@article{ojiambo_gent_mehra_christie_magarey_2017, title={Focus expansion and stability of the spread parameter estimate of the power law model for dispersal gradients}, volume={5}, ISSN={["2167-8359"]}, url={http://europepmc.org/abstract/med/28649473}, DOI={10.7717/peerj.3465}, abstractNote={Empirical and mechanistic modeling indicate that pathogens transmitted via aerially dispersed inoculum follow a power law, resulting in dispersive epidemic waves. The spread parameter (b) of the power law model, which is an indicator of the distance of the epidemic wave front from an initial focus per unit time, has been found to be approximately 2 for several animal and plant diseases over a wide range of spatial scales under conditions favorable for disease spread. Although disease spread and epidemic expansion can be influenced by several factors, the stability of the parameter b over multiple epidemic years has not been determined. Additionally, the size of the initial epidemic area is expected to be strongly related to the final epidemic extent for epidemics, but the stability of this relationship is also not well established. Here, empirical data of cucurbit downy mildew epidemics collected from 2008 to 2014 were analyzed using a spatio-temporal model of disease spread that incorporates logistic growth in time with a power law function for dispersal. Final epidemic extent ranged from 4.16 ×108 km2 in 2012 to 6.44 ×108 km2 in 2009. Current epidemic extent became significantly associated (P < 0.0332; 0.56 < R2 < 0.99) with final epidemic area beginning near the end of April, with the association increasing monotonically to 1.0 by the end of the epidemic season in July. The position of the epidemic wave-front became exponentially more distant with time, and epidemic velocity increased linearly with distance. Slopes from the temporal and spatial regression models varied with about a 2.5-fold range across epidemic years. Estimates of b varied substantially ranging from 1.51 to 4.16 across epidemic years. We observed a significant b ×time (or distance) interaction (P < 0.05) for epidemic years where data were well described by the power law model. These results suggest that the spread parameter b may not be stable over multiple epidemic years. However, b ≈ 2 may be considered the lower limit of the distance traveled by epidemic wave-fronts for aerially transmitted pathogens that follow a power law dispersal function.}, journal={PEERJ}, author={Ojiambo, Peter S. and Gent, David H. and Mehra, Lucky K. and Christie, David and Magarey, Roger}, year={2017}, month={Jun} }
 @article{cruz_magarey_christie_fowler_fernandes_bockus_valent_stack_2016, title={Climate Suitability for Magnaporthe oryzae Triticum Pathotype in the United States}, volume={100}, ISSN={["1943-7692"]}, DOI={10.1094/pdis-09-15-1006-re}, abstractNote={ Wheat blast, caused by the Triticum pathotype of Magnaporthe oryzae, is an emerging disease considered to be a limiting factor to wheat production in various countries. Given the importance of wheat blast as a high-consequence plant disease, weather-based infection models were used to estimate the probabilities of M. oryzae Triticum establishment and wheat blast outbreaks in the United States. The models identified significant disease risk in some areas. With the threshold levels used, the models predicted that the climate was adequate for maintaining M. oryzae Triticum populations in 40% of winter wheat production areas of the United States. Disease outbreak threshold levels were only reached in 25% of the country. In Louisiana, Mississippi, and Florida, the probability of years suitable for outbreaks was greater than 70%. The models generated in this study should provide the foundation for more advanced models in the future, and the results reported could be used to prioritize research efforts regarding the biology of M. oryzae Triticum and the epidemiology of the wheat blast disease. }, number={10}, journal={PLANT DISEASE}, author={Cruz, Christian D. and Magarey, Roger D. and Christie, David N. and Fowler, Glenn A. and Fernandes, Jose M. and Bockus, William W. and Valent, Barbara and Stack, James P.}, year={2016}, month={Oct}, pages={1979–1987} }
 @article{magarey_hong_fourie_christie_miles_schutte_gottwald_2015, title={Prediction of Phyllosticta citricarpa using an hourly infection model and validation with prevalence data from South Africa and Australia}, volume={75}, ISSN={["1873-6904"]}, DOI={10.1016/j.cropro.2015.05.016}, abstractNote={An hourly infection model was used for a risk assessment of citrus black spot (CBS) caused by Phyllosticta citricarpa. The infection model contained a temperature-moisture response function and also included functions to simulate ascospore release and dispersal of pycnidiospores. A validation data set of 18 locations from South Africa and Australia was developed based on locations with known citrus black spot prevalence. An additional 67 sites from Europe and the United States with unknown prevalence were also identified. The model was run for each location with 9 years of hourly weather data from the National Centers for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSR) database. The infection scores for the sites with known prevalence where ranked and a threshold for suitability in a given year was derived from the average score of the lowest ranked moderate prevalence site. The results of the simulation confirm that locations in Florida were high risk while most locations in California and Europe were not at risk. The European location with the highest risk score was Andravida, Greece which had 67% of years suitable for ascosporic infection but only 11% of years were suitable for pycnidiosporic infection. There were six other sites in Europe that had frequency of years suitable for ascosporic infection greater than 22% including Pontecagnano, Italy; Kekrya, Greece; Reggio Calabria, Italy; Cozzo Spadaro, Italy; Messina, Italy; and Siracusa, Italy. Of these six sites only Reggio Calabria had a frequency of years suitable for pycnidiosporic infection greater than 0%. These six sites are predicted to have prevalence similar or less than Messina, South Africa, i.e. low and occasional. Other sites in Europe would best be described as likely to have no prevalence based on very low simulated scores for both spore types. Although Andravida had a similar risk of infection to moderate locations in South Africa there was a difference in the seasonality of infection periods. The ascosporic infection period score was similar between the two sites, but Andravida had a much lower pycnidiosporic infection score in the middle of the period of fruit susceptibility than Addo, South Africa. In Europe favorable climatic conditions are discontinuous, i.e., there is a low frequency of suitable seasons. This raises doubts about the ability of the pathogen to persist at a location and cause disease loss when favorable seasons reoccur. These results suggest that Europe is less suitable for CBS than suggested by an earlier study produced by the European Food Safety Authority using a similar model. The findings from our model simulations suggest that only a few isolated locations in the extreme south of Europe are likely to have a low to marginal risk of P. citricarpa establishment.}, journal={CROP PROTECTION}, publisher={Elsevier BV}, author={Magarey, Roger D. and Hong, Seung Cheon and Fourie, Paul H. and Christie, David N. and Miles, Andrew K. and Schutte, Gerhardus C. and Gottwald, Timothy R.}, year={2015}, month={Sep}, pages={104–114} }