Cross Protection Against Citrus tristeza virus - a Review

Abstract

Tristeza, caused by Citrus tristeza virus (CTV) is now in its second century as one of the most destructive and most researched diseases of citrus. This review encompasses the early history of tristeza and its relationship to the 19 th century Phytophthora epidemic which caused worldwide destruction of citrus then grown primarily as seedlings. The sour orange then evolved as a highly regarded and popular Phytophthora-tolerant rootstock. However, this combination of sweet orange, mandarin or grapefruit on the sour orange rootstock was susceptible to a new highly destructive vector-transmitted disease aptly named tristeza. There are two primary vectors for CTV: Aphis gossypii and the more efficient Toxoptera citricida. When tristeza enters a country, sour orange ultimately will disappear as the primary rootstock. All attempts at cross protection to salvage sour orange as a rootstock have failed. In the presence of the efficient vector T. citricida, severe stem pitting isolates of CTV evolved, which began a second wave of tristeza destruction with symptoms of severe stem pitting, small fruit size, and low yield. By searching for trees which survived, followed by extensive experimentation, protective isolates of stem pitting/small fruit CTV were found in Brazil, Argentina, Australia, South Africa, India, and Japan and recently in Peru. It may take 10 to 15 yr or longer to find and develop effective protection against stem pitting tristeza as was done by Costa and Müller in Brazil, van Vuuren in South Africa and by Bederski in Peru. The protective CTV isolates developed by Müller and Costa have proven highly effective for over 40 years in Brazil. A new technique is presented for creating protective CTV isolates by attenuating severe isolates by passage through Passiflora gracilis or P. caerulea via aphid transmission. Some of these isolates have proven highly effective in the revival of the citrus industry in Peru. The spread of T. citricida into Central America, the Caribbean islands, Mexico, Florida, the Madeira Islands, northern Portugal and Spain should stimulate concern for all citrus growing areas where this aphid is still not present, and argues strongly for accelerated research on cross protection. For a complete picture slide show on tristeza cross protection see EcoPort slide show #103.