Impact of okra leaf curl disease on morphology and yield of okra

The impact of okra leaf curl disease on four accessions of a local okra cultivar and four commercial okra cultivars was assessed in field conditions from June to October in 2007 and 2008. Disease incidence varied from 68.5% to 72.5% among accessions of the local cultivar while commercial cultivars were much less infected (8.7–16.2%). Diseased plants of the local cultivar suffered more reduced growth (58% reduction) than those of the commercial cultivars (39.6%). The number of marketable fruits per plant, the fruit length, fruit diameter and fruit weight which were subject to reductions of 26–61%, 19–64%, 6–42% and 23–63%, respectively. The overall yield losses were significantly higher in accessions of the local cultivar (26–55%) than in the commercial ones (4.4–9.6%). The average economic losses for one hectare of crop were estimated at of 11,100 USD and 1950 USD, respectively for the local and the commercial cultivars.     

In vitro antioxidant, lipoxygenase and xanthine oxidase inhibitory activities of fractions from Cienfuegosia digitata Cav., Sida alba L. and Sida acuta Burn f. (Malvaceae)

In this study polyphenol content, antioxidant activity, lipoxygenase (LOX) and Xanthine Oxidase (XO) inhibitory effects of n-hexane, dichloromethane, ethyl acetate and n-butanol fractions of aqueous acetone extracts from S. alba L., S. acuta Burn f and Cienfuegosia digitata Cav. were investigated. The total phenolics, flavonoids, flavonols and total tannins were determined by spectrophotometric methods using Folin-ciocalteu, AlCl3 reagents and tannic acid, respectively. The antioxidant potential was evaluated using three methods: inhibition of free radical 2,2-diphenyl-1-picrylhydramzyl (DPPH), ABTS radical cation decolorization assay and Iron (III) to iron (II) reduction activity (FRAP). For enzymatic activity, lipoxygenase and xanthine oxidase inhibitory activities were used. This study shows a relationship between polyphenol contents, antioxidant and enzymatic activities. Present results showed that ethyl acetate and dichloromethane fractions elicit the highest polyphenol content, antioxidant and enzymatic activities.     

Abiotic transmission of Rice yellow mottle virus through soil and contact between plants

The roles of guttation fluid, irrigation water, contact between plants and transplantation into contaminated soil in the transmission of Rice yellow mottle virus (RYMV) were assessed. RYMV presence and infectivity were tested by Enzyme-Linked Immunosorbent Assay (ELISA) and by inoculation to susceptible rice cultivar BG90-2. The virus was readily detected in guttation fluid collected from infected rice plants. Transmission tests from this fluid led to high disease incidence (86.6%). Irrigation water collected at the base of infected plants growing in pots was less infectious, as inoculations led to disease incidences below 40%. No virus was detected and could be transmitted from field-irrigation water. Up to 44% healthy rice plants whose leaves were in contact with those of infected plants became infected but, no transmission occurred through intertwined roots. Transplantation of rice seedling into virus-contaminated soil also led to plant infection. However, virus survival in the soil decrease rapidly and infectivity was completely lost 14 days after soil contamination. Altogether, these results indicated that high planting densities of rice are likely to favour secondary spread of rice yellow mottle disease. Transplantation of rice seedlings not earlier than 2 weeks after soil preparation should prevent soil transmission of the virus. Although guttation fluid is highly infectious its contribution to virus infectivity in irrigation water is negligible as field-irrigation water was not found to be an infectious source for RYMV.     

Rice seedbeds as a source of primary infection by Rice yellow mottle virus

The effect of contamination of rice seedlings by Rice yellow mottle virus (RYMV) in seedbeds on the onset and spread of rice yellow mottle in the field was investigated. Rice seedlings were artificially contaminated in seedbeds at different rates (0.1, 0.5, and 2.5%) and pooled in bundles before transplantation, as done by farmers. RYMV was successfully transmitted through contaminated hands and bundling healthy and diseased seedlings together. Hand contamination was responsible for 4.5% infection. Disease incidence in the field after secondary spread reached 32% for 2.5% seedbed contamination rate but remained limited (less than 10%) for all other rates. Eradicating infected plants from seedbeds lessened disease incidence in the field. This technique may be used in conjunction with other prophylactic measures to efficiently control rice yellow mottle disease.     

