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Striga hermonthica; Sorghum landraces; Striga resistance

Introduction

Striga hermonthica (Del.) Benth., commoly known as Purple witch’s Weed, is a root parasitic flowering weed of the family Orbanchacea (Matusova  et al. 2005; Mohamed and Musselman 2008). It is an abundant and damaging parasitic weed found throughout the world (Ejeta and Gressel 2007; Oswald 2005; Parker 2009). It is particularly common in sub-Saharan Africa, including the Central, West and East Africa regions (Gethi and Smith 2004; Mohamed and Musselman 2008; Rodenburg et al. 2016) [1].

Striga hermonthica is known to severely infest various crops, including sorghum, maize, millets, tef, rice and even sugarcane (Addisu and Feleke 2021; Atera and Itoh 2011; Atera  et al. 2012; Kountche  et al. 2016; Parker 2012; Spallek  et al. 2013). It has severely affected the agricultural productivity of smallholder subsistence farmers in sub-Saharan Africa, including Ethiopia. It is also considered to be the most devastating biological barrier to cereal production in these regions (Omanya et al. 2004). Several studies have highlighted the widespread infestation of S. hermonthica in different parts of Ethiopia. In the Tigray region, Atsbha Gebreslasie et al. (2016) reported moderate to severe infestation S. hermonthica. Lemma Degebasa et al. (2022) reported S. hermonthica as the dominant species in eastern and western Hararghe, Oromia. S. hermonthica is a serious challenge to sorghum production in almost all districts of Benishangul Gumuz, Ethiopia (Mesfin and Girma 2022) [2,3].

The impact of S. hermonthica to sorghum production in Ethiopia is significant and widespread. Various studies have reported yield losses ranging from 65% to 100% in sorghum due to Striga infestation (Bayable & Marcantonio 2013; Ejeta et al. 2002; Haussmann et al. 2000; Lemma Degebasa et al. 2022; Tesso et al. 2007). In Benishangul Gumuz, S. hermonthica has been identified as the main factor affecting sorghum production (Mesfin and Girma 2022). The detrimental effects of S. hermonthica are not confined to Ethiopia, but are also felt in other parts of the Africa (Ejeta and Gressel 2007) [4].

These points highlight the need for developing effective strategies to manage and control S. hermonthica in order to mitigate its devastating impact on sorghum production in Benishangul Gumuz region and other affected areas in Ethiopia. The use of resistant crop varieties has been proposed as a practical and cost-effective long-term strategy for managing Striga (Hearne 2009; Mandumbu et al. 2019). Therefore, this study aimed to identify sorghum genotypes that are resistant to S. hermonthica [5].

Materials and Methods

Plant materials

The sorghum genotypes used in the study were selected from landraces collected from farmers’ fields in Ethiopia, specifically in Benishangul Gumuz and some parts of western Oromia. A total of 49 genotypes were included in the study, along with four released varieties. The resistant checks used in the study, Berhan and Framida, were obtained from Melkassa Agricultural Research Center. The inclusion of check varieties helped for the comparison sorghum genotypes and evaluation of genetic diversity and resistance in sorghum to S. hermonthica.

In the pot trials conducted for the study, a total of 48 sorghum genotypes were used. This included both negative (susceptible genotypes) and positive checks (resistant genotype). The susceptible checks used in the pot trials were Assosa-1 variety, which is commonly grown in the Benishangul Gumuz region, and Adukara variety. The resistant check used was Berhan variety [6].

Out of the initial 48 sorghum genotypes tested in pots, a total of 33 sorghum genotypes were selected for further evaluation in a sick-plot specifically designed to measure their resistance to S. hermonthica. Additionally, another resistant check, Framida variety, was included in this evaluation. The sick-plot trial was conducted at the Assosa Agricultural Research Center [7].

For further validation of the pot and sick-plot trials, seven sorghum genotypes, including resistant check varieties like Berhan and Framida, as well as promising resistant landraces such as ETSL102969, ETSL102970, and ETSL102975, alongside susceptible check varieties like Assosa-1 and ETSL102957 were selected and evaluated under hot-spot farmers’ fields in three locations at Assosa, Benishangul Gumuz, Ethiopia [8].

The Striga seeds used in the study were collected over a period of three years, from 2019 to 2021. These seeds were obtained from sorghum fields that were highly infested with S. hermonthica in various districts of Assosa Zone, specifically Bambasi, Abramo, and Ura districts. After collection, the seeds were stored in glass jars, kept in a dark environment at room temperature until they were needed for infesting the pots and sick plots [9].

