African Crop Science Journal African Crop Science Society ISSN: 1021-9730 EISSN: 2072-6589 Vol. 8, Num. 2, 2000, pp. 171-178

African Crop Science Journal, Vol. 8. No. 2, pp. 171-178

AN ALTERNATIVE METHOD OF SCREENING MAIZE FOR TOLERANCE TO STRIGA

V.O. Adetimirin, S.K. Kim¹ and M.E. Aken’Ova²
Department of Agronomy, University of Ibadan, Ibadan, Nigeria
¹International Institute of Tropical Agriculture (IITA), PMB 5320, Ibadan, Nigeria
Present address: International Agricultural Research Institute (IARI), Kyungpook National University, Taegu, South Korea
²Department of Plant Production, University of Venda, Pvt. Bag 5050, Thohoyandou, South Africa

(Received 21 October, 1998; accepted 23 February, 2000)

Code Number: CS00018

INTRODUCTION

Striga spp. are parasitic weeds of cereals and legumes. Presently, Striga is considered one of the major biological constraints to food production in African agriculture (M’Boob, 1994; Kroschel et al., 1996), with annual yield losses estimated at US$ 7 billion (M’Boob, 1989). Striga hermonthica (Del.) Benth., which infects cereals, is the most widespread species (Lagoke et al., 1991), and is responsible for the greatest losses (Ramaiah, 1991). Striga tolerant and resistant crop varieties can avert these losses, but are more effective if used in combination with other control measures (Adetimirin and Kim, 1997; Kim and Adetimirin, 1997a; Kim et al., 1994).

In Striga research, tolerance is the ability of maize plants to withstand the effects of the parasite already attached while resistance relates to the ability of a host plant to prevent attachment or emergence (Kim and Adetimirin, 1997a). Grain yield, the primary concern of small-scale farmers, correlates better with Striga tolerance than resistance (Kim, 1991). Field screening under Striga attack is required for the development of tolerant varieties. However, because of spacial variability of Striga seed distribution under natural infestation (Berner et al., 1996) and in Striga-sick plots (Efron, 1993), efficient progress for tolerance breeding can only be made under uniform artificial Striga seed infestation. The currently reco-mmended practice involves the application of Striga seed to every hill or stand of the crop. The various stages in the process of artificial infestation are (i) scooping out soil to create 5 cm deep holes, (ii) application into the holes of known quantities of Striga seed, and (iii) light covering of the applied Striga seed with soil. This process is completed prior to planting the crop.

Although effective, artificial infestation of every individual hill is labour intensive (Berner et al., 1996), and thus expensive. For maize that is planted at a standard spacing of 0.75 x 0.25 m, with one plant maintained per hill, the resulting plant population density of 53,333 plants ha-1 implies that artificial infestation would be required for 53,333 hills ha-1. Reduction of the labour requirement for screening under artificial infestation can be achieved by widening the spacing between hills thereby reducing the number of hills per unit area while increasing number of plants per hill. The objective of this study, therefore, was to investigate the effectiveness of screening that involves artificial infestation of more widely spaced individual hills while retaining the same plant population per unit area as that under standard spacing through the proportionate increase in the number of plants per hill. However, labour requirement for infestation and crop seed planting is reduced owing to the reduction in number of hills.

MATERIALS AND METHODS

Screenhouse and field experiments were carried out to compare an alternative method of Striga tolerance screening with the standard method currently in use. The standard method involves application of 4200 germinable Striga seeds into 5 cm deep holes of 8 cm diameter every 0.25 m on ridges spaced 0.75 m apart. Two to three maize seeds were planted in the holes after Striga seed infestation. Plants were thinned to one per hill (OMPH) two weeks after maize planting (WAP). The components of the standard method (OMPH) viz. quantity of Striga seeds applied per hill, hill spacing and number of maize plants per hill were modified to obtain the alternative method, TMPH. In TMPH, 8400 Striga seeds were applied in holes of similar dimensions as in OMPH but spaced 0.50 m apart on the ridges, and maize plants thinned to two per hill. The same maize plant population was therefore achieved under OMPH and TMPH. However, while in OMPH 53,333 hills ha-1 would be infested and planted with maize, in TMPH only 26,666 hills are involved. This should translate into a 50% reduction in labour requirement with the alternative method.

