Approaches to diagnosis and detection of cassava brown streak virus (Potiviridae: Ipomovirus) in fieldgrown cassava crop

Open Access JournalCassava brown streak disease (CBSD) has been a problem in the East African coastal cassava growing areas for more than 70 years. The disease is caused by successful infection with Cassava Brown Streak Virus (CBSV) (Family, Potyviridae: Genus, Ipomovirus). Diagnosis of CBSD has for long been primarily leaf symptoms-based. This is unreliable due to the irregular pattern and variability of the disease phenotype in roots and leaves. The suitable method to undertake reliable field diagnostic survey and derive acceptable analysis of the disease situation has never been standardized. Zigzag and diagonal approaches for disease assessment have been used successfully on other diseases infecting cassava such as Cassava mosaic disease but neither of them has ever been tested and proven suitable for CBSD assessment. In addition, the suitable sample for successful molecular detection of the causal virus has never been optimised. The number of samples to be collected from large plant stands which would be a true representation of the population has never been determined. The effect of sample bulking on possible detection or non detection of infection particularly when un-infected samples are combined with infected ones is not known. In this study, the comparative efficiencies of diagonal and zigzag approaches to CBSD field diagnosis were tested through surveys conducted in 20 randomly selected farmers’ fields in major cassava growing areas of the Coastal and Lake Zones in Tanzania. Using molecular diagnostic techniques, the plant parts which are suitable for Cassava brown streak virus (CBSV) detection were determined. Sample bulking was tested for rationalized laboratory detection of CBSV over large cassava stands. The study revealed that CBSD incidences and severities obtained using either diagonal or zigzag approach did not differ significantly. Suitable parts for CBSV detection were identified to be flowers, fruits, apical buds, young tender leaves, newly-opened leaves, youngest symptomatic leaves, the tender top-green portion of the stem and non-necrotic storage root tissues. CBSV was not detected in seeds. In bulked leaf samples, CBSV was detected from ratios of 1:1 up to 1:19 of CBSVinfected to CBSV-free tissues in cultivar Albert. It was concluded that either zigzag or diagonal can be used for CBSD field diagnosis. A choice of the suitable sample is of absolute necessity, and bulking of many samples for collective CBSV detection over a large crop stand is effective

Early detection of plant diseases using spectral data

Early detection of crop disease is an essential step in food security. Usually, the detection becomes possible in a stage where disease symptoms are already visible on the aerial part of the plant. However, once the disease has manifested in different parts of the plant, little can be done to salvage the situation. Here, we suggest that the use of visible and near infrared spectral information facilitates disease detection in cassava crops before symptoms can be seen by the human eye. To test this hypothesis, we grow cassava plants in a screen house where they are inoculated with disease viruses. We monitor the plants over time collecting both spectra and plant tissue for wet chemistry analysis. Our results demonstrate that suitably trained classifiers are indeed able to detect cassava diseases. Specifically, we consider Generalized Matrix Relevance Learning Vector Quantization (GMLVQ) applied to original spectra and, alternatively, in combination with dimension reduction by Principal Component Analysis (PCA). We show that successful detection is possible shortly after the infection can be confirmed by wet lab chemistry, several weeks before symptoms manifest on the plants

Research note: Effect of vine age and storage duration on regeneration potential of sweetpotato vines in drier areas of Tanzania

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Approaches To Diagnosis And Detection Of Cassava Brown Streak Virus (Potiviridae: Ipomovirus) In Field-Grown Cassava Crop

Cassava brown streak disease (CBSD) has been a problem in the East African coastal cassava growing areas for more than 70 years. The disease is caused by successful infection with Cassava Brown Streak Virus (CBSV) (Family, Potyviridae: Genus, Ipomovirus). Diagnosis of CBSD has for long been primarily leaf symptoms-based. This is unreliable due to the irregular pattern and variability of the disease phenotype in roots and leaves. The suitable method to undertake reliable field diagnostic survey and derive acceptable analysis of the disease situation has never been standardized. Zigzag and diagonal approaches for disease assessment have been used successfully on other diseases infecting cassava such as Cassava mosaic disease but neither of them has ever been tested and proven suitable for CBSD assessment. In addition, the suitable sample for successful molecular detection of the causal virus has never been optimised. The number of samples to be collected from large plant stands which would be a true representation of the population has never been determined. The effect of sample bulking on possible detection or non detection of infection particularly when un-infected samples are combined with infected ones is not known. In this study, the comparative efficiencies of diagonal and zigzag approaches to CBSD field diagnosis were tested through surveys conducted in 20 randomly selected farmers’ fields in major cassava growing areas of the Coastal and Lake Zones in Tanzania. Using molecular diagnostic techniques, the plant parts which are suitable for Cassava Brown Streak Virus (CBSV) detection were determined. Sample bulking was tested for rationalized laboratory detection of CBSV over large cassava stands. The study revealed that CBSD incidences and severities obtained using either diagonal or zigzag approach did not differ significantly. Suitable parts for CBSV detection were identified to be flowers, fruits, apical buds, young tender leaves, newly-opened leaves, youngest symptomatic leaves, the tender top-green portion of the stem and non-necrotic storage root tissues. CBSV was not detected in seeds. In bulked leaf samples, CBSV was detected from ratios of 1:1 up to 1:19 of CBSV-infected to CBSV-free tissues in cultivar Albert. It was concluded that either zigzag or diagonal can be used for CBSD field diagnosis. A choice of the suitable sample is of absolute necessity, and bulking of many samples for collective CBSV detection over a large crop stand is effective

Widespread occurrence and diversity of cassava brown streak virus (Potyviridae:Ipomovirus) in Tanzania

Cassava brown streak disease (CBSD) has been a problem in Tanzania since 1936. Existing literature indicated limited distribution of the disease to low altitudes, usually <100 m above sea level, but the current geographical distribution of the disease was not known. Whether a single or many strains for the virus exist in Tanzania had not been reported to date. In this study, CBSD was recorded from sea level to ≈1,800 m above sea level. In total, 2,730 cassava plants were assessed for CBSD leaf symptoms in 91 fields and root symptoms were assessed at 81 sites. A sample was taken from each site for laboratory screening for Cassava brown streak virus (CBSV). CBSD mean foliar and root incidences were 38 and 36%, respectively. Reverse-transcription polymerase chain reaction of a partial 3′-terminal coat protein (CP) region of CBSV indicated the presence of CBSV in 67 of the 91 (73%) samples. Forty-three amplicons were sequenced, and phylogenetic comparisons with nucleotide sequences from GenBank (National Center for Biotechnology Information database) suggested that one major clade of CBSV primarily exists in Tanzania. However, there was nucleotide sequence divergence of up to 19% among the 42 isolates. In all, 42 of the 43 sequences had 80 to 100% nucleotide identity with 6 previously reported CP-CBSV sequences (from Mozambique and Tanzania). In total, 13 of 42 isolates had <80% nucleotide identities with three previously reported Ugandan CBSV sequences. One isolate, FJ687177, shared <78% sequence identity with the other Tanzanian sequences but was closely related (93%) to Ugandan isolates. It is likely that isolate FJ687177 may belong to a less widely distributed recently described species (clade 2) of CBSV, named Ugandan cassava brown streak virus (UCBSV)