A historical note on two unreported obstacles for cross-protecting mature citrus trees against severe Citrus tristeza virus isolates.

Abstract

Letter to the Editor A historical note on two unreported obstacles for cross-protecting mature citrus trees against severe Citrus tristeza virus isolates. M Bar-Joseph 1* The S Tolkowsky Laboratory, ARO, The Volcani Center, Bet Dagan 50250, Israel. *Correspondence to: mbjoseph@gmail.com Citation: Bar-Joseph M. 2015. A historical note on two unreported obstacles for cross-protecting mature citrus trees against severe Citrus tristeza virus isolates. J Cit Pathol. iocv_journalcitruspathology_28534. During the years 1970 to 1986 the Israeli citrus industry had undertaken a costly and ambitious program for suppressing the outbreak of a Citrus tristeza virus (CTV) epidemic. For comprehensive reviews of CTV and the tristeza disease see Dawson et al. (2013), Moreno et al. (2008), and Bar-Joseph et al. (1989). The program, which involved millions of ELISA tests, was a coordinated effort of virus research, extension, and regulatory agencies funded by the local citrus marketing board and the Ministry of Agriculture. These major commitments of CTV control by a “search and destroy” strategy were based on statistical analyses indicating that CTV infection rates throughout citrus areas were low (Bar-Joseph et al. 1989). Because of the absence of reliable diagnostic methods to differentiate between mild and severe CTV isolates at early stages of infection, the program policy was to eliminate every tree that showed a positive ELISA reaction. An increase in the incidence of CTV detection during 1983 to 1985 and grower reluctance to cooperate with timely removal of symptomless infected trees, pointed to the need for reevaluating CTV infection rates. Results in 1986 suggested that despite the suppression efforts, which for economic reasons had been already reduced a few years earlier, there were about 50,000 undetected infected trees, spread over 5000 hectares (Bar-Joseph et al. 1989). It was also apparent from the high ratio of symptomless to declining trees that the majority of these sweet orange on sour orange rootstock trees were carriers of “mild” CTV isolates. Trees infected with such isolates remained symptomless for 5 to 10 years, even when the isolates were infecting a decline sensitive combination of sweet orange scions on sour orange rootstocks. Cost-benefit analyses indicated that locating these symptomless trees among the millions of citrus trees cultivated at that time in Israel would have involved testing and compensation costs far beyond the industry’s funding resources. Once the CTV suppression program came to a halt, testing the mild isolates from symptomless trees for their ability to protect trees in plots with severe decline causing isolates became an option. In 1988 a cross protection iocv_journalcitruspathology_28534 experiment to prevent decline was established in a mature (around 20 years old) Valencia orange grove grafted on sour orange. The experimental plot was part of the Yachin Company Morasha plantation located east of Tel Aviv. In this area an extremely severe isolate Mor-T (Ben-Zeev et al. 1988), belonging to the VT strain (Mawassi et al. 1993; Shalitin et al. 1994) was causing rapid decline of Minneola tangelo, Valencia, and Shamouti orange trees at early stages of natural infection. Trees of sensitive combinations infected by Mor-T were showing quick decline long before the virions pervaded the canopies to allow routine detection by ELISA (Ben-Zeev et al. 1988). For mild strain cross protection we used several CTV isolates belonging to the VT strain, including Ach-T, Miqveh-T, and Miqveh-127K (Mawassi et al. 1993; Shalitin et al. 1994), obtained from mature Shamouti orange trees on sour orange stocks that had been infected for several years with CTV, and did not show observable differences when compared with uninfected Shamouti orange trees. Screen house tests showed that, unlike the original VT and Mor-T isolates, these mild VT isolates did not induce seedling yellows when graft inoculated to sour orange seedlings. In addition, simultaneous placing of a Valencia bud and CTV inoculum buds on 1-year-old sour orange seedlings resulted in severe chlorosis of the sprouting Valencia shoot when the buds were infected with Mor-T, while similar Valencia buds on plants simultaneously grafted with the Ach-T and Miqveh-T inocula allowed the development of apparently normal shoots, similar to mock inoculated control plants. The cross protection experiment was conducted on a 2 ha plot, essentially still free of CTV infected trees, located west of the Morasha citrus planted area. The experimental design was simple: each of the protective isolates maintained in Citrus macrophylla, was graft-inoculated along entire rows of about 50 to 60 trees each, with at least 2 replications. Rows on the orchard edges and in the middle of the plot were left as unprotected controls. Trees were graft inoculated on about 1 cm lateral branches at a height of about 60 to 80 cm on 2 sides of each tree. The