Emerged Striga Plants Research Articles

Genetic gains from selection for high grain yield and Striga resistance in early maturing maize cultivars of three breeding periods under Striga-infested and Striga-free environments

Maize is the most important staple cereal crop in sub-Saharan Africa (SSA) and has great potential to rectify the food insecurity in the sub-region. A study was conducted at four locations two each in Nigeria and Benin in 2010 and 2011 to determine the rate of genetic improvement in the grain yield of 50 maize cultivars developed during three breeding periods, 1988–2000 (period 1), 2001–2006 (period 2) and 2007–2010 (period 2) under Striga-infested and Striga-free conditions. Under Striga infestation, mean grain yield ranged from 2537kgha−1 for the period 1 cultivars to 3122kgha−1 for period 3 cultivars with a corresponding annual genetic gain of 1.93%. When Striga-free, grain yield ranged from 3646kgha−1 for the period 1 cultivars to 4227kgha−1 for the period 2 cultivars with an annual genetic gain of 1.0%. The average annual rate of increase in grain yield was 41kgha−1 when Striga-infested and 34kgha−1 when Striga-free. The annual genetic gain in grain yield was 0.86% for period 1 cultivars, 2.07% for period 2 cultivars, and 2.11% for period 3 cultivars under Striga infestation. The increase in grain yield under Striga infestation was associated with significant decrease in Striga damage and the number of emerged Striga plants at 8 and 10 weeks after planting (WAP), an improvement in ear aspect, lodging resistance, plant height, and increase in the number of ears per plant (EPP) from the period 1 to the period 3 cultivars. For the emerged Striga plants, the annual genetic gains of −0.63% for 8WAP and −0.57% for 10WAP were obtained from the period 1 to the period 2 cultivars. The increase in genetic gains was 0.70% for EPP and −0.65% for ear aspect. The increase in grain yield from period 1 to period 3 cultivars when Striga-free was associated with increased days to anthesis, increased plant and ear heights, improvement in ear aspect, and husk cover and increased EPP. Cultivars 28, 14, 29, and 26 were the most outstanding across Striga-infested environments. Substantial progress has been made in breeding for high yielding, Striga resistant/tolerant cultivars during the past three decades.

Relative changes in genetic variability and correlations in an early-maturing maize population during recurrent selection

Four cycles of S(1) family recurrent selection to improve grain yield and resistance to Striga hermonthica have been completed in TZE-Y Pop STR C(0.) In order to determine whether or not to continue with the recurrent scheme, it was desirable to evaluate the amount of residual genetic variance and associated parameters in the population. The objective of this study was to characterize the relative changes in the levels of the genetic variances, heritability estimates and genetic correlation coefficients, and to predict future gains from selection for grain yield, Striga resistance and other agronomic traits. Fifty S(1) families, derived from each cycle, were evaluated under Striga-infested and Striga-free conditions at Mokwa, Ikenne and Abuja, Nigeria, in 2005 and 2007. Under Striga infestation, genetic variances for grain yield, days to anthesis, plant height and Striga damage generally increased in the advanced cycles of selection. In contrast, the genetic variances for days to silk, anthesis-silking interval, ears per plant, ear aspect and number of emerged Striga plants decreased with selection. The advanced cycles of selection significantly out-yielded the original cycle in both research environments. Heritabilities for grain yield, Striga damage and number of emerged Striga plants were significantly greater than zero. The realized gains from selection for grain yield under Striga infestation (52kgha(-1)cycle(-1)) and Striga-free conditions (130kgha(-1)cycle(-1)) were remarkably lower than the predicted gains (350 and 250kgha(-1)cycle(-1), respectively). Adequate genetic variability exists in cycle 4 of the scheme to ensure future gains from selection.

