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Harald Scherm

Harald Scherm was born and raised on a farm near Kulmbach, Germany. He earned an undergraduate degree in agricultural sciences from the Technical University of Munich in 1990 and a Ph.D. degree in plant pathology from the University of California, Davis, in 1994. Following postdoctoral work at Iowa State University, he joined the Department of Plant Pathology at the University of Georgia, Athens, in 1996, where he is currently an associate professor.

Dr. Scherm’s research is on pathogen biology and disease management in fruit crops. A major thrust has been on mummy berry disease of blueberry, caused by the fungus Monilinia vacciniicorymbosi. To improve disease control, he and his students and postdoctoral associates investigated inoculum dynamics and disease development and management in relation to host phenology and environment. In addition, postharvest studies were conducted to improve detection of infected fruit in packinghouses. More basic research focused on interactions between conidia, pollen, and the pistil of open flowers where secondary infection occurs. These investigations have resulted in a series of seven publications in Phytopathology and Plant Disease between 2000 and 2002.

Field observations revealed that many blueberry plantings with a high incidence of fruit mummification at harvest in early summer harbor very low densities of pseudosclerotia on the ground in late fall, suggesting that oversummer survival is surprisingly low. Results of laboratory and field experiments indicated that survival was related primarily to maturity of pseudosclerotia at the time of fruit drop in early summer and was greatest for pseudosclerotia containing mature entostroma. Since the proportion of mature entostroma in infected fruit at fruit drop can vary considerably among southern blueberry cultivars, there may be cultivar-related differences in survival among cultivars that show similar levels of fruit infection. Further research revealed that survival and performance of pseudosclerotia could be reduced by application of fertilizer and commonly used herbicides. Experiments with mechanical cultivation showed that single passes with commonly used tillage implements can reduce apothecial emergence from pseudosclerotia by about 50%. This conclusion was based on results obtained in simulated field conditions and on commercial farms combined with a mathematical risk assessment model. Simulations with this model suggest that more effective risk reductions may be obtained by multiple cultivation passes that result in deep burial of pseudosclerotia combined with shallow tillage near the plant rows where most pseudosclerotia are located. This risk assessment approach provides a novel way of evaluating the effect of tillage practices on sclerotial pathogens.

The timing of apothecium emergence from pseudosclerotia is a critical event in the mummy berry disease cycle. In developing a temperature-based model for apothecium emergence, Dr. Scherm found that only a minimal amount of wintertime chilling was needed for development of viable apothecia, indicating effective adaptation of the pathogen to the low-chill environment of Georgia. There was a negative relationship between the number of chill-hours received and the number of degree-days needed for apothecium production. Thus, pseudosclerotia are well adapted to form apothecia following cold winters (high chill-hours, low degree-days) as well as warm winters (low chill-hours, high degree- days). Based on these results, Dr. Scherm developed a model to predict apothecium emergence by simultaneously monitoring chill-hour and degree-day accumulation during the winter. This model is being evaluated to improve scouting programs and more accurately target management of primary infection.

Knowledge of dispersal patterns of ascospores and conidia is important for assessing the risk of infection from nearby infested blueberry plantings or wild Vaccinium species. Dr. Scherm’s research group recorded gradients of primary and secondary infection from inoculum point sources to understand the contribution to disease spread by different dispersal mechanisms and determine the potential for long-distance spread. As expected, primary infection gradients were longer downwind than upwind, demonstrating the dominant role of wind for ascospore dispersal. However, secondary infection gradients were longer upwind, suggesting that factors other than wind are important in conidial dispersal. This could include dispersal by bees, which have been shown previously to harbor conidia of M. vaccinii-corymbosi. A key role of pollinators in conidial dispersal, along with evidence for upwind foraging preferences of bees, could explain these longer upwind gradients.

M. vaccinii-corymbosi is one of the few fungal plant pathogens capable of infecting open flowers via the gynoecial pathway. Innovative research in Dr. Scherm’s laboratory documented that both flower age and the timing and sequence of inoculation and pollination affect infection. Infection decreased significantly with flower age and by application of pollen at least one day before inoculation. Stigmatic xudates from blueberry flowers was shown to strongly enhance conidial germination. The extent to which conidia–pistil interactions during flower infection mimic processes that occur during pollination and fertilization is currently under investigation.

Mechanical harvesting and sorting procedures do not effectively separate infected from healthy fruit. Because of a near-zero tolerance for mummified fruit, fruit loads harboring even low numbers of pseudosclerotia are appraised at lower quality grades, resulting in severe economic penalties to producers. To improve estimates of disease incidence in fruit loads, a new method to detect and enumerate pseudosclerotia was evaluated. The method consisted of visual symptom assessment of intact fruit using a comprehensive pictorial key describing symptoms. This method was considerably more accurate and precise compared with previous methods. In addition, a sequential sampling plan was derived to calculate the minimum number of samples needed to accurately determine disease incidence with minimum expenditure of time.

Dr. Scherm has served as an associate editor for Phytopathology and Plant Disease and is presently a senior editor of Phytopathology. He has also served as a member of several committees of APS and as a panel member or reviewer for several national and international grant programs. In addition, he currently teaches an undergraduate course in introductory plant pathology and a graduate course in advanced disease management. Dr. Scherm’s productivity, the significance of his publications, and his society contributions well qualify him as this year’s recipient of the Lee M. Hutchins Award.