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Asiatic citrus canker (ACC) foliar lesions were evaluated on progenies of 84 seed-source genotypes ("parent genotypes") from the Citrus Variety Collection (CVC) of the University of California at Riverside (UCR) of Citrus trifoliata and hybrids between C. trifoliata and other Citrus species and hybrids. Trees were planted Aug. 2013 in a completely randomized design at the Fort Pierce U.S. Department of Agriculture (USDA) grove. Plants were assessed visually Aug. 2017, Sept. 2019, and Sept. 2020 for distinctive ACC lesion incidence and severity. Progeny were compared by parent genotypes using nonparametric analysis. Incidence of ACC [percentage of leaves displaying symptoms, verified by quantitative polymerase chain reaction (qPCR) to be associated with Xanthomonas citri pv. citri] across parent genotypes ranged from 8% to 80% (mean, 49%) of leaves affected in 2017, from 4% to 58% (mean, 29%) in 2019, and 8% to 46% (mean, 25%) in 2020. In 2017, of 49 C. trifoliata parent genotypes, only four separated from the two highest ACC-incidence statistical categories [Citrus Research Center (CRC) 3345, 3484, 3888, and 4017]. whereas 29 of the 35 C. trifoliata hybrids displayed lower ACC incidence, which separated from the two highest statistical categories. In 2019, of the C. trifoliata, only six separated from the highest ACC-incidence statistical category (CRC 3330, 3484, 3547, 3549, 3876, and 3888), whereas all 35 C. trifoliata hybrids displayed lower ACC incidence and separated from the highest statistical category, and 26 hybrids separated from 18 of the C. trifoliata. In 2020, only three C. trifoliata separated from the highest ACC-incidence statistical category (CRC 2861, 3549, and 3888) and 20 hybrids separated from 18 of the C. trifoliata. By parent genotype, ACC incidence correlated substantially between each pair of the 3 years, with r2 values of 0.39, 0.57, and 0.65. Of 34 hybrids validated, similar numbers had C. trifoliata, grapefruit (C. 3aurantium var. racemosa), and sweet orange (C. 3aurantium var. sinensis) chloroplasts. Chloroplast type affected ACC incidence and severity, but not in a consistent manner. Near-isogenic groups within C. trifoliata, as determined by DNA markers, were associated with some statistically different ACC sensitivity. Overall, hybrids of C. trifoliata with other citrus types displayed markedly reduced ACC sensitivity compared with C. trifoliata, indicating that this trait is readily overcome through breeding. [ABSTRACT FROM AUTHOR]

The U.S. Department of Agriculture citrus scion breeding program is urgently working on developing huanglongbing (HLB; pathogen Candidatus Liberibacter asiaticus)-tolerant cultivars with excellent fruit quality and productivity when HLB-affected. The slow process of assessing new citrus hybrids is a major impediment to delivery of these much-needed cultivars. We generate thousands of hybrids each year, germinate the seedlings, grow them for 2 years in the greenhouse, plant them at high density in a field where the disease HLB is abundant, grow them for 5 to 10 years, and make selections based on tree performance and fruit quality of these HLB-affected trees. Based on promising reports of accelerated citrus growth when grown in a metallized reflective mulch (MRM) system, we tested the hypothesis that the MRM system may accelerate growth and selection of new hybrid seedlings compared with conventional soil culture (CSC). In the MRM system, tree rows are covered with a layer of metallized plastic film and drip irrigation is installed beneath the plastic. In 2 years of analysis, tree canopy volume was significantly greater with MRM in 2020 (27% greater than CSC) but not in 2021, and MRM tree height was greater in 2021 (7% greater than CSC). Mortality was significantly greater with MRM in both 2020 and 2021(in 2021: 32% vs. 17% under CSC), and MRM trees had more chlorotic leaves. Because of staff limitations, plant debris and soil were not routinely cleared from MRM, thus diminishing any benefit from the reflective surface. Better maintenance might have resulted in more sustained evidence of MRM growth benefits. With the current resource availability, the MRM system does not appear to accelerate the assessment of hybrid seedling trees. [ABSTRACT FROM AUTHOR]

