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2023 Lee M. Hutchins Award: Clive Bock​

Clive H. Bock was born in Nairobi, Kenya, completing his BA and MA in Botany at the University of Oxford, and an MSc in Crop Production at the University of Bath in the UK. He earned his PhD in Plant Pathology at the University of Reading. Between 1995 and 2010, he worked in various research capacities at organizations in England, India, Australia, and the U.S. In 2010, he joined the USDA-ARS Southeastern Fruit and Tree Nut Research Station, Byron, GA, as a Research Plant Pathologist with primary responsibility for diseases of pecan. Bock is an adjunct professor at the University of Georgia.

Bock's research focuses on pathogen population biology, epidemiology, and disease management. He is an authority in phytopathometry, the measurement of plant diseases, with a focus on use of ordinal scales and standard area diagrams (SADs). This research is documented in numerous articles in APS journals (42 articles in Phytopathology, Plant Disease, and Plant Health Progress combined) and elsewhere (an additional 95 articles).

Bock's research program is aimed at alleviating disease impact to pecan, a native nut tree to North America. Scab (caused by Venturia effusa) is the major disease of pecan in the southeastern U.S. Scab-susceptible cultivars are widely grown and fungicide resistance is an issue, with as many as 10-20 fungicide sprays applied each season. Apart from a few landmark studies, there was a lack of foundational knowledge of the pathogen and the disease, and alternative, practical approaches to manage it were wanting. Bock and colleagues demonstrated that scab in tall pecan trees was more severe in the upper canopy of fungicide-treated trees (Plant Disease 97:626), whereas in non-treated trees, scab tended to be more severe low in the canopy. Studies of disease distribution were supported by research on orchard air-blast spray distribution using spray cards and metallic tracers in pecan canopies up to heights of 19 m (Plant Disease 99:916). Spray coverage declined sharply at 12-15 m, a serious limitation of sprayer technology. As a control option in tall pecan trees, research using aerial fungicide application demonstrated that low-volume aerial applications provided excellent control in the upper and mid-canopy (Plant Disease 104:2014). Using air-blast sprayers, some growers favor applying higher spray volumes at slower speeds, believing this improved coverage and scab control. Work by Bock showed that higher spray volumes only resulted in improved coverage low in the canopy but did not improve coverage at heights >12-15 m (Plant Disease 105: 2509). Disease control was equivalent at all heights tested whether fungicides were applied at 470-1870 liter/ha up to the maximum speed tested, 3.2 km/h. Furthermore, lower application volumes and faster speed results in substantial savings for the grower (Plant Disease 105:3909).

Given the height and complex canopy architecture of pecan trees, Bock explored the advantages of mechanical hedge-pruning to better manage scab throughout the tree profile. Hedge-pruning and topping removes the portion of the trees at heights >15 m, ensuring fruit production occurs in the zone of efficacious air-blast applied fungicide sprays (Plant Disease 101:785). Many growers are now hedge-pruning pecan with the primary aim of improving scab management.

The population genetics of V. effusa was entirely uncharacterized, yet that information is critical to assess pathogen reproduction and inoculum sources. Furthermore, V. effusa was characterized as reproducing solely asexually. Bock and colleagues were first to sequence a genome of the scab pathogen and to develop a chromosome-level reference genome, providing valuable resources for future advances. Additionally, he has developed various molecular markers for scab and performed several studies describing the population genetic diversity and population structure of V. effusa in the southeastern U.S. and South America (Plant Disease 98:916; Phytopathology 107:607; Phytopathology 108:1326; Phytopathology 112:2224). The studies indicated that V. effusa underwent regular recombination and had the hallmarks of a sexually reproducing fungus. In further collaborative studies, Bock identified the mating types of V. effusa (Phytopathology 108:837). MAT1-1 and MAT1-2 were in equilibrium in most populations, providing further evidence for sexual reproduction. Recent collaborative work pairing isolates of opposite mating types in vitro identified the hitherto unknown sexual stage, which raises further epidemiological questions about the disease that remain to be addressed. On the host side, Bock collaborated with pecan breeders in characterizing a provenance collection of ~900 trees for susceptibility to scab, identifying areas in the tree's native range that would be most likely to harbor scab resistance (Plant Disease 100:1937).

In the area of phytopathometry, Bock has conducted extensive research on plant disease severity estimation and measurement in fruit and nut trees aimed at improving accuracy of visual estimates. He characterized error in visual estimates of citrus canker severity (Plant Disease 92:430; Plant Disease 93:412) and the limitations to automated image analysis of the disease (Plant Disease 93:660). Further work has demonstrated the value of SADs in various systems, including for estimating pecan scab and citrus canker, as well as factors that can affect their accuracy (Plant Disease 98:1543; Phytopathology 98:1543; Plant Disease 104:2440). Ordinal scales are widely used to estimate disease in fruit crops and other systems. Based on data from citrus canker, Bock applied simulation modeling and hypothesis testing to demonstrate the limitations of using the Horsfall-Barratt scale compared with estimates made directly using the percentage scale (Phytopathology 100:1030). Subsequent collaborative research developed an improved ordinal scale design (Phytopathology 104:575) and provided a fundamental understanding of rater bias in disease estimation as it impacts hypothesis testing (Phytopathology 106:1451). This provides an informed approach for plant pathologists to select or develop disease assessment scales. Although incidence is far easier to collect than severity, the two measures do not always have a straightforward relationship, as documented by Bock for pecan scab (Plant Disease 103:2865).

Bock currently serves in his second term as a Senior Editor for Phytopathology. He has served as a member/Chair/vice-Chair of several APS committees. Bock is a deserving nominee recipient of the Lee M. Hutchins Award based on his groundbreaking research on pecan scab during the past 11 years, much of which has been published in APS journals.