Journal of Citrus Pathology

The Citrus tristeza virus (CTV) strains present in an area can determine the type and severity of disease produced. Using real time RT-qPCR, we screened a series of representative samples collected throughout Florida from 1964 to 2002 for CTV strain presence. We found that contrary to previous reports, the historical samples were infected with both the T30 and T36 strains, the latter often at low titer. The VT strain was rare, with a scattered distribution. We compared this to samples collected in 2014 and 2015, and found that T30 and T36 are prevalent in nearly all commercial groves; VT strain incidence has increased markedly. With changing cultural practices, such as an increase in sour orange rootstock use, tristeza disease continues to be a significant threat to the Florida citrus industry.

Citrus Bacterial Canker (CBC) is a serious disease that affects production of almost all commercial citrus cultivars in subtropical citrus growing regions worldwide. In this study, the effectiveness of monthly foliar sprays of wettable powder formulation Serenade MAX of Bacillus subtilis QST 713, alone or as tank mixes with copper hydroxide on CBC disease development was evaluated under greenhouse and uncovered nursery conditions. The QST 713 as a tank mix with copper hydroxide reduced significantly the disease severity and incidence, followed by the copper hydroxide treatment, compared to the control. The disease incidence on leaves of inoculated trees treated with a combination of copper with QST 713 was never higher than 19%, whereas, the disease incidence reached 43% for non-sprayed trees. It was possible to reduce the number of copper sprays up to 6 sprays per season when it was mixed with the bio-fungicide QST 713 to effectively control CBC compared with 8 sprays of copper hydroxide alone per season. Based on our results, the application of QST 713 not only may have potential for CBC management in conjunction with copper hydroxide or other disease control but also may reduce the frequency or rate of copper sprays in citrus groves.

Viroid species identified in citrus induce a range of symptoms in this host as well as in non-citrus hosts. Currently, 7 citrus viroid species are recognized including Citrus bent leaf viroid (CBLVd), Hop stunt viroid (HSVd), Citrus dwarfing viroid (CDVd), Citrus bark cracking viroid (CBCVd), Citrus viroid V (CVd-V), Citrus viroid VI (CVd-VI) and Citrus exocortis viroid (CEVd). Cachexia-inducing variants of HSVd and CEVd, which causes exocortis, are considered severe pathogens of citrus, whereas other viroid species induce less severe symptoms such as stunting, either singly or in combination. Some viroid species, such as CDVd, have previously been used to deliberately induce stunting for high density planting of citrus, but studies on the effects of commercial orchards are limited. Research on the effect of viroid species requires the isolation of single viroid species. Biological isolation of single species from naturally infected citrus is challenging since viroids seldom occur as single infections and are often found in combination with various other pathogens. The production of single viroid species clones for in vitro transcription enables the generation of a single viroid species inoculum for research applications and circumvents the need for maintenance of sources in plants. Complete genomes of 7 viroid strains including CBLVd, 2 HSVd variants (CVd-IIa and CVd-IIb), CDVd, CBCVd, CVd-V and CEVd were cloned with a leading T7 promoter sequence to facilitate in vitro transcription. Circularized RNA transcripts were successfully used to transfect ‘Etrog’ citron (Citrus medica) by slash-inoculation that developed typical citrus viroid symptoms.