Germination stimulation of striga gesnerioides seeds by hosts and nonhosts

Cowpea (Vigna unguiculata) is an important source of protein in Africa, but production is hindered by the parasitic plant Striga gesnerioides. Crop rotation with non-host cultivars, selected to stimulate parasite seed germination, is being used successfully to control other Striga spp. and may have potential to control S. gesnerioides. Little information has been available on nonhosts of S. gesnerioides that are capable of stimulating germination of parasite seeds. A laboratory procedure was used to evaluate species and cultivars for their ability to stimulate S. gesnerioides seed germination. Genotypes of all Vigna spp. tested stimulated parasite seed germination. Some genotypes of the non-host species Cajanus cajan, Lablab purpureus, Sphenostylis stenocarpa, and Sorghum bicolor also stimulated parasite seed germination. One cultivar of Sorghum bicolor stimulated significantly more germination than any other cultivar or species. Control of S. gesnerioides through rotation with selected non-host cultivars has potential if selection is done with the parasite isolate(s) from the locality of intended use. When seeds of single-plant isolates of S. gesnerioides were tested against roots of seedlings from Sorghum bicolor landraces and from a susceptible cowpea cultivar, only specific isolate and plant combinations resulted in parasite seed germination. These specific interactions have broader implications for parasite survival

Potential of imazaquin seed treatment for control of Stiga gesnerioides and Alectra vogeliii in cowpea (Viigna unguiculata)

The herbicide imazaquin was tested for efficacy in striga gesnerioides and Alectra vogelii control when applied as a cowpea (Vigna unguiculata) seed treatment. Four sets of experiments were conducted in the laboratory and screenhouse. Two cowpea cultivar, three geographical isolates of S. gesnerioides from two hosts, and two geographical isolates of A. vogeli from two hosts were used. Preliminary experiments indicated cowpea seed treatments of 5-min duration in aqueous solutions of the ammonium salt of imazaquin, ranging from 1.8 to 7.2 mg a.i./ml, fit our test criteria of 50% germination and observable radical growth inhabitation. Treated cowpea seeds were dried and planted in soil-filled pots infested with 3,000 germinable S. gesnerioides or A. vogeli seeds. All experiments showed imazaquin seed treatments resulted in increased total cowpea dry weight in S. gesnerioides- infested pots in all experiments. Increases were significant at P<0.05 in two experiments. Observation in vitro, combined with screenhouse data, showed apparently normal parasite germination and attachment, indicating post attachment demise of both parasites. Increasing imazaquin rates led to delays in cowpea flowering; and increased soak times, at 3.6 mg a.i./ml and higher concentrations, led to reductions in cowpea seedling emergence. By prolonging seed soak times at an imazaquin concentration of 1.8 mg/ml, good parasite control was obtained. The lower rate at longer soak durations would provide both economical ($2.31 to$3.85 per hectare) control and the flexibility in treatment necessary for implementation on African farms. Field trials on farmer fields are under way to tailor specific seed treatment recommendations

ALSinhibiting herbicide seed treatments control Striga hermonthica in ALSmodified corn (zea mays)

Seed treatments with two acetolactate synthase (ALS)-inhibiting herbicides, the sulfony- lurea herbicide nicosulfuron and the imidazolinone herbicide imazaquin, controlled the parasitic weed Striga hermonthica in corn. The XA-17 gene in ALS-modified P3180IR corn strongly reduced corn injury from herbicide seed treatments while another ALS modification was not effective. Combining seed treatment of ALS-inhibiting herbicides and ALS-modified corn with the XA-17 gene may offer a practical means for African growers to control Striga hermonthic

Reproductive ability of hybrids of Striga aspera and Striga hermonthica

Striga aspera and S. hermonthica are sympatric in Africa. Each may serve as virulent gene reservoirs for the other if they hybridize and their hybrids are virulent and fertile. Intraspecific and interspecific crosses were made within and between the species, and reproductive success was determined. Freshly harvested seeds from the parental and F1 crosses were tested over time for germination. Chromosome counts from shoottip squashes of seedlings of S. aspera were determined as n = 18, and as n = 19 for S. hermonthica. Hybridization results indicated that S. aspera and S. hermonthica could be intercrossed and their hybrids successfully backcrossed to either parent. Reproductive success in all crosses ranged from 68 to 95% and seeds of all the crosses were viable, germinated on exposure to a synthetic germination stimulant, and were pathogenic on maize. Seeds from the hybrids and backcrosses were less viable and germinable than either parent, suggesting that the two species were not 100% compatible. Results suggest that the two species are closely related, but are separate taxa, perhaps at the subspecies level. Seed dormancy for both species and the F1 hybrids was less than 84 days after pollination. Germination of S. hermonthica seeds reached 31% at 28 days after pollination. Frequently occurring seed germination peaks were observed for all the seeds tested

Evaluation of variability in Striga aspera, Striga hermonthica and their hybrids using morphological characters and random amplified polymorphic DNA markers

Striga aspera and Striga hermonthica are recognized as separate species, but their close morphological similarity causes difficulty in distinguishing between them in areas where they coexist in Africa. In this study, crosses between the species were made using randomly selected morphologically typical parental plants collected from different locations in Nigeria. Genetic analysis of both species and their reciprocal F1hybrids were determined using cluster analysis of DNA pro Æles derived from genetic polymorphism (RAPD)-polymerase chain reaction (PCR) markers. Principal component and hierarchical cluster analyses were used to separate parental and hybrid populations based on 13 morphological characteristics. Morphological data from wild samples of both species were compared with the hand-pollinated parental, F1and F2hybrids, and back-crosses. Results showed that S. Aspera and S. hermonthica were genetically and morphologically distinct. Morphological and genetic analyses revealed two major clusters: a S. Aspera cluster and S. hermonthica cluster. Genetically, the F1hybrids showed closer a unity to their maternal parents, while morphologically, the F1hybrids formed distinct clusters intermediate to the parents. Most F2plants and back-crosses were morphologically similar to S. hermonthica. Comparative morphological analysis of wild and hand-pollinated populations showed some samples from the wild clustered with the hybrids, suggesting that hybrids may exist in nature

Macrophomina phaseolina on the tropical cover crops Mucuna pruriens var. utilis

During the rainy season of 1991, severely reduced plant stands and severely stunted plants were observed in several fields of the cover crop Mucuna pruriens (L.) DC. var. utilis (Wallich ex Wight) Baker ex Burck at the International Institute of Tropical Agriculture in Ibadan, Nigeria. Initial outbreak of symptoms followed a brief dry period, and subsequently more than 75% of the crop was lost. When the sites were replanted, the mucuna was again severely affected, although there was no dry period. Stunted plants had necrotic crowns and numerous necrotic lesions along the roots and runners. Only Macrophomina phaseolina (Tassi) Goidanich was consistently isolated from these lesions. Infestations of sterilized soil with a drench of a homogenized M. phaseolina culture, isolated from a symptomatic plant, resulted in poor seedling emergence in the screenhouse. Necrotic root symptoms were evident on emerged seedlings as well as on older mucuna plants taken from infested pots. M. phaseolina was reisolated and Koch's postulates proved. Tropical farm management increasingly relies on M. p. utilis as a rotational cover crop to restore fertility after cereal cultivation. This first report of the pathogenicity of M. phaseolina on mucuna indicates a potentially serious threat to this rotation