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​2023 Fellow: John Hartung​

 John S. Hartung was born in Pittsburgh, Pennsylvania. He received his B. Sc. in Biology from Grove City College (Pennsylvania) in 1974, M. Sc. and Ph. D. degrees from Michigan State University in 1980 and 1985, respectively. His interest in plant pathology began when he was a Peace Corps volunteer doing agriculture extension work and rice paddy development in Sierra Leone. This work included the introduction of green revolution rice cultivars developed by the International Rice Research Institute (IRRI) and the Republic of China and interactions with plant pathologists from IRRI observing his farmers paddies for virus diseases. His professional research has been on fruit diseases. In his Master's thesis he demonstrated that post-harvest decays of blueberries are caused by Colletotrichum gloeosporioides infection during bloom, leading to a change in fungicide control strategies. As part of his Ph.D. dissertation, he cloned a polysaccharide depolymerase gene from a bacteriophage that infected Erwinia amylovora and transformed the pathogen with this gene to demonstrate that intact extracellular polysaccharide was not necessary for fire blight disease.

Dr. Hartung's research focus at USDA ARS has been with high consequence exotic pathogens of citrus, performed under quarantine conditions, starting with Xanthomonas citri (citrus canker). His studies of the genomic structures of Xanthomonads found in diseased Florida citrus in comparison with the genomes of X. citri collected from around the world clearly showed that the disease found in citrus nurseries in Florida was not in fact, citrus canker. Shortly thereafter Dr. Hartung used similar methods to determine that a separate introduction of X. citri had occurred in FL, which was followed by regulatory actions by USDA APHIS and the state of Florida. In cooperation with international researchers in France, a plasmid sequence was identified in all X. citri strains, and the partial plasmid sequence was used for one of the first PCR-based assays for a plant pathogenic bacterium. Similar methods of genome analysis and development of PCR-based assays were used to establish the etiology of citrus variegated chlorosis and coffee leaf scorch disease caused by Xylella fastidiosa subsp. pauca, in Brazil. Prior to the introduction of 'Ca. Liberibacter asiaticus' (CLas) in Florida, he maintained this pathogen in the Exotic Pathogens of Citrus Collection to develop a sensitive and specific qPCR assay for the detection of CLas. This assay was used by APHIS to confirm the discovery of CLas in Florida in 2005 and remained the standard of detection for several years thereafter. The qPCR assay was also used to establish the irregular distribution of CLas in infected citrus, which creates sampling problems for diagnostic methods. His analysis of genome sequence data from CLas led to the identification and cloning of an outer membrane protein from CLas.  This protein was expressed in E. coli, purified and used to immunize rabbits and utilized for simple tissue print assays for the detection of CLas in citrus. This assay is inexpensive and scales well for many samples, and nicely complements the standard PCR-based assays used to detect this pathogen for research and diagnostic purposes. A team led by Dr. Hartung used machine learning and convoluted neural network methods to automate and standardize the scoring of images of these tissue prints.

The USDA maintains an herbarium at Beltsville which includes samples of citrus diseases, some more than a century old. Dr. Hartung's laboratory performed DNA extractions on samples

submitted as 'citrus canker' in the 20th century from all over the world. Polymorphisms within the sequence data recovered from these canker lesions were used to document the origin of X. citri and subsequent global distribution with infected trifoliate rootstocks from Japan in the early 20th century. Similar work was done to identify the pathogen associated with herbarium samples from Florida and South America which were deposited as 'citrus leprosis'. Following RNA sequencing, assembled reads revealed that the original citrus leprosis samples from Florida belonged to a unique, and now apparently extinct clade of the nuclear form of citrus leprosis (Dichorhavirus), while other samples from South America belonged to the most prevalent cytoplasmic form of leprosis (Cilevirus).

Dr. Hartung's collection of graft transmissible pathogens of citrus was used to determine the complete genome sequence of the Citrus yellow mosaic virus. A full-length infectious clone of this virus was developed and utilized to establish the etiology of this disease by Agrobacterium- mediated infection. In collaboration with other researchers from the University of Florida, the collection was used to document diversity among many strains of Citrus tristeza virus (CTV), including the presence of a previously unknown 'trifoliate resistance breaking' strain in Puerto Rico. Long term collaborative work with domestic and international researchers led to the discovery of remarkable and previously unknown diversity within citrus leprosis viruses in Colombia, Mexico, Hawaii, and Florida. Viruses in both genera (Dichorhavirus and Cilevirus) are transmitted by Brevipalpus mites and cause very similar disease symptoms in citrus, a remarkable example of convergent evolution.

Dr Hartung has published 110 peer reviewed publications, 1 United States patent, 8 invited reviews and book chapters and 5 extension publications. He has made 32 invited presentations and has received international research fellowships from Japan, France and Brazil. Dr. Hartung has been active in the International Organization of Citrus Virologists and has been a member of the APS Biochemistry and Bacteriology Committees and the WCC-20 Viral and Subviral Pathogens Committee. Dr. Hartung has been a reviewer of numerous articles for APS journals and many other journals and has served as Associate Editor of both 'Plant Disease' and 'Phytopathology'. Dr. Hartung has provided expert advice on exotic citrus pathogens to USDA-APHIS, the Florida Department of Agriculture, the California Department of Food and Agriculture (Pierce's Disease), FAPESP (Brazil; Functional Genomics), Uruguay (Tree Fruit Germplasm Quarantine). Dr. Hartung's outstanding contributions in basic and applied sciences have advanced the knowledge of Citrus Pathology and the diagnostic technologies for high consequence citrus pathogens, and coupled with his extensive service to citrus growers, regulators and the international research community make him highly worthy of recognition as an APS Fellow.