Citrus Blight and Other Diseases of Recalcitrant Etiology

Abstract

Several economically important diseases of unknown or recently determined cause are reviewed. Citrus blight (CB), first described over 100 years ago, was shown in 1984 to be transmitted by root-graft inoculations; the cause remains unknown and is controversial. Based on graft transmission, it is considered to be an infectious agent by some; others suggest that the cause of CB is abiotic. Citrus variegated chlorosis, although probably long present in Argentina, where it was considered to be a variant of CB, was identified as a specific disease and shown to be caused by a strain of Xylella fastidiosa after if reached epidemic levels in Brazil in 1987. Citrus psorosis, described in 1933 as the first virus disease of citrus, is perhaps one of the last to be characterized. In 1988, it was shown to be caused by a very unusual virus. The cause of lettuce big vein appears to be a viruslike agent that is transmitted by a soilborne fungus. Double-stranded RNAs were associated with the disease, suggesting it may be caused by an unidentified RNA virus. Rio Grande gummosis, dry rot root, peach tree short life, and some replant diseases may be diseases of complex etiology. Various microorganisms have been isolated from trees with these diseases, but the diseases may be attributable in part to environmental factors. Determination of the cause of these diseases of complex etiology has proven difficult, in part, because they affect only mature trees.

Identification and Genomic Characterization of a New Virus ( Tymoviridae Family) Associated with Citrus Sudden Death Disease

“…Several citrus diseases have been related to virus infection (13). They caused severe economical losses for the world citrus industry in the past and are still a problem.…”

Different changes of soil nematode communities in replant and continuous-planting peach orchards and their indicative value for peach replant problem

“…The common symptoms of peach replant problem are expressed as stunted growth, low productivity and a decline in tree vigor, all of which could shorten the economic viability of a peach orchard. There are confl icting reports in the literature concerning the complex etiology of this problem, and the replant problem likely results from a combination of diseases and disorders of complex etiologies, and cannot be attributed to any one factor (Derrick & Timmer, 2000). The failure of the replanted trees has been linked to the activity of insects and nematodes as well as to the effect of poor plant nutrition, toxic agents, and spray residues (Koch, 1955).…”

Introduction of a citrus blight-associated gene into Carrizo citrange [Citrus sinensis (L.) Osbc. � Poncirus trifoliata (L.) Raf.] by Agrobacterium-mediated transformation

“…The p12 open reading frame (ORF) is significantly similar to that of an expansin from Arabidopsis thaliana (At-EXP2 accession U30481). The p12 is also similar in size and sequence to a hypothetical protein and a blight-associated protein homolog gene in Arabidopsis (Derrick and Timmer 2000) that has been classified as an g expansin (Li et al 2002). Expansins are responsible for loosening cell walls during longitudinal growth (McQueen-Mason et al 1992;Shcherban et al 1995), and known expansins are approximately three times larger than p12.…”

“…The disease is a problem wherever citrus is grown in hot and humid areas. Current estimated annual tree losses to CB are 650,000 for Florida and 10 million for Brazil (Derrick and Timmer 2000). The cause of CB is not known, but it can be transmitted by root grafting, suggesting that an infectious agent is involved (Timmer et al 1991).…”

“…The cause of CB is not known, but it can be transmitted by root grafting, suggesting that an infectious agent is involved (Timmer et al 1991). Derrick and Timmer (2000) and Timmer and Bhatia (2003) have published recent reviews on the history and other aspects of CB. Symptoms of trees with CB include a general decline, which is characteristic of a number of disorders of citrus.…”