The Journal of Citrus Pathology in an international, peer-reviewed, open access, online publication. The Journal of Citrus Pathology welcomes reports on research from all branches of pathology on all diseases of citrus and related fields. The journal accepts original contributions in basic and applied research on citrus diseases, pathogens and disease-associated agents, including graft-transmissible agents, viruses, viroids, bacteria, phytoplasmas and other wall-less bacteria, fungi, oomycetes, and nematodes as well as any agents affecting citrus biology. This on-line IOCV publication by eScholarship ensures the distribution of critical information for citrus health and hosts occasional invited autobiographies and biographies of pioneer leaders of the field of citrus pathology.
Volume 5, Issue 1, 2018
Huanglongbing (HLB), also known as citrus greening, is one of the most destructive diseases of citrus worldwide. It is caused by unculturable phloem-limited bacteria that belong to the Candidatus Liberibacter genus including Ca. L. asiaticus (CLas), Ca. L. africanus, and Ca. L. americanus. Currently, there is no effective control strategy for HLB and no known cure for the disease. We have previously generated transgenic ‘Duncan’ grapefruit and ‘Hamlin’ sweet orange expressing the Arabidopsis NPR1 (AtNPR1) gene, which encodes a master regulator of systemic acquired resistance. Characterization of the transgenic lines indicated that overexpression of AtNPR1 confers resistance to citrus canker, another serious bacterial disease in citrus. In this study, we intensively screened these transgenic lines for resistance or tolerance to HLB under greenhouse conditions. Three independent transgenic lines (one line in the ‘Duncan’ grapefruit background and two lines in the ‘Hamlin’ sweet orange background) only occasionally displayed mild HLB symptoms in the presence of C Las and have kept growing normally over a period of nine years. Significantly, all vegetatively propagated progeny plants of these lines have retained the same levels of HLB tolerance. Furthermore, immunoblot analysis revealed that the three transgenic lines accumulate high levels of AtNPR1 protein. These results, together with the previous finding that the same three transgenic lines are resistant to citrus canker, demonstrate that overexpression of the AtNPR1 protein in citrus is able to provide robust tolerance to HLB.
Efficient disease management is critical in the production of citrus; a crop that is susceptible to several plant pathogens. The ongoing battle with citrus greening has led to a shift in cultural practices, which could lead to a resurgence of previously controlled diseases. Here we investigated the presence of several common citrus-infecting viruses and viroids (Citrus leaf blotch virus, Apple stem grooving virus (synonym: Citrus tatter leaf virus), Citrus exocortis viroid, Hop stunt viroid (synonym: Citrus viroid II), and Citrus dwarfing viroid (synonym: Citrus viroid III) in Florida citrus groves. All five viruses and viroids are still present, with varying incidence. It would be prudent to take them into consideration when developing citrus disease management strategies.
In the P.R. China, ten graft-transmissible pathogens have been identified towards citrus, including Candidatus Liberibacter asiaticus, Citrus tristeza virus (CTV), Citrus tatter-leaf virus (CTLV), Citrus exocortis viroid (CEVd), Citrus yellow vein clearing virus (CYVCV),Satsuma dwarf virus (SDV), Citrus vein enation virus (CVED), Citrus psorosis virus (CPsV), Citrus cachexia viroid (CCaVd) and Citrus chlorotic dwarf virus (CCDV). Of these pathogens, the first five cause damage in field citrus trees, whereas the latter five were occasionally detected from the imported citrus materials or field trees. The research progresses about HLB, CTV, CYVCV etc. within recent three years have briefly been reviewed.