Response of exotic sorghum (Sorghum bicolor [L.] Moench) cultivars to planting date under natural infestation of Striga hermonthica (Del) Benth. in the Sudan savanna zone of northeast Nigeria

Sorghum (Sorghum bicolor [L.] Moench) is one of the most important cereal crops grown in the dry savannas and semi-arid regions of West Africa under rain-fed conditions. Although total sorghum production in Nigeria has increased over the years, the average grain yield on farmers' fields has remained low. A two-year trial was conducted to evaluate the effect of planting dates on three exotic sorghum cultivars (P9402, P9405, and PSL985061) and an improved local cultivar (KSV 8) under natural infestation with Striga hermonthica in northeast Nigeria. At Nzuda, the highest yield was obtained when planting was carried out on 19 July 2006 and on 5 July 2007. The decline in yield in 2007 as planting dates increased was largely due to early cessation of rain, which affected grain yield. At Sabon-Gari, KSV 8 and P9402 produced comparable grain yields that were higher than the yields of other cultivars at all planting dates. Grain yields of KSV 8 and P9402 were greater for 5 July than for the other planting dates and decreased significantly as the planting date was delayed, probably due to long maturity period. The highest grain yields were obtained from P9405 and PSL985061 when planted on 12 July and 19 July, respectively. KSV 8 had the highest number of emerged Striga plants at both locations. The number of emerged Striga plants under KSV 8 was 4.0–5.1 times higher at Nzuda and 2.4–2.6 times higher at Sabon-Gari than the number on the exotic cultivars at those locations. Among the exotic cultivars, differences in the number of emerged Striga plants were not significant. Farmers in the zone would maximize sorghum yield if improved cultivars are planted at the appropriate dates in Striga infested fields.

Effects of Recurrent Selection for Grain Yield and Striga Resistance in an Extra‐Early Maize Population

ABSTRACTStriga hermonthica (Del.) Benth threatens maize (Zea mays L.) production in sub‐Saharan Africa. In an extra‐early population, S1 families derived from four cycles of recurrent selection for grain yield and Striga resistance were evaluated with and without Striga infestation at three locations in Nigeria for 2 yr to determine relative changes in genetic variances, heritabilities, and genetic correlations for yield and other traits. Under infestation, yield was not correlated with other traits at C0 but was significantly correlated with ears per plant (EPP), Striga damage, and emerged Striga plants in advanced cycles. Genetic correlations between yield and most traits were significant in C0 when Striga‐free but was reduced in advanced cycles. Genetic variances and heritabilities for yield and ear aspect increased in advanced cycles. Genetic variances decreased with selection for emerged Striga counts and EPP under infestation. A response to selection for improved yield, Striga emergence and EPP is expected in subsequent cycles. Low genetic variances and heritabilities for Striga damage and low predicted gain cycle−1 for all traits except numbers of emerged Striga plants suggest a need to introgress novel resistance genes into the population for faster progress from selection for increased yield.

Pathogenicity of Fusarium isolates to Striga hermonthica in Burkina Faso

Striga hermonthica (Del.) Benth. is an important constraint to cereal crop production in Burkina Faso, of which sorghum (Sorghum bicolor L. Moench) is the most important component. Native Fusarium species to use as bio-control agents to S. hermonthica has been investigated. Fifty one Fusarium isolates obtained from diseased plants of S. hermonthica were evaluated for their pathogenicity against Striga under controlled environmental conditions. Of 51 Fusarium isolates, 14 were pathogenic to S. hermonthica but their virulence differed. These 14 isolates were evaluated for their effects on Striga seed germination in the laboratory and their ability to kill emerged Striga plants growing in greenhouse pots. Spores of Fusarium sp. isolates 150a-M, 125b-Za, 6-Fa, Fusarium equiseti isolates 5-Kou, 31-Kom, 32-Or, 13-Ba and Fusarium oxysporum isolate 34-Fo reduced Striga germination by 78 to 96% compared to the untreated control. The study showed that at the rate of 33 mg mL(-1), metabolites of Fusarium sp. isolates 125b-Za, 6-Fa, F. equiseti 5-Kou and F. oxysporum 34-Fo prevented Striga seed germination. In addition to these four isolates, Fusarium sp. isolates 141b-O, 150a-M and F. equiseti isolate 32-Or were effective at 67 mg mL(-1). Percentage of Striga mortality ranged from 17-37% between 14 and 28 days after inoculation with spores of F. oxysporum 34-Fo and F. equiseti 5-Kou. Striga dry biomass was reduced by 84 and 78% for the respective isolates compared to the untreated control with Striga. Sorghum yield was improved by 84 and 99% with Fusarium sp. 6-Fa and F. oxysporum 34-Fo, respectively, compared to the control without Striga. The use of Fusarium spores and metabolites against Striga offers different possibilities of bio-herbicides formulation that can be combined with other controls methods in the integrated Striga management. Further studies will be carried out under field conditions to assess the efficacy and safety of these Fusarium isolates to environment and humans and evaluate low cost strategies for transfer to subsistence farmers.