Assessments of the resistance of citrus germplasm to huanglongbing (HLB) can be expedited by inoculating plants under laboratory or greenhouse settings with the HLB bacterium, Candidatus Liberibacter asiaticus (CLas). Consistent rapid screening is critical to efficiently assess disease resistance among plant materials; however, a number of factors may govern the efficacy of such inoculations. Despite the rapidity at which HLB can spread in a grove, it often takes 8 to 10 months for high levels of CLas and HLB symptoms to develop even in highly susceptible sweet orange. Therefore, two experiments were conducted to assess factors that might influence efficiency in screening for HLB resistance. In one experiment, three test citrus genotypes ('Kuharske', previously shown to be HLB resistant; rough lemon, previously shown to be HLB tolerant; and 'Valencia', HLB susceptible) were bud grafted using CLas-infected buds from four different source genotypes. All bud source genotypes had similar levels of CLas titer, but citron, rough lemon, and Volkamer lemon were hypothesized to be better bud inoculum sources as they are more tolerant of HLB than 'Valencia'. Among the three test genotypes over all sources of infected buds, inoculations of 'Kuharske' resulted in lower CLas titers and fewer HLB symptoms than inoculations of rough lemon or 'Valencia'. Inoculations of rough lemon resulted in higher CLas titers and more pronounced HLB symptoms when it was inoculated using infected buds from rough lemon or 'Valencia'. Grafting 'Valencia' with infected buds from Volkamer lemon resulted in less disease than when 'Valencia' was grafted with infected citron, rough lemon, or 'Valencia' buds. Overall, these results suggest that the source of CLas-infected buds used to graftinoculate some genotypes will influence disease development. Trunk cross-sectional area increase for the year following infection was 33 higher in 'Kuharske' and rough lemon, compared with 'Valencia'. 'Kuharske' had very low levels of CLas (30 CLas/µg DNA), whereas 'Valencia' (43,000 CLas/µg DNA) and rough lemon (6700 CLas/ µg DNA) had relatively high levels. As an alternative to graft-inoculating plants with CLas-infected buds, plants can be subjected to infestations of CLas-infected Asian citrus psyllid (ACP) as occurs naturally. Of interest is if transmission rates of CLas and the development of HLB in a genotype are greater when the ACP have been feeding on the same host genotype. An experiment was therefore conducted to assess transmission of CLas by ACP reared on CLas-infected rough lemon to five different genotypes ('Carrizo', 'Flame' grapefruit, rough lemon, 'Temple', and 'Valencia'). These assessments were made using a detached leaf assay recognized as a faster method of gauging transmission rates of CLas than using whole plants. Higher percentages of ACP died when they were transferred from infected rough lemon to healthy 'Carrizo', and lower percentages died when they were transferred to rough lemon or 'Flame'. However, CLas transmission by infected ACP occurred to at least some leaves of each genotype in each of the five different assays, with 70% or more leaves of each genotype becoming infected in at least one assay. Over all assays, there was relatively little variation among genotypes in the percentage of leaves becoming CLas infected and in the titer of CLas developing in infected leaves. However, there were relatively large differences in transmission rates among individual assays unrelated to differences among test genotypes. Because of the rapidity of the detached leaf assay, efforts are merited to improve consistency of this inoculation method. [ABSTRACT FROM AUTHOR]

A test population consisting of progenies of 92 seed-source genotypes (hereafter called "parent genotypes") of Citrus and Citrus relatives in the field in east-central Florida was assessed after natural freeze events in the winters of 2010 and 2011. Eight seedlings per parent genotype were planted in a randomized complete block design; however, as a result of mortality, the number of plants assessed in some genotype groups was reduced at some or all sampling dates. The citrus diseases huanglongbing and citrus canker were endemic in the planting and may have influenced tree response to cold temperatures. Unusually low temperatures (near -4 °C each winter) for east-central Florida were experienced during the trial period. Defoliation and dieback were significantly greater in the winter of 2011 than in the winter of 2010. The winter in 2011 was preceded by a period of extraordinarily low temperatures in mid-December with no period of cool temperatures to allow trees to acclimate. In 2010 the average defoliation was 53% ± 28% and less than 13% of the trees exhibited any noticeable dieback, whereas in 2011, the average defoliation and dieback were 93% ± 17% and 51% ± 35%, respectively. Within the genus Citrus, several progenies were identified that had 16% to 24% dieback in 2011 and these were from parent genotypes C. reticulata (CRC 2590) (23%), C. sinensis (CRC 3858) (24%), C. maxima (CRC 3945) (16%), C. hassaku (CRC 3907 and 3942) (16% and 17%), C. aurantium (CRC 628 and 2717) (18% and 7%), C. taiwanica (CRC 2588) (21%), and C. neo-aurantium (C. obovoidea + C. unshiu graft chimera) (CRC 3816) (23%). Within other genera in the Aurantiodeae, Poncirus trifoliata (CRCs 301,3957,3549, and 4007), Severinia buxifoiia (CRC 1497), Bergera koenigii (CRC 3165), and Glycosmis pentaphylla (CRC 3285) had the least amount of dieback, all at less than 23%. The two species within the Toddalioideae subfamily of the Rutaceae (Casimiroa edulis and Zanthoxylum ailanthoides) had among the least amount of dieback (1% and 8%, respectively). When considered by groups, the Citrons and Australian natives had the greatest amount of dieback in 2011, 68% and 65%, respectively. The trifoliates (Poncirus and hybrids) had the least dieback, ranging from 4% to 40%. The information from this study may be useful in germplasm enhancement and Citrus breeding targeting greater cold tolerance. [ABSTRACT FROM AUTHOR]