Study sites, trial design and procedures

A trial was established at the Assosa Agricultural Research Center (AsARC), situated in Assosa Zone, Benishangul Gumuz region, Ethiopia. The pot trials were laid out in randomized complete block design having two replications in 2020 and 2021 under the Lath-house condition of AsARC. Sand/peat/compost (1:3:1 v/v) mix soil was used to fill 96 pots. Each pot was infested with 4 mg S. hermonthica seeds at 5 cm depth and covered with a thin layer of the mix soil. After a 10 day delay to precondition the Striga seed, six sorghum seeds of each genotype were sown in the pot and later thinned to three plants per pot [10].

A total of 33 sorghum genotypes were also evaluated in S. hermonthica sick plot of AsARC in 2022. The trial site was ploughed twice and furrows with 70 cm spacing were prepared. The trial was laid out in RCBD having two replications. The furrows within the trial unit (2 m x 1.40 m) were uniformly infested by S. hermonthica seeds collected during 2021 cropping season, Ethiopia. The infested Striga seeds were covered with a thin layer of soil and preconditioned for 10 days. After preconditioning, sorghum genotypes were sown within the furrows at a rate of 10 kg ha^-1. Apart from Striga, other weeds were hand weeded when observed. Other agronomic practices were done as recommended for the areas.

The validation trial was designed in RCBD with three replications. Farmers’ fields are considered as a replication. The seven chosen sorghum genotype were validated using plot sizes of 4.20 m X 4.05 m for each genotypes. Other than Striga, weeds were manually pulled when they were noticed. We followed the areas’ recommended other agronomic procedures [11].

Data collection and analysis

Data on sorghum and Striga parameters were collected. Data on sorghum included days to 50% anthesis, days to maturity, plant height, number of leafs, biomass and dry mater (g/pot). Striga data includes emerged Striga count and Striga height were carefully inspected at weekly intervals starting from 7th weeks after crop emergence (WACE) to 12th WACE. Also, Striga biomass and dry matter was recorded [12].

Data analysis

The maximum above ground Striga was determined as suggested by Rodenburg et al. (2006). The area under Striga number progress curve (ASNPC) was calculated as suggested by (Haussmann et al. 2012) as follows:

Where n is the number of Striga recording dates, Yi is the Striga count at the ith assessment date, and ti is the number of days after sowing at the ith assessment date.

Collected data were subjected to analysis of variance using randomized complete block procedure of R software. The mode used was

Where, Yij is observed value for the experimental unit in the jth replication (r) assigned to the ith genotype, j = 1, 2, . . ., r and i = 1, 2, . . ., genotype, µ is the overall mean, α is the effect due to the ith treatment, β is the effect due to the jth block, and εij is the error term where the error terms, are independent observations from an approximately normal distribution with mean equal to zero and constant variance σ2ε [13].

Independent sample t-test was used to assess the significant difference between sorghum genotypes’ performance against S. hermonthica. Treatment means were separated using Fisher’s least significant difference procedure at 5% probability level [14] (Table 1).

Table 1: List and sources of 49 sorghum genotypes used in the study

Results and Discussion

Response of Sorghum landraces to S. hermonthica

Pot-trial

The mean number of emerged Striga plants per pot across all sorghum genotypes was 12.13, while the mean number of Striga plant per sorghum plant was 4.04. This indicates that overall, there is an adequate level of Striga infestation in the study to determine the resistance of sorghum genotypes to Striga (Table 2).

Table 2: The response of Sorghum landraces to artificially infested S. hermonthica (at 12th WACE) in 2020 to 2021 under pot experimentation at Assosa, Benishangul Gumuz, Ethiopia

The results of the pot experiment results indicate that there is a highly significant difference (P <0.0001) in the response of sorghum landraces to S. hermonthica infestation. The average number of emerged Striga plants per pot ranged from 0.25 for the Berhan variety (resistant check) to 29.75 for the ETSL102973 landrace. Similarly, the number of Striga plant per sorghum plant ranged from 0.08 for Berhan to 9.92 for ETSL102973. These findings suggest that ETSL102973 had significantly higher Striga emergence compared to Berhan, indicating its susceptibility to S. hermonthica. On the other hand, sorghum landraces ETSL102969, and ETSL102970 showed reduced Striga emergence compared to other landraces [15].

Sick plot trial

The sick-plot experiment revealed that the mean number of emerged Striga plants per plot was significantly varied (P<0.05) among sorghum landraces at 12 WACE. Sorghum genotypes exhibited varied Striga emergence per plot that ranged from 3.0 for the Berhan and ETSL102969 to 148.5 for the ETSL102944 sorghum landrace. Likewise, emerged Striga plants per sorghum plant ranged from 0.22 for the ETSL102966 to 5.48 for the ETSL102954. The result exposed that the resistant check Berhan and sorghum landrace ETSL102969 sustained the lowest (3.0) Striga emergence. Sorghum landraces ETSL102970, ETSL102975, ETSL19001, and ETSL100053 also exhibited lower Striga emergence compared to the resistant check variety Framida in the sick-plot trial. The results are consistent with the findings from a previous pot trial [16-20].