Resistance screening of sorghum and effects of varieties and herbicide application on reproduction of Striga hermonthica

The obligate parasitic witchweed, [Striga hermonthica (Del.) Benth] is the greatest biological constraint to cereal production in sub-Saharan Africa. Sorghum [Sorghum bicolor (L.) Moench], a widely grown cereal in the region is an important host crop that has coevolved with the parasite. Several methods to control striga have been proposed, but with limited success. Genetic control of striga is effective, although sources of resistance are limited in most crops. Studies were initiated on striga–infested fields in view to identify sources of resistance to Striga hermonthica and to investigate the response of selected sorghum cultivars and some herbicides on the emergence of striga. In one experiment, eleven sorghum cultivars were screened for resistance to Striga hermonthica. Cultivars of sorghum differed significantly with respect to number of emerged striga plants at 60 days after planting and at harvesting. The varieties S 35 and CS 54 constantly recorded low number of striga plants than the popular resistant variety Framida. In the herbicide application trials, the numbers of emerged striga per square meter were much reduced following spraying with 2,4-D. Although the urea treatmenthad high grain yield, the number of emerged striga plant per unit area was also very high. Application of the appropriate herbicide at the right stage of striga emergence and growth, combined with the use of resistant/tolerant varieties may inhibit striga development. The results reported in this study offer promise in simultaneously sustaining high grain yields and suppressing the development of striga in farmers’ fields.Key words: Herbicides, Sorghum bicolor, host resistance, Striga hermonthica, sustainable production

Efficacy of Herbicide Seed Treatments for Controlling Striga Infestation of Sorghum

Witchweed (Striga spp.) infestations are the greatest obstacle to sorghum [Sorghum bicolor (L.) Moench] grain production in many areas in Africa. The objective of this study was to evaluate the efficacy of herbicide seed treatments for controlling Striga infestation of sorghum. Seeds of an acetolactate synthase (ALS) herbicide–tolerant sorghum hybrid were treated with two ALS‐inhibiting herbicides. Treatments included three rates of imazapyr (IMI), three rates of metsulfuron‐methyl (MET), and an untreated control group. In greenhouse trials, observations at 32, 46, and 60 d after planting showed that seeds treated with the highest herbicide rates had the fewest Striga attachments and the greatest delay in attachment. All plants in the untreated group died at or before sorghum flowering; however, herbicide seed treatments, particularly metsulfuron, reduced Striga emergence and significantly increased sorghum grain yield and dry matter production. Field studies comparing seed treatments produced similar results with delayed Striga emergence and fewer emerged Striga plants in herbicide‐treated plots. These studies indicate that herbicide seed treatments may provide a highly effective tool for managing Striga in sorghum.

Combining ability of maize inbred lines containing genes from Zea diploperennis for resistance to Striga hermonthica (Del.) Benth

AbstractMaize (Zea mays) is a staple food crop in sub‐Saharan Africa, but its production is threatened by Striga hermonthica (Del.) Benth. Transfer of resistance genes from wild relatives may increase resistance to S. hermonthica in tropical maize. The objective of this study was to determine the combining ability of resistance to S. hermonthica among lines containing Zea diploperennis and tropical germplasm. Forty‐five diallel crosses of 10 inbred lines were evaluated in an alpha‐lattice design with and without artificial Striga infestation at two locations each in the Republic of Benin and Nigeria for 3 years. Results of analyses showed that only general combining ability (GCA) mean square was significant (P = 0.01) for number of emerged Striga plants (NESPP), while both GCA and specific combining ability (SCA) mean squares were significant for host damage score (HDS) and grain yield under Striga infestation. The ratio of GCA to SCA mean squares for the three traits varied from 3.5 to 57.5. Although GCA × environment interaction was significant for the three traits, two inbred lines containing Z. diploperennis (ZD 551) and tropical (TZL TC 87) germplasm had negative and significant GCA effects for NESPP and HDS and positive GCA effects for grain yield under Striga infestation in the two countries. Correlation between NESPP and HDS was strong and significant (r = 0.87, P = 0.01). Our results highlight the importance of harnessing useful genes from wild relatives to improve resistance to S. hermonthica in adapted maize germplasm.