Screenhouse trial. The screenhouse trial was carried out at the International Institute of Tropical Agriculture (IITA), Ibadan, Nigeria (latitude 7o3’N, longitude 3o54’E). A total of 44 families of 14 single cross hybrids including one tolerant hybrid, 9022-13, and one susceptible hybrid, 8338-1, both as checks; and 119 families of 14 inbred lines were evaluated under the two screening methods. These materials were selected in field trials in Mokwa (latitude 9o18’N; longitude 5o4’E). Originally, the screenhouse trial was conducted to verify the performance of selected families prior to off-season seed multiplication and recombination of proven families. However, the need for a cheaper method of screening was the basis for testing another screening method.

Ridges were made with hoes and maize planted three days after Striga seed infestation. Each hybrid and inbred family was planted to one hill under each of the screening methods. Each screening method was in one replication due to limited screenhouse space. Fertiliser was not applied and weeds other than Striga were hand-pulled. At weekly intervals from 8 to 12 WAP, observations on maize damage score (MDS), and number of Striga shoots per maize plant (SNUMP) were made. Height of maize was measured at 12 WAP. Maize damage was scored on a scale of 1 to 9, where 1 = normal maize growth and 9 = complete maize damage or death (Kim, 1994). Leaf blotches with necrotic spots, plant stunting, stem diameter, ear and tassel size reduction, stalk lodging and husk opening are considered in the damage scores (Kim and Adetimirin, 1997b). Data from the inbreds were analysed separately from those of hybrids as possible differences in response to the two methods, indicated by the method x genotype interaction cannot be detected due to the lack of true error term. Analysis of variance was carried out assuming screening methods and genotypes to be random effects. Screening methods and genotypes were each tested against method x genotype interaction for significance.

Field trials. Field studies were conducted in two years. The first year study was carried out at Abuja (latitude 9o18’N; longitude 7o20’E) and the second at Mokwa. Both locations are in the southern Guinea Savanna zone in Nigeria where Striga poses a serious threat to maize cultivation. The Abuja soil was a sandy loam alfisol. Soil pH (H20 : Soil, 1:1) was 5.1, organic carbon 5.8 g kg-1, total nitrogen 0.68 g kg-1, available phosphorus (Bray –1) 41.5 mg kg-1. Exchangeable cation (NH40AC, cmol kg-1) was 1.69 for Ca, 0.46 for Mg and 0.28 for K. In Mokwa, the soil was a sandy loam ultisol. Soil pH (H20 : Soil, 1:1) was 6.4, organic carbon 7.9 g kg-1, total nitrogen 0.51 g kg-1, available phosphorus (Bray-1) 5.3 mg kg-1. Exchangeable cation (NH40AC, cmol kg-1) was 1.90 for Ca, 0.70 for Mg and 0.20 for K.

Two hybrids viz. 9022-13, tolerant and 8338-1, susceptible, and two inbred lines viz. TZi 12 (tolerant) and TZi 10 (susceptible) were evaluated under the two screening methods described above. Land was ploughed, harrowed and ridged. The experiment was a split-plot with four replicates. Genotypes were assigned to main plots and screening methods to sub-plots. The main plots were arranged in a randomised complete block design. Screening methods were randomly assigned to sub-plots. The plants were established in 3 m long rows with 4 rows per plot. A total of 60 kg N ha-1, which is half the recommended rate for maize in this zone was applied as low N enable better Striga development. Nitrogen was applied in two split doses of 30 kg ha-1 at 3 and 8 WAP. Phosphorus and potassium were applied at land preparation, each at the elemental rate of 30 kg ha-1. Removal of weeds other than Striga was carried out throughout the duration of crop growth. In Abuja, data were collected on MDS (10 WAP) and maize height. In Mokwa, additional data collected include SNUMP (10 WAP), ear height, ear aspect score, number of ears per plant and grain yield. Data were collected from plants in the two innermost rows. Ear aspect was assessed on a scale of 1 to 9, where 1 = ears well formed, and 9 = ears poorly formed. Maize yield was adjusted to 15% moisture content. Data were analysed as split plot.

RESULTS AND DISCUSSION

In the screenhouse, hybrids under TMPH showed more damage symptoms at 8, 9 and 10 WAP, hence MDS at these times were significantly higher (P < 0.05) than those for OMPH (Table 1). At 11 and 12 WAP, differences in MDS between plants under OMPH and TMPH were not significant. Maize damage caused by Striga can be best assessed at the early stages of grain filling (Kim, 1994). This period corresponded to about 10 WAP for the hybrid genotypes used in this study, which are of intermediate maturity (110 – 120 days). At this time, MDS under TMPH was significantly (P < 0.05) correlated (r = 0.38, n = 44) to MDS under OMPH. With the inbred lines, MDS of plants under TMPH was significantly higher at 8 and 9 WAP but not beyond (Table 1). At 9 WAP, Spearman rank-order correlation (r) between MDS ranks of 119 inbred lines under both screening methods was highly (P < 0.01) significant (r = 0.39, n = 119). Thus, the relative severity of damage symptoms developed by the maize genotypes under OMPH and TMPH were similar.

