Molecular biodiversity of cassava begomoviruses in Tanzania: evolution of cassava geminiviruses in Africa and evidence for East Africa being a center of diversity of cassava geminiviruses

Cassava is infected by numerous geminiviruses in Africa and India that cause devastating losses to poor farmers. We here describe the molecular diversity of seven representative cassava mosaic geminiviruses (CMGs) infecting cassava from multiple locations in Tanzania. We report for the first time the presence of two isolates in East Africa: (EACMCV-[TZ1] and EACMCV-[TZ7]) of the species East African cassava mosaic Cameroon virus, originally described in West Africa. The complete nucleotide sequence of EACMCV-[TZ1] DNA-A and DNA-B components shared a high overall sequence identity to EACMCV-[CM] components (92% and 84%). The EACMCV-[TZ1] and -[TZ7] genomic components have recombinations in the same genome regions reported in EACMCV-[CM], but they also have additional recombinations in both components. Evidence from sequence analysis suggests that the two strains have the same ancient origin and are not recent introductions. EACMCV-[TZ1] occurred widely in the southern part of the country. Four other CMG isolates were identified: two were close to the EACMV-Kenya strain (named EACMV-[KE/TZT] and EACMV-[KE/TZM] with 96% sequence identity); one isolate, TZ10, had 98% homology to EACMV-UG2Svr and was named EACMV-UG2 [TZ10]; and finally one isolate was 95% identical to EACMV-[TZ] and named EACMV-[TZ/YV]. One isolate of African cassava mosaic virus with 97% sequence identity with other isolates of ACMV was named ACMV-[TZ]. It represents the first ACMV isolate from Tanzania to be sequenced. The molecular variability of CMGs was also evaluated using partial B component nucleotide sequences of 13 EACMV isolates from Tanzania. Using the sequences of all CMGs currently available, we have shown the presence of a number of putative recombination fragments that are more prominent in all components of EACMV than in ACMV. This new knowledge about the molecular CMG diversity in East Africa, and in Tanzania in particular, has led us to hypothesize about the probable importance of this part of Africa as a source of diversity and evolutionary change both during the early stages of the relationship between CMGs and cassava and in more recent times. The existence of multiple CMG isolates with high DNA genome diversity in Tanzania and the molecular forces behind this diversity pose a threat to cassava production throughout the African continent

Plant-soil interactions of sludge-borne heavy metals and the effect on maize (Zea mays L.) seedling growth

The use of sewage sludge as an organic fertiliser under South African conditions is an alternative disposal route to sacrificial land disposal. However, the lack of research done under South African conditions and the conservative nature of the heavy metal guidelines, when interpreted as total metal content is limiting the agricultural use of sludge. A glasshouse experiment, which forms part of a greater project, was conducted to characterise soil-plant interactions of the main sludge-borne heavy metals (Pb, Cd, Zn and Cu) in two sludges (low metal and high metal) to different soil types (clayey, loamy and sandy) on maize seedlings. Growth differences, heavy metal accumulation in plant parts and soil-metal concentrations (total and potentially available) were determined. The low metal sludge treatment showed the highest yield for maize seedlings when compared to controls (soil unamended and inorganic fertiliser added). The amendment of sludge to the soil did indicate higher heavy metal content, although the increase was not as predicted, owing to the difficulty of obtaining a representative sample in the soil. Except for Cd, heavy metal values in the soils (at the beginning and end of experiment) exceeded guidelines due to very high background values in the soil. No negative effects of heavy metal contamination in plant parts of the crops could be proven. Results showed that application of sludge to different soils could be useful in order to increase crop growth over a 28 d period in the glasshouse. Soil, plant and water quality monitoring, together with the prevention of metals entering the plant, is a prerequisite in order to prevent potential health hazards of sludge application to agricultural land. WaterSA Vol.27(1) 2001: 71-7

Mycoflora and Fumonisin Mycotoxins Associated with Cowpea [Vigna unguiculata (L.) Walp] Seeds

ArticleCowpea seed samples from South Africa and Benin were analyzed for seed mycoflora. Fusarium species detected were F. equiseti, F. chlamydosporum, F. graminearum, F. proliferatum, F. sambucinum, F. semitectum, and F. subglutinans. Cowpea seed from South Africa and Benin and F. proliferatum isolates from Benin, inoculated onto maize patty medium, were analyzed for fumonisin production. Samples were extracted with methanol/water and cleaned up on strong anion exchange solid phase extraction cartridges. HPLC with precolumn derivatization using o-phthaldialdehyde was used for the detection and quantification of fumonisins. Cowpea cultivars from South Africa showed the presence of fumonisin B1 at concentrations ranging between 0.12 and 0.61 μg/g, whereas those from Benin showed no fumonisins. This is believed to be the first report of the natural occurrence of FB1 on cowpea seed. Fumonisin B1, B2, and B3 were produced by all F. proliferatum isolates. Total fumonisin concentrations were between 0.8 and 25.30 μg/g, and the highest level of FB1 detected was 16.86 μg/g

Histopathological Study of the Alternaria solani Infection Process in Potato Cultivars with Different Levels of Early Blight Resistance

Histopathological analyses of the infection process of Alternaria solani were accomplished in three potato cultivars with different levels of early blight resistance. Leaflets of Aracy (resistant), Delta (moderately resistant) and Bintje (susceptible) cultivars were inoculated at the beginning of the flowering stage. In order to study the effect of leaf age, leaf samples were collected separately, in the lower, middle and upper third of the plants. Conidia germination, appressoria formation, penetration and number of penetration sites exhibiting hypersensitive response (HR) were qualitatively and quantitatively assessed at 6, 12, 24, 48 and 72 h after inoculation. Penetration occurred most frequently through the junctions of the epidermal cells. Penetrations through stomata were rarely observed. There was no association between the events and the resistance levels, except for the number of penetration sites showing HR that was highest in Aracy, intermediate in Delta and lowest in Bintje. Similarly, the number of sites with HR was the only event associated with the leaf age. Regardless of cultivar resistance levels, the number of the penetration sites with HR was higher in leaves in the upper part of the plant. These results suggest that HR may be one of the mechanisms associated with age‐related and genetic resistance to early blight in potato

Growth and sporulation of Stemphylium vesicarium, tha causal agent of brown spot of pear, on herb plants of orchard lawns

The inoculum sources of ascospores of Pleospora allii and of conidia of its anamorph Stemphylium vesicarium were investigated in relation to the brown spot disease epidemiology on pear. Dead and living leaves of three pear varieties (Abate Fetel, Conference and William), seven grasses (Poa pratensis, Festuca rubra, Festuca ovina, Lolium perenne, Digitaria sanguinalis and Setaria glauca) and Trifolium repens, which are used in pear orchard lawns, were inoculated with conidia of Stemphylium vesicarium virulent on pear and incubated under controlled-environment. Stemphylium vesicarium was always re-isolated from dead leaves of the considered plants, but not from symptomless green or yellowish living leaves. The fungus was occasionally re-isolated from leaf segments showing unspecific necrosis. Inoculation of pear leaves with isolates from grasses demonstrated that the fungus did not lose pathogenicity. Pseudothecia, ascospores and conidia were produced on all the dead inoculated leaves; differences between specimens were found for phenology of pseudothecia, their density and size, and for the number of conidia produced. Pseudothecia were produced faster in the lawn species than in pear leaves, and their density was higher, especially for S. glauca, L. perenne and P. pratensis. Ascospore maturation and ejection was more concentrated for the pseudothecia developed on pear leaves than for those on F. ovina and S. glauca. All the lawn species produced more conidia than pear leaves