Durability of natural and transgenic resistances in rice to <Emphasis Type="Italic">Rice yellow mottle virus</Emphasis>

A monogenic recessive resistance to Rice yellow mottle virus (RYMV) found in the Oryza sativa indica cultivar Gigante and in a few Oryza glaberrima cultivars provided a higher level of resistance than either a polygenic partial resistance found in some japonica cultivars which delayed symptom expression or transgenic resistances which were partial and temporary. This high resistance was overcome by several isolates, but the percentage of such virulent isolates in the fields was low. There was no relationship between the virulence of an isolate towards the high resistance and its aggressiveness in other cultivars. Isolates with either of the two components of pathogenicity – virulence and aggressiveness – were found in each strain and in all regions of Africa, in both wild and cultivated grass species. There was no loss of fitness of resistance-breaking (RB) isolates as they were not counter-selected, impaired or outperformed after serial passages in susceptible cultivars, even in mixture with avirulent quasi-isogenic wild type isolates. Resistance breaking was highly dependent on the amount of virus inoculated and on the mode of transmission. Implications of these results for the durability of the resistances to RYMV and for the development of integrated disease management strategies are discussed.     

Rice Yellow Mottle Virus is Seed-borne but not Seed Transmitted in Rice Seeds

Seed transmission of two pathogroups of rice yellow mottle virus from Soudano-Sahelian areas of West Africa was studied in several rice genotypes. The virus was detected by enzyme-linked immunosorbent assay and the highly susceptible rice variety BG 90-2 was used for infectivity assays. In most of the rice genotypes studied (17 out of 21), rice yellow mottle virus was detected in all seed parts including glumella, endosperm and embryo at a rate ranging from 65 to 100%. Nevertheless, no seed-borne infection was found. Infectivity of the virus decreased throughout the process of seed formation suggesting inactivation of the virus as a result of seed maturation and desiccation. It was concluded that rice yellow mottle epidemics do not develop from seed-borne infections in rice seeds.     

Emergence of Resistance-breaking Isolates of Rice yellow mottle virus during Serial Inoculations

Two sources of resistance to Rice yellow mottle virus were challenged in host passage experiments. Pronounced changes in pathogenicity occurred over serial passages of virus isolates inoculated to partially or highly resistant cultivars. The changes encompassed the known existing pathogenic variability of field isolates. Ultimately, the high resistance of the Oryza indica cv. Gigante was overcome and the partial resistance of the O. sativa japonica cv. Azucena broke down. The effect was resistance-specific as different isolates overcame partial and high resistance, and may also be allele-specific as different isolates overcame the resistance of cultivars carrying the same resistance gene. The ability of isolates to break resistance was not linked to a high initial pathogenicity of the isolates and did not result in higher virus content in the infected plants. Implications for resistance breeding and deployment are discussed.     

Potential to harness superior nutritional qualities of exotic baobabs if local adaptation can be conferred through grafting

Baobab leaves form an important part of the local diet in Sahel countries and elsewhere in Africa. Existing leaf nutritional data and agroforestry performance information are based solely on Adansonia digitata L., the baobab of continental Africa. The introduction potential of Adansonia species from the center of diversity in Madagascar and from Australia remains untapped. To assess this potential, the mineral contents and B₁ and B₂ vitamin levels of dried baobab leaves were determined for five-year old trees of A. digitata, A. gibbosa (A. Cunn.) Guymer ex D. Baum, A. rubrostipa Jum. & H. Perrier (syn. A. fony Baill.), A. perrieri Capuron and A. za Baill. grown in an introduction trial in Mali. Nutritional data were evaluated against survival and vigor to identify promising germplasm. Leaf vitamin and crude protein contents were highest in the Madagascar species, especially A. rubrostipa (B₁ 88 mg 100 g⁻¹, B₂ 187 mg 100 g⁻¹, protein 20.7% dry weight). However, the local species far outperformed the introductions in survival, tree height, basal diameter and resistance to termites. We suggest grafting as a way of harnessing the vigor of well-adapted local baobab varieties to the superior nutritional profiles of A. rubrostipa and others. Cross-species grafting tests in Adansonia were successful, thus creating new agroforestry possibilities with different scion/rootstock combinations.