Cassava mosaic begomoviruses (CMBs) cause cassava mosaic disease (CMD) across Africa and the Indian subcontinent. Like all members of the geminivirus family, CMBs have small, circular single-stranded DNA genomes. We report here the discovery of two novel DNA sequences, designated SEGS-1 and SEGS-2 (for sequences enhancing geminivirus symptoms), that enhance symptoms and break resistance to CMD. The SEGS are characterized by GC-rich regions and the absence of long open reading frames. Both SEGS enhanced CMD symptoms in cassava (Manihot esculenta Crantz) when coinoculated with African cassava mosaic virus (ACMV), East African cassava mosaic Cameroon virus (EACMCV), or East African cassava mosaic virus-Uganda (EACMV-UG). SEGS-1 also overcame resistance of a cassava landrace carrying the CMD2 resistance locus when coinoculated with EACMV-UG. Episomal forms of both SEGS were detected in CMB-infected cassava but not in healthy cassava. SEGS-2 episomes were also found in virions and whiteflies. SEGS-1 has no homology to geminiviruses or their associated satellites, but the cassava genome contains a sequence that is 99% identical to full-length SEGS-1. The cassava genome also includes three sequences with 84 to 89% identity to SEGS-2 that together encompass all of SEGS-2 except for a 52-bp region, which includes the episomal junction and a 26-bp sequence related to alphasatellite replication origins. These results suggest that SEGS-1 is derived from the cassava genome and facilitates CMB infection as an integrated copy and/or an episome, while SEGS-2 was originally from the cassava genome but now is encapsidated into virions and transmitted as an episome by whiteflies.IMPORTANCE Cassava is a major crop in the developing world, with its production in Africa being second only to maize. CMD is one of the most important diseases of cassava and a serious constraint to production across Africa. CMD2 is a major CMD resistance locus that has been deployed in many cassava cultivars through large-scale breeding programs. In recent years, severe, atypical CMD symptoms have been observed occasionally on resistant cultivars, some of which carry the CMD2 locus, in African fields. In this report, we identified and characterized two DNA sequences, SEGS-1 and SEGS-2, which produce similar symptoms when coinoculated with cassava mosaic begomoviruses onto a susceptible cultivar or a CMD2-resistant landrace. The ability of SEGS-1 to overcome CMD2 resistance and the transmission of SEGS-2 by whiteflies has major implications for the long-term durability of CMD2 resistance and underscore the need for alternative sources of resistance in cassava.

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.

SUMMARYThe aim of the present investigation was to evaluate the antifungal activities of plant extracts which can be used to control bean and cowpea anthracnose. Acetone, ethyl acetate and water extracts of Ipomoea batatas, Carica papaya, Allium sativum, Syzygium cordatum, Chlorophytum comosum and Agapanthus caulescens were screened in vitro for their antifungal activities against Colletotrichum lindemuthianum and Colletotrichum dematium of common bean and cowpea using the agar disc infusion and microtitre double-dilution techniques.The same extracts were then tested for antifungal activity in vivo as seed treatments against anthracnose disease. The water extracts of Carica and Syzygium were active against C. lindemuthianum and had minimum inhibitory concentrations (MICs) of 1·56 mg/ml. Syzygium, Allium and Chlorophytum water extracts were active against C. dematium and MICs were 3·13, 6·25 and 12·5 mg/ml, respectively. The MICs of Allium, Syzygium and Agapanthus acetone extracts were 0·78, 3·13 and 6·25 mg/ml, respectively, against C. lindemuthianum and 0·78, 6·25 and 3·13 mg/ml against C. dematium. Agapanthus water extracts and all the acetone extracts tested in vivo effectively reduced the incidence and severity of bean anthracnose disease in the greenhouse. Agapanthus acetone, Allium water, and both acetone and water extracts of Carica and Syzygium performed well in vivo in reducing cowpea anthracnose disease and compared well with reductions due to the application of the synthetic fungicide fludioxonil + mefenoxam (the commercial product Celest ® XL) applied at 25 gai/l and also with levels in the non-inoculated control. The Agapanthus, Carica, Syzygium and Allium extracts were active on both Colletotrichum spp. in vitro and also reduced anthracnose disease of bean and cowpea and are potential seed treatments in anthracnose disease control. The easy seed treatment process and the accessibility of plants used in the present study could lead to high adoption of the use of the plant extracts as seed treatments by resource-poor, smallholder farmers.

Damping-off and stem rot disease-causing

No abstract

Conidial germination of Alternaria porri, formation of prepenetration structures, penetration of the onion leaf surface, and the postpenetration processes were studied using light, scanning electron, and transmission electron microscopy. Ninety-six percent of conidia germinated at 25 °C within 24 h of inoculation. Each conidium formed several germ tubes that grew in any direction across the leaf surface. Each germ tube usually terminated in a bulbous appressorium formed directly on the epidermal cell (52.4% of appressoria) or on a stoma (48.6% of appressoria). Following direct penetration of the outer epidermal cell wall or the stoma, bulbous primary hyphae developed below the appressoria. Secondary hyphae developed from the primary hyphae within 48 h after inoculation and grew within the intercellular spaces penetrating mesophyll cells. The changes in ultrastructure of cells in close proximity to hyphae and of infected cells are described. Key words: Allium cepa, electron microscopy, infection process, purple blotch.

Background: Plant viral diseases present major constraints to crop production. Effective sampling of the viruses infecting plants is required to facilitate their molecular study and is essential for the development of crop protection and improvement programs. Retaining integrity of viral pathogens within sampled plant tissues is often a limiting factor in this process, most especially when sample sizes are large and when operating in developing counties and regions remote from laboratory facilities. FTA is a paper-based system designed to fix and store nucleic acids directly from fresh tissues pressed into the treated paper. We report here the use of FTA as an effective technology for sampling and retrieval of DNA and RNA viruses from plant tissues and their subsequent molecular analysis.

The time for dewatered sludge to reach stability in terms of respiration after application to agricultural land was investigated and found that dewatered sludge applied at 16 ton/ha (dry) reached the background respiration rate of the soil, 50h after administration. Laboratory seedling experiments showed this stabilisation did not effect the effectivity of the sludge as a soil conditioner (growth of the maize seedlings) since seedling growth was similar whether seedlings were planted directly after, or 14 days after sludge was administered (after stabilisation). The beneficial effects of the sludge was significantly enhanced when doubling the dosage to 16 ton/ha in loam soil without heavy metal and salt accumulation in the soil or plant foliage. Laboratory studies of seedlings grown in 8, 24 and 72 ton/ha amended silica sand illustrated that the inorganic fertiliser (Hoagland's solution) affected seedling growth better in terms of root, leaf mass and shoot length but showed chlorosis. Apart from Zn the heavy metal concentrations in the seedling foliage were still similar to the positive control even at 72 ton/ha, clearly indicating the benefits of attempting to keep metals from entering the wastewater treatment plant.