Result of this study revealed that the resistant sorghum genotypes were early maturing with maturity period of 125 days for the Berhan, 142 days for ETSL102970, and 144 days for ETSL102969. This argued with the finding of Ayana et al. (2019) that reported early maturing sorghum genotypes showed resistance to S. hermonthica. Franke et al. (2006) was also reported that earlier maturing sorghum genotypes had positive response to Striga stress.

Moreover, S. hermonthica resistant sorghum genotypes of this study were shorter plant heights that ranged from 101.96 cm for ETSL102969 to 139.79 cm for Berhan. Similarly, they have lower number of leaves per plant that ranged from 3.45 for ETSL102969 to 5.87 for Berhan  [21-23].

As illustrated the genotypes ETSL102970, Berhan, and ETSL102969 have lower ASNPC values compared to other genotypes, indicating a slower or less severe emergence of Striga plants in these resistant sorghum genotypes. On the other hand, genotypes ETSL102957 and ETSL102944 exhibit higher ASNPC values, suggesting a higher incidence and more rapid emergence of Striga plants in these susceptible sorghum genotypes. The resistant checks (Berhan and Framida) also demonstrate low ASNPC values, indicating their resistance to S. hermonthica infestation. These findings further support the potential resistance of sorghum genotypes ETSL102970, Berhan, and ETSL102969 against S. hermonthica infestation in Assosa, Benishangul Gumuz region of Ethiopia (Figure 1,2&3).

Figure 1: Reaction of sorghum genotypes to area under S. hermonthica emergence progress curve in 2022 at Assosa, Benishangul Gumuz, Ethiopia.

Figure 2: Top five sorghum genotypes with lowest emerged Striga count per sorghum plant from 2020 to 2022 at Assosa, Benishangul Gumuz, Ethiopia.

Figure 3: Top five sorghum genotypes with highest emerged Striga count per sorghum plant for 2020 to 2022 at Assosa, Benishangul Gumuz, Ethiopia.

Validation trial at hot-spot farmer’s fields

As illustrated in the results of the validation trial confirm that the resistant check Berhan and sorghum landrace ETSL102969 have the lowest number of emerged S. hermonthica plants per 4m x4m plot. Additionally, sorghum landrace ETSL102970 has a lower number of emerged S. hermonthica plants compared to the second resistant check Framida. The number of Striga per sorghum plant is also low for Berhan, ETSL102969, and ETSL102970 [24].

On the other hand, susceptible checks ETSL102957 and Assosa-1 variety have the highest count of emerged S. hermonthica plants. These findings indicate that Berhan ETSL102969, and ETSL102970 exhibit promising resistance against S. hermonthica infestation. Overall, this validation trial confirms that these sorghum landraces possess comparable or good resistance to S. hermonthica when compared to resistant checks like Berhan and Framida [25] (Table 3).

Table 3:  Validation of Striga hermonthica resistant sorghum landraces at hot spot farmers’ fields in 2023 at Assosa, Benishangul Gumuz, Ethiopia

The promising sorghum landraces ETSL102969 and ETSL102970 have been found to have higher yielder yield compared to resistant checks in a validation trial. Additionally, the bold seed size and white seed colour of sorghum landrace ETSL102969 are desirable traits by the local farming communities. These traits can be advantageous for the breeding program, as they can be combined with the Striga resistance trait to develop sorghum varieties that have both resistance to Striga infestation and the preferred seed characteristics. By incorporating these additional traits into the breeding program, there is a higher probability of obtaining F-generations that exhibit both white colour and bold sized seeds, along with resistance to S. hermonthica. This would not only benefit the farmers but also contribute towards improving productivity and enhancing market value for sorghum in the region [26].

Conclusions

In conclusion, sorghum landraces ETSL102969 and ETSL102970 possess good resistance to S. hermonthica, even outperforming the resistant checks. This suggests that these landraces could be valuable sources of resistance in breeding programs aimed at developing Striga resistant sorghum varieties. Moreover, the fact that sorghum landrace ETSL102969 has the additional advantage of white seed colour, which is highly preferred by farmers, makes it an even more promising candidate for breeding programs. By incorporating this desirable trait along with the Striga resistance trait from ETSL102969 and the enhanced resistance from ETSL102970, it may be possible to develop new sorghum lines that exhibit both Striga resistance and white seed colour.

Acknowledgements

Authors thanks Ethiopian Institute of Agricultural Research for financial support and Assosa agricultural Research Center for vehicle support during S. hermonthica seed collection and field works.

Disclosure Statement

No potential conflict of interest is expected by the authors.

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