International Institiute for Tropical Agriculture, Ibadan, Nigeria

Background and objectives
The International Institute of Tropical Agriculture (IITA) aims to improve the nutritional status and well-being of low-income people in the humid tropics and subtropics of sub-Saharan Africa. In collaboration with National Agricultural Research Systems (NARS), IITA conducts research on the improvement of food crops and cropping practices. The major food crops are cassava, yam, plantain and banana, cowpea, maize and soyabean, as well as herbaceous legumes which are used for farming systems improvement. While the exchange of germplasm to and from NARS is a major activity, great care must be taken to ensure that no pests or pathogens are inadvertently transferred. IITA has a Germplasm Health Unit whose main aim, in collaboration with National Plant Quarantine Services (NPQS) and IITA scientists, is to ensure the health of germplasm entering and leaving IITA.

Materials and methods
For the international movement of vegetatively propagated crops (cassava, yam, banana and plantain), virus-tested in vitro plantlets or propagules derived from them (minitubers and ministakes produced under quarantine screenhouses) and true seeds are used. For cowpea, soyabean, maize and herbaceous legumes, true seeds are the means of germplasm exchange. ln vitro plantlets are produced through meristem culture to eliminate viruses and other diseases and pests. Regenerated plants are tested for viruses in collaboration with NPQS by symptom expression, the use of indicator plants, ELISA, electron microscopy, immunosorbent electron microscopy and polymerase chain reaction. Prior to packaging, cultures are examined for possible contamination by fungi and bacteria.

For production of disease-free seed, germplasm is planted in areas free from diseases (e.g. downy mildew-maize, Striga-maize, Striga-cowpea) and, if possible, in quarantine screenhouses (e.g. Vigna spp). Pre-harvest inspection of the plants is done in collaboration with NPQS. Harvested seeds are cleaned, dried and fumigated. Cassava seeds are also treated with hot air. A sub-sample of the seed lots is tested for pests, nematodes, fungi, bacteria and Striga by visual inspection, seed-washing tests, agar and blotter tests and seed maceration and streaking on differential media. For viruses in legumes, growing-out tests and ELISA are performed. Seeds are then dressed with fungicides and insecticides and packaged for export.

Imported germplasm is treated in the same manner after the materials are released by the NPQS. Seed samples are randomly taken for various tests, the germplasm is planted in quarantine screenhouses for observation and further testing before planting out in the field. Follow up inspections are also carried out.

Results and conclusions
About 40 genotypes of cassava and 10 of yam are disease-tested and certified yearly. IITA has more than 340 selected cassava genotypes and more than 45 yam genotypes available for distribution. Virus-tested germplasm has been distributed to collaborators in more than 40 countries. During 1997, 6767 yam plantlets, 6927 yam minitubers and 21,040 cassava plantlets were distributed to NARS collaborators. In 1996 over 360 imported seed lots of cowpea, soyabean, maize, velvet beans and cassava, and 131 yam accessions were tested. Over 330 seed lots of various crops species were processed for international distribution and about 1500 Vigna lines were cleaned from seedborne virus infection. Through the introduction of IITA's improved germplasm to NARS in various countries in sub-Saharan Africa, high-yielding and disease-resistant varieties have been selected and released to farmers, thus contributing immensely to the alleviation of poverty in the region.