Registration of Striga‐Resistant and Drought‐Tolerant Tropical Early Maize Populations TZE‐W Pop DT STR C4 and TZE‐Y Pop DT STR C4

Two maize (Zea mays L.) populations, TZE‐W Pop DT STR C4 (Reg. No. GP‐574, PI 654407), white‐ grained, flint/dent, and TZE‐Y Pop DT STR C4 (Reg. No. GP‐575, PI 654408), yellow‐grained, flint/dent with drought tolerance and moderate levels of resistance to the flowering parasitic witchweed, Striga hermonthica (Del.) Benth, were developed at the International Institute of Tropical Agriculture (IITA). The two breeding populations were released to national maize programs in west and central Africa as source germplasm in 2004 for the development of Striga‐resistant synthetic varieties, parental inbred lines, and hybrids. The populations were released for their superior grain yield under Striga‐infested and noninfested conditions. They also have good levels of resistance to maize streak virus, tropical lowland rust (incited by Puccinia polysora Underw.), and blight [caused by Bipolaris maydis (Nisikado & Miyake) Shoemaker]. In multilocation trials in Benin Republic and Nigeria, 2006 and 2007, TZE‐W Pop STR C4 outyielded the Striga‐susceptible check TZE Comp. 4 by 44% under Striga infestation and 12% when noninfested. Under the same conditions, the increased yield for TZE‐Y Pop DT STR C4 was 42% (Striga infested) and 16% (noninfested). The check variety TZE Comp. 4 suffered the highest Striga damage and supported the highest number of emerged Striga plants when Striga infested.

Efficacy of Pesta granular formulation of Striga-mycoherbicide Fusarium oxysporum f. s p. strigae Foxy 2 after 5-year of storage

An important goal for mycoherbicide research and development is to produce a viable and consistently efficacious product with a long shelf life. The efficacy of 5-year-old Pesta granules of Fusarium oxysporum Foxy 2 in comparison to freshly prepared Pesta granules in controlling the root parasitic weed Striga hermonthica was studied. Pesta granules were made by encapsulating fresh chlamydospores of Foxy 2 in a matrix composed of durum wheat-flour, kaolin, and sucrose. Samples of Pesta granules were stored at 4°C over a period of 5 years. Two g per pot (4 kg soil) of each granular preparation were incorporated pre-planting to sorghum together with Striga seeds. The application of the 5-year-old Pesta granules (2.4 × 105 Cfu g–1) as well as the freshly prepared Pesta granules (1.5 × 106 Cfu g–1) of Foxy 2 sharply reduced the total number of healthy emerged S. hermonthica shoots and/or caused severe disease on the emerged Striga plants compared to the control. None of the emerged plants reached flowering stage in Foxy 2-treated pots, whereas in the control treatment 19.2% of the Striga plants flowered. Maximum efficacy (i.e., 100% reduction in healthy emerged Striga shoots compared to the control treatment) of both Pesta preparations in controlling Striga was achieved, indicating similar potential of both old and fresh formulations. The result revealed that encapsulation of Foxy 2 into “Pesta” formulation did not adversely affect its virulence and efficacy even after 5 years of storage under cold conditions. This represents an important attribute to support wide-scale deployment, storage, handling and delivery of Striga-mycoherbicides Pesta technology in Africa.

