Sweet potato (Ipomoea batatas) is ranked seventh in global food crop production and is the third most important root crop after potato and cassava. Sweet potatoes are vegetative propagated from vines, root slips (sprouts), or tubers. Therefore, virus diseases can be a major constrain, reducing yields markedly, often more than 50%. The main viruses worldwide are Sweet potato feathery mottle virus (SPFMV) and Sweet potato chlorotic stunt virus (SPCSV). Effects on yields by SPFMV or SPCSV alone are minor, or but in complex infection by the two or other viruses yield losses of 50%. The orthodox way of controlling viruses in vegetative propagated crops is by supplying the growers with virus-tested planting material. High-yielding plants are tested for freedom of viruses by PCR, serology, and grafting to sweet potato virus indicator plants. After this, meristem tips are taken from those plants that reacted negative. The meristems were grown into plants which were kept under insect-proof conditions and away from other sweet potato material for distribution to farmers after another cycle of reproduction.

Potatoes are an important crop in Mediterranean countries both for local consumption and for export to other countries, mainly during the winter. Many Mediterranean countries import certified seed potato in addition to their own seed production. The local seeds are mainly used for planting in the autumn and winter, while the imported seed are used for early and late spring plantings. Potato virus Y is the most important virus in Mediterranean countries, present mainly in the autumn plantings. The second important virus is Potato leafroll virus, though in recent years its importance seems to be decreasing. Potato virus X, Potato virus A, Potato virus S, Potato virus M, and the viroid, Potato spindle tuber viroid, were also recorded in several Mediterranean countries. For each virus the main strains, transmission, characterization of the virus particle, its genome organization, detection, and control methods including transgenic approaches will be discussed.

Tomato plants transformed with a cDNA clone encoding the inhibitor-of-virus-replication (IVR) gene were partially resistant to Botrytis cinerea. This resistance was observed as a significant reduction in the size of lesions induced by the fungus in transgenic plants compared with the lesions on the nontransgenic control plants. This resistance was weakened when plants were kept at an elevated temperature, 32 degrees C, before inoculation with B. cinerea compared with plants kept at 17 to 22 degrees C prior to inoculation. Resistance correlated with the presence of IVR transcripts, as detected by reverse transcription-polymerase chain reaction. This is one of the few cases in which a gene associated with resistance to a virus also seems to be involved in resistance to a fungal disease.

1. Mechanisms of recognition in dominant R gene mediated resistance - Peter Moffett 2. RNA silencing, an antiviral mechanism - Tibor Csorba, Vitantonio Pantaleo & Jozsef Burgyan 3. Local Lesions and Induced Resistance - G. Loebenstein 4. Recessive resistance to plant viruses - V. Truniger and M.A. Aranda 5. Towards a quarter century of pathogen-derived resistance and practical approaches to plant virus disease control - John Gottula and Marc Fuchs 6. Genetically Engineered Virus-Resistant Plants In Developing Countries: Current Status And Future Prospects - D.V.R. Reddy, Mysore R. Sudarshana, Marc Fuchs, N. Chandrasekhara Rao and George Thottappilly

A transition from dry farming to irrigated crops occurred in the mid 1950s in Israeli agriculture. This was due to the development of major irrigation schemes and the establishment of numerous small-holder farms, but was accompanied by outbreaks of virus diseases in vegetable and ornamental crops. These viral epidemics led to an intensification of the research efforts in plant virology. The major research fields and achievements are described and prospects for the future are outlined.

In this synopsis several mechanisms of natural resistance of plants to viral infections is outlined. Engineered resistance mechanisms are not treated in this article.

A QUICK method for detecting vein-clearing virus1 by paper chromatography has been developed.