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Matthew L. Richardson and Dewey M. Caron

Various instruments and contract services can be used to calculate degree-days. This study compared instruments and services to the Wescor Biophenometer, an instrument used by cooperators of the Southeast Pennsylvania IPM Research Group (SE PA IPM RG) throughout Delaware and southeastern Pennsylvania for 10 years. Instruments evaluated in the study were the Wescor Biophenometer Datalogger, Avatel HarvestGuard, Avatel Datascribe Junior, Davis Weather Monitor II, Accu-Trax, and the HOBO H8 Pro Temperature Data Logger. The services were SkyBit and national weather data. Different combinations of instruments and services were used at three locations in Pennsylvania and four locations in Delaware over a 2-year period. We checked the degree-day accumulation of each instrument and service weekly and made statistical comparisons among the instruments and services at each site. To further construct a comparison of the instruments, we noted distinctive qualities of each instrument, interviewed the manufacturers, and received feedback from SE PA IPM RG members who used the instruments. We evaluated the instruments' algorithms, durability, cost, temperature sampling interval, ease of use, time input required by the user, and other distinctive factors. Statistically, there were no significant differences in degree-day accumulations between the Biophenometer, Harvest-Guard, Datascribe, Weather Monitor II, Skybit, or weather service data. However, cost and time required to access/interpret data and personal preference should be major considerations in choosing an instrument or service to measure degree-days.

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Ed Stover, Sharon Inch, Matthew L. Richardson and David G. Hall

The citrus disease huanglongbing (HLB) has become endemic in Florida, with estimates that greater than 80% of citrus trees are currently infected. Although there are no commercial citrus varieties with strong HLB resistance, some field tolerance has been observed in trees exposed to the disease after they were mature. There is great urgency to identify citrus which may permit economic citrus production where HLB is endemic. Therefore, the objective of this study was to assess field tolerance to HLB. To expedite the trial due to urgency, nursery trees were purchased on rootstock varieties as available. The trial included the following unbalanced scion/rootstock combinations: ‘Hamlin/Kinkoji’, ‘Hamlin/Cleopatra’, ‘Temple/Cleopatra’, ‘Fallglo/Kinkoji’, ‘Sugar Belle/Sour Orange’, ‘Tango/Kuharske’, and ‘Ruby Red/Kinkoji’, with most comparisons based solely on scion/rootstock combinations. A randomized complete block experiment was established at Fort Pierce, FL, in Sept. 2010. All trees exhibited symptoms of HLB and tested positive for the Candidatus Liberibacter asiaticus (CLas) bacterium by Oct. 2012, with similar titers [directly assessed as cycle threshold (Ct) using quantitative polymerase chain reaction (qPCR)] measured for all scion/rootstocks at most sample dates, but early titer development in ‘Ruby Red/Kinkoji’ was significantly lower than several other scion/rootstocks. Across all time-points, ‘Fallglo/Kinkoji’ had the lowest rating of distinctive HLB mottling and ‘Ruby Red/Kinkoji’ had the highest rating, but ‘SugarBelle/Sour Orange’ had the highest percentage of leaves affected. After 5 years, ‘SugarBelle/Sour Orange’ and ‘Tango/Kuharske’ had the greatest overall increase in trunk diameter, and were among the healthiest in overall appearance. In Oct. 2015, ‘SugarBelle/Sour Orange’ and ‘Tango/Kuharske’ trees had significantly greater fruit load (80–88 fruit/tree) followed by ‘Temple/Cleopatra’ and ‘Fallglo/Kinkoji’ (31–35 fruit/tree) while ‘Hamlin/Kinkoji’, ‘Hamlin/Cleopatra’, and ‘Ruby Red/Kinkoji’ produced less than 20 fruit per tree. Despite becoming infected by CLas in less than 2 years after planting, the trees continued to grow and all scion/rootstocks displayed increasing fruit production, although very low in ‘Hamlin/Kinkoji’, ‘Hamlin/Cleopatra’, and ‘Ruby Red/Kinkoji’. Growth and fruit production in the highest performing scion/rootstocks were likely less than would be expected for healthy trees, but these results are promising, with markedly better response of some scion/rootstocks with mandarin hybrid scions compared with trees with sweet orange or grapefruit scions. Larger fully replicated trials are underway. It is noteworthy that the most pronounced HLB symptoms and higher early pathogen titer, which are the two criteria most widely used in assessing HLB resistance, were not associated with the lowest growth and cropping, and focus on early symptomatic traits when screening for resistance may obscure important disease tolerance.