Biological control of Striga hermonthica by Cubitermes termite mound powder amendment in sorghum culture

Striga hermonthica (Del.) Benth is an obligate root hemi-parasite of several cereals. Its effect on cereal crops is the main constraint for food production in sub-Saharan Africa. Various control methods have been already proposed, but the infestation by these parasitic plants persists. An appropriated method for Striga management adapted for the African farmer is very much needed. In this study, amendment of soil infested by this phytoparasite with Cubitermes mound powder is proposed as chemical amendment and natural microbial inoculum, to promote plant growth and reduce damage by S. hermonthica on sorghum ( Sorghum bicolor L.). The influence of Cubitermes mound powder on the development of several microbial groups (arbuscular mycorrhizal fungi, actinomycetes, saprophytic fungi) was investigated in a pot experiment with sorghum cultured in a sandy soil infested by S. hermonthica. In the amended soil, sorghum growth and mycorrhizal colonization of sorghum plants were significantly greater than in the control treatment. Mycorrhizal colonization was negatively correlated with the number of emerged Striga plants per pot and positively correlated with sorghum growth. The relationship with substrate-induced respiration (SIR) responses showed that amended soil was characterized by its response to hydroxybutyric acid (catabolic marker of mycorrhizal colonization) and non-amended soil by its response to phenylalanine. We noted that the number of emerged Striga plants in amended pots was significantly decreased. Since Cubitermes mound suspensions did not affect Striga seed germination under axenic conditions, it suggests that the amendment with Cubitermes powder reduces S. hermonthica infestation indirectly, i.e. via its effect on the indigenous soil microflora. Overall, it appears that management of Cubitermes mounds is a promising strategy to consider for effective protection of sorghum from Striga infestation.

The role of infection time in the differential response of sorghum cultivars to Striga hermonthica infection

Summary Striga hermonthica is an important parasitic weed that severely reduces yields of sorghum in sub‐Saharan Africa. Pot experiments with the sensitive sorghum cultivar CK60‐B and the tolerant Tiémarifing were conducted in 1999 and 2000 to investigate the role of infection time on the interaction between sorghum and Striga hermonthica. Timing of Striga inoculation was used to establish delays of one and two weeks in first attachment of the parasite. In 1999, early Striga inoculation resulted in a relatively early first Striga attachment on CK60‐B. Although first infection of Tiémarifing occurred one week later, an identical final number of emerged Striga plants was observed. Plants of CK60‐B were more severely affected and supported a higher total Striga biomass. Only with this cultivar the interaction between host and parasite was significantly affected by delayed infection. Parasite biomass was most sensitive and already significantly reduced following a 1‐week delay in infection time. With a further 1‐week delay, an additional reduction in parasite biomass was accompanied by a strong and significant increase in total and panicle dry weight of the host plant. In 2000, first infection of CK60‐B was relatively late and occurred simultaneously with first infection of Tiémarifing and no significant effect of delayed infection on Striga biomass or host‐plant performance was observed. The results indicate that the influence of delayed infection strongly depends on actual infection time and confirm that earlier observed differences in time of first infection between the two cultivars do contribute to the more tolerant response of Tiémarifing to Striga infection.

Assessment of reactions of diverse maize inbred lines to Striga hermonthica (Del.) Benth

Abstract Striga hermonthica is the most widespread and destructive obligate root parasite infecting maize and other cereals in Africa. Maize inbred lines supporting reduced S. hermonthica emergence can form an important basis for developing Striga‐resistant maize cultivars. Twenty new inbred lines selected for field resistance to S. hermonthica, and five inbred checks with known resitance, tolerance and susceptibility reactions to S. hermonthica were evaluated in pots, greenhouse and field experiments under artificial Striga infestation for 3 years. The experiments were conducted to determine the extent of variation in parasite attachment to the roots of these lines and its relationship with emerged Striga plants and other traits. Significant differences (P < 0.0001) were detected among the inbred lines for the numbers of attached and emerged Striga plants and the results were consistent across test environments. Also, the lines exhibited significant differences for Striga damage symptom ratings and other traits recorded in the field. Parasite attachment to the roots was significantly correlated with emerged Striga count in the screenhouse (r = 0.67–0.68, P < 0.001) and in the field (r = 0.82–0.84, P < 0.0001) and with levels of grain yield reduction due to Striga (r = 0.71, P < 0.0001). Regression analysis of the numbers of attached parasites on the first principal component axis scores that integrated several traits recorded in the field was significant (P < 0.0001) and accounted for 62% of the total variation in numbers of attached parasites. The new inbred lines and the resistant inbred check were the least affected by S. hermonthica and exhibited yield losses of 0–37% compared with the yields of the tolerant and the susceptible inbred checks, which were reduced by 40–85%. Sixteen new inbred lines supported significantly fewer attached parasites compared with the susceptible inbred check. Some of these lines also supported significantly fewer emerged parasites and sustained lower damage symptoms and percentage yield losses due to Striga compared with the susceptible inbred check. These inbred lines would be useful in breeding programmes for developing resistant maize cultivars.