In spring 1999, a commercial NAA (1-naphthaleneacetic acid) preparation for trunk sprout inhibition was compared with a corrugated plastic trunk wrap, aluminum foil wrap, bimonthly hand removal of sprouts, use of NAA preparation plus bimonthly hand removal when sprouts appeared, and a nontreated control. Three recently planted groves on three different rootstocks [Midsweet' orange (Citrus sinensis)] un Swingle citrumelo (Citrus paradisi x Poncirus trifoliata), 'Valencia' orange on Volkamer lemon (Volk, Citrus limon), and 'Minneola' tangelo (Citrus paradisi x C. reticulata) on Smooth Flat Seville (SFS, Citrus hybrid) received each of the treatments in a randomized complete block experimental design with trees blocked by initial height and circumference. Every 2 months, sprouts were counted on each tree and removed from the hand removal treatments. After 1 year, all sprouts were removed and counted and height and circumference of trees was determined. Across all experiments, 82% to 100% of nontreated trees produced trunk sprouts and all sprout control methods significantly reduced sprouts per tree. NAA treatments were never significantly less effective at sprout suppression than the wraps at the P = 0.05 level, although in two experiments, wraps were more effective than NAA at P = 0.10. Time of sprout appearance varied between the three experimental blocks. Plastic and foil trunk wraps enhanced development of trunk circumference compared with nontreated controls in 'Midsweet'/Swingle and 'Valencia'/ Volk. Greater trunk circumference resulted from use of wraps versus NAA in all three experiments, which appeared unrelated to differential sprout suppression. In these experiments, it appears that either wraps enhanced tree development beyond the suppression of sprouts or NAA influence on tree metabolism somewhat reduced trunk growth. The economics of the sprout suppression methods are also discussed. [ABSTRACT FROM AUTHOR]

Huanglongbing (HLB) is a serious and devastating disease of citrus caused by Candidatus Liberibacter spp. and vectored by the Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Psyllidae). The disease has the potential to greatly limit the production of citrus in Florida and other citrus-growing regions worldwide. Current control of D. citri and HLB is inadequate, but the identification and incorporation of D. citri resistance traits from uncultivated Citrus spp. and Citrus relatives is seen as a potential disease management strategy. In this study, seedlings of 87 Rutaceae seed-source genotypes, primarily in the orange subfamily Aurantioideae, were assessed in the field for their propensity in a free-choice situation for infestations of natural south Florida populations of D. citri. The majority of test populations surveyed hosted all three life stages of D. citri; however, there were significant differences among the test populations in the mean ranks for 19. citri eggs (F = 3.13, df = 86, P < 0.0001), nymphs (F = 9.01, df 86, P < 0.0001), and adults (F = 4.21, df 86, P < 0.0001). The only sample test population that was completely avoided by all life stages of D. citri was seedlings of Casimiroa edulis Llave et Lex, commonly known as white sapote, which was one of the few plants included in the study belonging to the Rutaceae subfamily Toddalioideae. Although not completely avoided, very low levels of D. citri were found on two surveyed test populations of Poncirus trifoliata (L.) Raf, seedlings of 'Simmon's trifoliate' and 'Little-Leaf'. Poncirus trifoliata, the trifoliate orange, readily forms hybrids with Citrus spp., is commonly incorporated into rootstock varieties, and has been used in breeding-advanced scion material. The identification of partial resistance in P. trifoliata to D. citri could prove useful in future citrus breeding efforts aimed at reducing the incidence and spread of HLB. Determining if there is true resistance to D. citri in this and other germplasm sources with a low propensity for infestations in free-choice situations will require no-choice experiments. [ABSTRACT FROM AUTHOR]