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Ed Stover, Randall Driggers, Matthew L. Richardson, David G. Hall, Yongping Duan and Richard F. Lee

Xanthomonas citri ssp. citri (Xcc) is the causal agent of Asiatic citrus canker (ACC), a commercially important disease in Florida citrus as well as in many other regions. In this study we evaluated occurrence of foliar lesions from ACC on progenies of 94 seed-source genotypes (hereafter called “parent genotypes”) of Citrus and Citrus relatives in the field in east–central Florida to identify the relative susceptibility to ACC. Eight seedlings per parent genotype were planted in a randomized complete block planting, but the number of plants assessed in some genotype groups was reduced by mortality at some or all sampling dates. Plants experienced ambient exposure to high Xcc inoculum pressure and plants were assessed in Sept. 2010, July 2011, Oct. 2011, May 2012, and Sept. 2012. The incidence and severity of ACC lesions were assessed and evaluated using non-parametric analyses to compare progeny from the 94 parent genotypes. Progeny of 14 parent genotypes did not exhibit ACC symptoms at any date. All were in genera other than Citrus with only Microcitrus and Eremocitrus being cross-compatible with Citrus. The kumquat hybrid C. halimii, two accessions of C. reticulata, C. nobilis, and C. sunki were the only Citrus species in the group that had a low severity (percent total leaf area showing symptoms) on each date of assessment. The aforementioned accessions had an average incidence and severity of ACC lesions of less than 4% in 2011 and 2012, but 26% to 38% in 2010 when no chemical control for ACC was applied in the adjoining citrus groves at our field site. Fourteen of the 16 progeny of C. reticulata and related parent genotypes were in the group with the lowest incidence and severity of ACC on two or more assessment dates. However, for analysis of only the most symptomatic leaves on each plant, 10 C. reticulata parent genotype progenies were in the most resistant category on the Sept. 2012 assessment despite having a low incidence and severity of ACC symptoms overall. Progeny of Poncirus and its hybrids as well as those of C. maxima, C. limon, and related species were the most severely diseased at all assessment dates. There were few instances in which progeny of different accessions of the same species had markedly different responses to ACC: progeny of C. reticulata ‘Fremont’ displayed more severe ACC compared with several other C. reticulata groups and C. aurantium ‘Zhuluan’ displayed much lower incidence and severity of ACC compared with several other accessions designated C. aurantium. Information on ACC susceptibility in diverse Citrus and Citrus relatives may prove useful for breeding programs aimed at reducing ACC susceptibility and will be of value to researchers interested in mechanisms of ACC resistance and susceptibility.

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Matthew L. Richardson, Catherine J. Westbrook, David G. Hall, Ed Stover, Yong Ping Duan and Richard F. Lee

The citrus leafminer, Phyllocnistis citrella Stainton (Lepidoptera: Gracillariidae), is a key pest in most citrus-growing regions worldwide. Adult citrus leafminers oviposit primarily on young elongating flush of Citrus as well as other Rutaceae and some ornamental plants. Larvae feed on the epidermal cell layer of developing leaves and injury to leaves provides a pathway for infection by the bacterium Xanthomonas citri subsp. citri (Hasse), the causal agent of Asiatic citrus canker. In this study, we quantified abundance of citrus leafminer larvae on progeny of 87 seed parent genotypes of Citrus and Citrus relatives (family Rutaceae) in the field in East–central Florida to identify those that have low abundance of leafminers. Progeny from the 87 parent genotypes varied in abundance of the leafminer. Progeny of 15 parent genotypes had a high mean abundance of more than six leafminers per flush shoot. All but one of these genotypes were in the Citrus genus. Progeny of 16 parent genotypes had zero, or nearly zero, leafminers, but none were from the Citrus genus. However, many of these 16 genotypes were from genera closely related to true citrus (subtribe Citrinae) and are sexually compatible with Citrus. Progeny of two parent genotypes in the subfamily Toddalioideae and Glycosmis pentaphylla (Retz.) Corr. also had a low abundance of leafminer. Glycosmis pentaphylla also is a poor host for the Asian citrus psyllid, Diaphorina citri Kuwayama, and has biochemical resistance to the citrus weevil, Diaprepes abbreviatus (L.), so this genotype as well as others identified as poor hosts for the leafminer may prove useful in breeding programs aimed at reducing the abundance of multiple insect pests on citrus.