Cultural control measures to diminish sorghum yield loss and parasite success under Striga hermonthica infestation

Prospects of reducing Striga hermonthica (Del.) Benth. parasitism by means of cultural control measures were assessed. In a pot experiment, deep planting, the use of transplants and shallow soil-tillage, strongly delayed and reduced Striga infection of a sensitive and a tolerant sorghum cultivar. Application of all three measures combined resulted in a four-week delay in first emergence of the parasite, a reduced number of emerged Striga plants and a decimation of Striga dry weight. With the sensitive cultivar CK-60B a yield loss of 92% was reduced to 26%, whereas with the tolerant cultivar Tiemarifing application of the cultural control measures completely cancelled out a yield loss of 28%. Evaluation of these cultural measures under field conditions in Mali resulted in a strong reduction in Striga-infection level (85%). At the same time, the measures did not result in delayed first emergence of the parasite and had no beneficial effect on crop yield. It was argued that the presence of a natural Striga soil seed bank, with seeds throughout the tilth, might be an important reason for the reduced effectiveness of the proposed cultural control measures under field conditions.

Possibilities for control of Striga hermonthica in Sorghum (Sorghum bicolor) using neem (Azadiractha indica) and parkia (Parkia biglobosa)-based products

Striga hermonthica is a major threat to cereal production in Nigeria. Management approaches for control of Striga are generally based on cultural control strategies and host plant resistance. The use of plant products for the control of S. hermonthica is limited. Seed and leaf powders of neem and fruit and fruit peel powders of parkia trees were evaluated under screenhouse and field conditions in the Nigerian Savanna. Screenhouse evaluation of the plant materials showed that neem seed powder was the most effective, with only 16.5% of Striga emergence. This was followed by parkia fruit powder and parkia fruit peel powder, with 29.1% and 38.8% Striga emergence respectively. In the field, all plant materials significantly reduced Striga emergence. The lowest number of emerged Striga plants was observed on plots treated with neem seed powder (with 1.7 emerged Striga per 3m2 while the control plots had 30.3 emerged Striga plants per 3m2). Crop plants treated with neem seed powder did not show any reaction to Striga infestation, while parkia fruit powder and parkia fruit peel powder had reaction scores of 1.7 and 2.0 respectively when compared to neem leaf powder with 3.0 and the control with 4.7. Treatment by all the plant products however, significantly increased grain yield, which was highest in the treatment with neem seed powder. Our results highlight the possibilities of using these plant products for the control of S. hermonthica.

Conditioning effects of Striga hermonthica seed on field performance of maize

Seeds of the obligate parasite Striga hermonthica, germinate in response to stimulants in the root exudate of maize and other host crops. Under laboratory conditions, conditioning (defined as treatment with moisture) of Striga seeds for one to three weeks before exposure to germination stimulant is required for optimum germination. The objective of this study was to determine if maize performance and Striga emergence, both indices of resistance, were influenced by the interval between Striga seed infestation and maize planting, under high parasite inoculum (3000–6000 germinable seeds) as practiced in maize resistance breeding field nurseries. Four conditioning treatments were investigated: maize planted immediately after Striga seed infestation, and maize planted after one, two and three weeks after infestation. Experimental design was randomized complete block with four replications. Differences among the four conditioning treatments were not significant for maize damage score, height, stalk lodging, number of ears harvested, grain yield and number of emerged Striga plants. Thus, maize planted on the same day as Striga infestation showed similar response as maize planted one to three weeks after infestation. Using high seed inoculum, maize can be planted immediately after Striga infestation in maize breeding nurseries. Combining Striga seed infestation and maize planting into a single field operation is convenient and reduces cost of screening.

