Cassava brown streak disease (CBSD) is a severe virus disease of cassava and prevalent in the eastern regions of Africa. The disease is characterized by distinct vein chlorosis and streak symptoms on leaves and stems and necrosis of storage roots. This necrosis can encompass large areas of the root, rendering it inedible so that the entire cassava harvest can be lost. African cassava varieties are susceptible to either of the two viruses causing the disease, cassava brown streak virus (CBSV) and Uganda cassava brown streak virus, and while there are less sensitive varieties, all cassava eventually succumb to the disease. The lack of CBSD resistance in African cassava varieties prompted this search for new sources of virus resistance in the diversity of South American cassava germplasm held in the collection at International Center for Tropical Agriculture, Columbia. Our search for CBSD resistance in South American cassava germplasm accessions revealed that most of the 238 South American cassava lines infected with CBSV established systemic virus infections with moderate to severe disease symptoms on leaves and stems. Fifteen cassava accessions did not become virus infected, remained free of symptoms, and CBSV was undetected by qRT-PCR. When tuberous roots of those lines were examined, necrotic tissue was found in eight lines and CBSV was detected. The remaining seven cassava accessions remained clear of symptoms on all tissues and organs and were virus free. A broad spectrum of virus resistance also including other virus isolates was confirmed for the breeding lines DSC167 and DSC118. While detailed infection experiments with other cassava lines selected for resistance are still ongoing, this indicates that the resistance identified may also hold against a broader diversity of CBSVs. Taken together, we present the results of a comprehensive study on CBSV resistance and susceptibility in cassava germplasm accessions from South America. The virus resistance in cassava germplasm identified provides compelling evidence for the invaluable contribution of germplasm collections to supply the genetic resources for the improvement of our crops.

/pdf/resistance-against-cassava-brown-streak-viruses-from-africa-5a2cqolz2a.pdf

Resistance Against Cassava Brown Streak Viruses From Africa in Cassava Germplasm From South America

A method for generating virus-free cassava plants to combat viral disease epidemics in Africa

Cassava brown streak disease (CBSD), caused by cassava brown streak ipomoviruses (CBSIs), has become the most debilitating biotic stress to cassava production in East and Central Africa. Lack of CB...

/pdf/assessing-the-degeneration-of-cassava-under-high-virus-4th6ebldbd.pdf

Assessing the Degeneration of Cassava Under High-Virus Inoculum Conditions in Coastal Tanzania

Cassava is a staple food for people across sub-Saharan Africa. Over the last 20 years, there has been an increased frequency of outbreaks and crop damage in this region caused by the cassava-adapted Bemisia tabaci putative species. Little is known about when and why B. tabaci adults move and colonize new cassava crops, especially in farming systems that contain a mixture of cultivar types and plant ages. Here, we assessed experimentally whether the age and variety of cassava affected the density of B. tabaci. We also tested whether the age and variety of the source cassava field affected the variety preference of B. tabaci when they colonized new cassava plants. We placed uninfested potted “sentinel” plants of three cassava varieties (Nam 130, Nase 14, and Njule Red) in source fields containing one of two varieties (Nam 130 or Nase 14) and one of three age classes (young, medium, or old). After two weeks, the numbers of nymphs on the sentinel plants were used as a measure of colonization. Molecular identification revealed that the B. tabaci species was sub-Saharan Africa 1 (SSA1). We found a positive correlation between the density of nymphs on sentinel plants and the density of adults in the source field. The density of nymphs on the sentinels was not significantly related to the age of the source field. Bemisia tabaci adults did not preferentially colonize the sentinel plant of the same variety as the source field. There was a significant interactive effect, however, between the source and sentinel variety that may indicate variability in colonization. We conclude that managing cassava source fields to reduce B. tabaci abundance will be more effective than manipulating nearby varieties. We also suggest that planting a “whitefly sink” variety is unlikely to reduce B. tabaci SSA1 populations unless fields are managed to reduce B. tabaci densities using other integrative approaches.

/pdf/african-cassava-whitefly-bemisia-tabaci-cassava-colonization-1oybxltox7.pdf

African Cassava Whitefly, Bemisia Tabaci, Cassava Colonization Preferences and Control Implications

Genotype by environment cultivar evaluation for cassava brown streak disease resistance in Tanzania

Cassava varieties resistant to cassava mosaic disease (CMD) and cassava brown streak disease (CBSD) are needed for the food and income security of the rural poor in eastern and southern Africa (ESA). The International Institute of Tropical Agriculture led five national cassava breeding programs (Malawi, Mozambique, Kenya, Tanzania and Uganda) in virus-cleaning and exchanging elite cassava germplasm resistant to both diseases. This paper documents the experiences and lessons learned from the process. Thirty-one clones (25 elite, two standard and four national) were submitted by the five breeding programs to the Natural Resources Institute and Kenya Plant Health Inspectorate Services for virus cleaning and indexing. Subsequently, ca 75 in-vitro virus-indexed plantlets per clone were sent to Genetic Technologies International Limited (GTIL), a private tissue culture (TC) lab in Kenya, and micro-propagated to produce ≥1500 plantlets. After fulfilling all the formal procedures of germplasm exchange between countries ≥300 plantlets per clone were sent to each partner country. National check clones susceptible to CMD/CBSD were sent only to their countries of origin. In each country, the in-vitro plantlets were acclimatized under screen house conditions and transferred to clean isolated sites for field multiplication. All the clones were cleaned of the viruses, except Tomo. The cleaning process was slow for F19-NL, NASE1, and Kibandameno and TC micro-propagation at GTIL was less efficient for Pwani, Tajirika, NASE1, and Okhumelela than for the other clones. Difficulties in cleaning recalcitrant clones affected the timeline for establishing the multi-site evaluation trials in target countries. The initiative is the one of the kind to successfully clean and exchange elite germplasm as a joint action to combat CBSD in ESA. Adequate preparation in terms of infrastructure and personnel are critical to successfully receiving and adapting the indexed in-vitro plants as new germplasm.