Throughout the sampling period (8 to 12 WAP), SNUMP of hybrids under TMPH was higher than under OMPH. However, the difference was significant only at 8 WAP (Table 2). The signi-ficantly higher SNUMP observed under TMPH at 8 WAP suggests a faster establishment and emergence of Striga under TMPH. Although average number of Striga seeds per plant was similar in OMPH and TMPH, twice the root mass of maize and Striga seeds in one hill of OMPH were concentrated within similar soil volume in one hill of TMPH. Thus, more Striga seeds would have been in close contact with maize root under TMPH. Such close contact is necessary for successful Striga seed stimulation and germination as well as attachment to host root. It has been estimated that distances between host root and Striga seed must not exceed 3 mm for Striga seed germination and successful attachment (Worsham, 1987). SNUMP of the inbreds under TMPH was also higher than SNUMP under OMPH at all the sampling periods considered except 12 WAP. In both OMPH and TMPH, SNUMP of hybrids was reduced at 12 WAP. A decline in the numbers of the parasites, notably at 12 WAP, often before the parasite flowers, has been observed on maize (Efron, 1993; Adetimirin, 1995).

Across screening methods, correlation between MDS at any two of the sampling intervals was significant for both hybrids and inbred lines (Table 3). This also applied to SNUMP. In effect, the relative response with respect to MDS and SNUMP of maize genotypes can be reliably assessed between 8 and 12 WAP, the sampling interval considered in this study. In general, r values between MDS and SNUMP were either low or not significant. Therefore, the degree of damage cannot be reliably inferred from the number of emerged Striga plants and vice-versa. In all cases, r values between MDS and plant height were significant (P < 0.01) and negative. This indicates that for most of the genotypes used in this study, a common effect of S. hermonthica parasitism was reduction in plant height.

In Abuja, average MDS was 4.9 and 5.0 for OMPH and TMPH, respectively, but the difference was not significant. Similarly, the difference in mean height between maize plants under OMPH (1.2 m ) and TMPH (1.3 m) was not significant. In Mokwa, differences between the two methods were also not significant for maize height, ear height, number of ears per plant, ear aspect score (data not shown), MDS and grain yield (Table 4). Whereas SNUMP was higher for TMPH than OMPH in the screenhouse, in the field SNUMP was significantly lower for TMPH than OMPH. Maize growth in the field was more vigorous than in the screenhouse. Striga seedling development is dependent on the density of Striga attaching to host root below soil surface. Kunisch et al. (1991) reported that, although number of underground Striga attachments increases with increase in density of the host plant, the probability of emergence actually reduces. In the present study, this density-regulating phenomenon was more pronounced in the field where maize growth was more vigorous, as maize and Striga seed density per hill with TMPH was higher than under OMPH. This indicates that, in addition to the density of host plant and Striga seed in the soil, other factors such as plant vigour influence SNUMP.

Screening method x genotype interaction was significant only for MDS. The two hybrids had higher MDS under TMPH while MDS for TZi 12, a tolerant inbred line, was higher under OMPH (Table 4). Maize damage score of TZi 10, the susceptible inbred was similar under both methods. Of the two widely used indices of genetic defence against Striga, i.e., SNUMP and MDS, only MDS was significantly (P < 0.01) correlated with maize plant height, ear attributes of maize and grain yield (Table 5), thus confirming MDS to be a good indicator of maize tolerance to Striga.

The labour-saving screening method, TMPH, gave similar or even higher values of the indices of tolerance/susceptibility, and therefore appeared more sensitive. For MDS in the screenhouse, where significantly higher values were obtained under TMPH, significant Spearman rank-order correlation of genotypes under both methods indicated that TMPH gives similar results as OMPH in terms of assessing the relative performance of maize genotypes under Striga. Therefore, in view of its reduced labour requirement TMPH could be used for screening maize for tolerance to S. hermonthica. The International Institute of Tropical Agriculture (IITA), Ibadan, Nigeria, which maintains about 8 ha of Striga breeding nurseries in different locations, now employs TMPH. It will also be useful for national programmes of many developing countries where resources are often limited.

ACKNOWLEDGEMENT

This study was carried out as part of the first author’s Ph.D. research at the International Institute of Tropical Agriculture (IITA), Ibadan, Nigeria. We thank IITA for use of her facilities.

REFERENCES

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