops under Striga infestation. This is due to the fact that nitrogen fertilizer reduced the severity of Striga attack while simultaneously increasing the host performance (Lagoke and Isah, 2010).

CONCLUSION

Sorghum variety SAMSORG 3 was resistant to Striga hermonthica in this study. There was an improvement in the performance of sorghum varieties when Striga was mixed with Glomus indicating that Arbuscular Mycorrhizal fungi is effective in S. hermonthica management. It was further enhanced when higher levels of fertilizer are used especially with those sorghum varieties found to be susceptible to S. hermonthica parasitism.