Trends in 20^th century citrus research in Israel

The Israeli citrus industry reached its peak around 1975, with an area of ca. 43,000 ha and an export of almost 1 million t annually. While the ‘Shamouti’ orange was still the dominant cultivar, its low productivity and insufficient quality in new planting areas led to an increasing awareness of the need for a change. Internationally, the soft, easy peeling mandarin hybrid cultivars were on the rise. Thus, three major challenges were identified during the 2^nd half of the 20^th century, leading to intense research activity: a) genetic improvement and diversification of easy peeling cultivars; b) the fight against the spread of the Tristeza viral disease; c) augmenting productivity by plant growth regulators (PGR) and horticultural manipulations. The genetic improvement plan, led by Vardi and Spiegel-Roy, consisted mainly of extensive hybridization of mandarins, searching for productive and seedless cultivars. Several marketable selections were achieved and the most successful was ‘Orri’ mandarin hybrid, which received international recognition and is grown commercially in Israel and elsewhere. Little was known during the 1960^th about the virus causing the tristeza disease. Partial characterization of the citrus tristeza virus (CTV) and a few other similar thread-like particles, led to the establishment of the new Closterovirus group, consisting of plant viruses of different particle lengths, sharing structural similarities, cytopathology, and mode of vector transmission. The finding in 1970 of natural CTV transmission in Israel, prompted the local citrus industry to establish an ambitious CTV suppression program, which was later considerably expanded by the development of ELISA for CTV diagnosis. Partial cloning of a local CTV isolate and molecular hybridization allowed us to establish that the CTV-VT strain consists of both pathogenic and mild CTV isolates. The productivity research led by Monselise and his students was inspired by classic plant hormone theories and their implementation through use of plant growth regulators (PGR), mainly synthetic auxins and gibberellins (GAs). Almost every citrus’ developmental phenomenon – fruit set, fruit abscission, peel thickness, fruit coloration – can be enhanced or delayed by PGR. Combinations of girdling and GAs to enhance fruit set are used commercially worldwide. The impact of these achievements on current citriculture will be evaluated.