Responses of Mid-Altitude Maize Inbred Lines to Striga hermonthica Infestation

The incidence of the parasitic weeds, Striga spp. in the savanna and the mid-altitude ecology of Africa is on the increase. Eight inbred lines of maize developed for the mid-altitude ecology and a susceptible check (TZi 10) were evaluated in Mokwa, Nigeria for two seasons to determine their performance under Striga hermonthica, the most widespread species of Striga in Africa. The experiment was a split-plot with three replications; infestation treatments (Striga-infested and non-infested) and maize inbreds constituted the main and sub-plots, respectively. There were no significant differences among the inbred lines for number of underground attachments of Striga seedlings to maize roots. However, differences in number of emerged Striga plants were significant (P < 0.01). Damage scores at 10 WAP and reductions in height and biomass at 12 WAP varied significantly (P < 0.01) among the maize lines tested. Damage score was significantly related to the severity of height and biomass reduction by Striga (r = 0.70 − 0.73). Differences in response of the lines to Striga manifested fully after silking. Thus, resistance was expressed at adult plant stage. Damage scores and number of emerged Striga plants were lower (P < 0.05) for TZMi 102, TZMi 302 and TZMi 306 than the susceptible check. These lines demonstrated potential as sources of resistance to Striga. Genetic variation for resistance to S. hermonthica is therefore present among the mid-altitude inbreds tested.

Spatial distribution and population dynamics of Striga hermonthica seeds in naturally infested farm soils

Densities and spatial distribution in soil of seeds of Striga hermonthica were analysed for four naturally infested farm fields in Western Kenya. A revised method for extraction of Striga seeds from soil was used, combining centrifugation with existing techniques based on flotation. Tests showed that 85% of Striga seeds were retrieved from soil samples. In all fields the majority of seeds were found in the plough layer (0 – 20 cm). New seeds entering the soil from the surface after seed shedding created a strong gradient with depth. Downward penetration from the soil surface was larger in sandy soil than in clay soil. In tilled soils no significant vertical density gradient was found within the plough layer. At a fine scale (0.2 m) seed densities showed little horizontal variation, but significant differences in seed densities in the horizontal plane were found at larger scale distances (several m) between locations in all fields. At 125 days after sowing the estimated average number of seeds produced per emerged Striga shoot was 4,827, excluding an approximately similar amount of seeds present in maturing capsules. The estimated average number of seeds produced per mature Striga seed capsule was 1188. Large seasonal fluctuations in the Striga seedbank were measured. An average net increase of 88,825 Striga seeds m-2 (equivalent to 340%) was calculated from seedbank analyses in 16 sorghum plots. The level of Striga infestation in one field had decreased by 62% from 34,250 seeds m-2 to 13,125 seeds m-2 after keeping it fallow for a year. A sharp decline in Striga seed density was found in samples taken at increasing distances from highly infested fields, irrespective of wind direction or slope, suggesting very limited dispersal of Striga seeds by wind or water. Parasite emergence was non-linearly related to initial Striga seed densities in the soil, but this relationship was only observable at the scale of individual plant holes. Seed production was also non-linearly related to numbers of emerged parasites, when measured at plot scale (25 m2), but not at the scale of individual plant holes. In the fields we studied, seed densities below levels of 13,000 Striga seeds m-2 could be considered to suppress the number of emerging parasites. However, if two or three emerged Striga plants per m-2 were left to seed, enough seeds would be produced to keep the seedbank in balance.

Fusarium nygamai, a potential bioherbicide for Striga hermonthica control in sorghum

Abstract Glasshouse trials were performed to investigate the control of the parasitic weed Striga hermonthica by Fusarium nygamai and the performance of the host plant sorghum (Sorghum bicolor) using different inoculum substrates and inoculum amounts of the fungus. Optimal constant and alternating temperatures for the growth of the fungus were 25°C and 30 20° C , respectively. Striga incidence was decreased up to 100% when the fungus was incorporated into the soil preplanting. Emerged Striga plants at different stages of growth up to the flowering stage were killed by the fungus when the fungus was applied postemergent. In root-chamber trials none of the Striga seeds germinated when 10 ml inoculum suspension of 8 × 106 spores/ml of F. nygamai was applied on seeds of the parasitic weed sprinkled on the surface of filter paper. F. nygamai has potential as a bioherbicide for Striga control. Further studies regarding its performance under field conditions and its safety to the environment and humans should be assessed.