/pdf/exchanging-and-managing-in-vitro-elite-germplasm-to-combat-501hf6r1g9.pdf

Exchanging and managing in-vitro elite germplasm to combat Cassava Brown Streak Disease (CBSD) and Cassava Mosaic Disease (CMD) in Eastern and Southern Africa

Bacterial wilt (BW), caused by the Ralstonia solanacearum species complex (RSSC), is the most destructive potato disease in Kenya. Studies were conducted to (i) determine the molecular diversity of RSSC strains associated with BW of potato in Kenya, (ii) generate an RSSC distribution map for epidemiological inference, and (iii) determine whether phylotype II sequevar 1 strains exhibit epidemic clonality. Surveys were conducted in 2018 and 2019, in which tubers from wilting potato plants and stem samples of potential alternative hosts were collected for pathogen isolation. The pathogen was phylotyped by multiplex PCR and 536 RSSC strains typed at a sequevar level. Two RSSC phylotypes were identified, phylotype II (98.4%, n = 506 (sequevar 1 (n = 505) and sequevar 2 (n = 1))) and phylotype I (1.6%, n = 30 (sequevar 13 (n = 9) and a new sequevar (n = 21))). The phylotype II sequevar 1 strains were haplotyped using multilocus tandem repeat sequence typing (TRST) schemes. The TRST scheme identified 51 TRST profiles within the phylotype II sequevar 1 strains with a modest diversity index (HGDI = 0.87), confirming the epidemic clonality of RSSC phylotype II sequevar 1 strains in Kenya. A minimum spanning tree and mapping of the TRST profiles revealed that TRST27 '8-5-12-7-5' is the primary founder of the clonal complex of RSSC phylotype II sequevar 1 and is widely distributed via latently infected seed tubers.

Ralstonia Strains from Potato-Growing Regions of Kenya Reveals Two Phylotypes and Epidemic Clonality of Phylotype II Sequevar 1 Strains.

Tomato (Solanum lycopersicum L.) is a high value horticultural crop in Kenya. Nutritionally, the crop is rich in niacin, carotene, thiamine, and vitamin C. Mwea in Kirinyaga County is one of the major tomato growing areas in Kenya. Tomato production in Kenya is hindered by losses due to diseases caused by pathogens that include plant parasitic nematodes (PPN). Among the plant parasitic nematodes, the root-knot nematode (RKN) is the most predominant in Mwea. This study investigated the soil parameters that influence the distribution of the RKN for the purpose of mapping their distribution. This is important since it ensures that nematicide application is only to specific sites where these nematodes are found thereby saving on input costs as well as protecting the environment. The study consisted of surveys conducted in geo-referenced tomato production fields in seven (7) tomato production sites in Tebere and Mwea. Sampling was done in a stratified random manner in both rainfed and irrigated tomato production fields in both dry and rainy seasons for determination of the spatial and temporal distribution of nematodes, respectively. Nematode extraction was done using the centrifugal floatation technique and identification done to genera level using morphological features. Soil characteristics determining PPN distribution patterns in tomato production fields of Mwea were measured using standard methods. Soil parameters measured included soil pH, electrical conductivity, elevation and soil texture. The PPN incidence and diversity was determined using the Shannon-Weiner species diversity index. Detrended canonical correspondence analysis (DCCA) was performed to interpret and summarize major patterns of variation within the soil variable data and to estimate the ability of each soil variable to reflect variance in the entire PPN data set. Fourteen (14) genera of nematodes were identified with the diversity between 0.6 and 1.2. RKN distribution differed significantly (p<0.05) among the sites. Among the soil samples analyzed, 81% were positive for RKN infestation. There was an insignificant difference (p>0.05) between nematode densities in the dry and rainy seasons. Rainfed fields exhibited a significantly higher (p<0.05) RKN population densities compared to irrigated fields. The study established a great variability in the soil parameters in the area. The RKN distribution pattern, density and abundance were inversely correlated (p<0.05) with the soil pH and positively correlated (p<0.05) with soil EC. Based on the inverse relationship between soil pH and RKN distribution in the Mwea ecosystem, maps of nematode distribution and soil pH were developed. This enables the possibility of a site specific system for management of RKN

Development of a Site-Specific Management Strategy for Managing Root-Knot Nematodes Affecting Tomatoes in Kenya

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Exchanging and managing in-vitro elite germplasm to combat Cassava Brown Streak Disease (CBSD) and Cassava Mosaic Disease (CMD) in Eastern and Southern Africa

Ralstonia Strains from Potato-Growing Regions of Kenya Reveals Two Phylotypes and Epidemic Clonality of Phylotype II Sequevar 1 Strains.

Development of a Site-Specific Management Strategy for Managing Root-Knot Nematodes Affecting Tomatoes in Kenya