Citrus tristeza virus (CTV) isolates collected from the Lower Rio Grande Valley in south Texas and east Texas were characterized using citrus indicators and molecular methods. The citrus indicators were Mexican lime (Citrus aurantifolia), sour orange (C. aurantium), sweet orange (C. sinensis) grafted to sour orange, Duncan grapefruit (C. × paradisi), and Madam Vinous sweet orange, with some CTV isolates additionally indexed using the Texas commercial grapefruit cvs. Rio Red and Star Ruby, and Marrs and N-33 sweet orange. Severity ratings used 11 biotype groups or cumulative mean relative indices. Molecular characterization was carried out using poly- and monoclonal antibodies, seven strain-specific probes and single-stranded conformational polymorphism, and all were based on the CTV major coat protein or gene. All Texas CTV isolates produced vein clearing symptoms on inoculated Mexican lime plants. Over half of the CTV isolates tested were placed in biotype groups IX and X (causing decline of sweet orange on sour orange, seedling yellows on sour orange and grapefruit seedlings, and stem pitting of grapefruit or sweet orange), and one isolate was in biotype I (mild).
Elsinoë fawcettii causes citrus scab and E. australis causes sweet orange scab and there are different pathotypes of each species. Citrus scab is widely distributed, whereas sweet orange scab is limited mostly to southern South America. In 2010, E. australis was detected by PCR in Texas for the first time in the USA and subsequently found in commercial areas of Texas. In this study, cultures were obtained in Texas mostly from diseased fruit with symptoms similar to "late-season windscar" and identified as the Natsudaidai pathotype of E. australis by sequencing of the PCR amplification products and sequencing of the translation elongation factor and ITS regions. Inoculations of detached leaves and fruit of grapefruit, sweet orange, and tangerine produced scab-like symptoms on all organs and species. The fungus was re-isolated from the inoculated organs and PCR tests on re-isolated fungal colonies and lesions confirmed the presence of E. australis. The disease represents sweet orange scab of citrus with a new scab disease symptomatology, but the host range, ecology and epidemiology of this pathotype are still not completely understood. Thus far, Natsudaidai pathotype has been reported only from South Korea.
Citrus tatter leaf virus (CTLV) (Apple stem grooving virus) is mechanically-transmitted in citrus, causing bud-union crease in trees budded on trifoliate orange and its hybrids, but is symptomless in most scions. Seed transmission of a strain of CTLV has been reported in Lilium longiflora and Chenopodium quinoa. In order to test whether CTLV is seed transmitted in citrus, seed was collected from four adjacent CTLV-infected citrus trees of different species, namely Clementine mandarin, Meyer lemon, Eureka lemon and Meiwa kumquat. The resulting 355 seedlings and the four parent trees were tested for CTLV presence by RT-PCR using three primer sets. The four parents and two of the 136 Eureka lemon seedlings were found to be CTLV positive. This is the first report of CTLV seed transmission in citrus. Cloning and sequencing of the coat protein gene amplified using primer set TL1 showed that the sequence from the seedlings had an 89.7% homology with the parent tree, but 100% homology with the Meyer lemon and Meiwa kumquat trees, suggesting a filtering effect during transmission through the Eureka lemon parent as reported elsewhere.
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