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Location: Subtropical Plant Pathology Research

Title: Prediction of Phyllosticta citricarpa using an hourly infection model and validation with prevalence data from South Africa and Australia

item MAGAREY, R - North Carolina State University
item HONG, S - North Carolina State University
item FOURIE, P - Citrus Research International (CRI)
item HOLTZ, T - Animal And Plant Health Inspection Service (APHIS)
item FOWLER, G - Animal And Plant Health Inspection Service (APHIS)
item TAKEHUCHI, Y - North Carolina State University
item CHRISTIE, D - North Carolina State University
item MILES, A - University Of Queensland
item SCHUTTE, T - Citrus Research International (CRI)
item Gottwald, Timothy

Submitted to: Crop Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/15/2015
Publication Date: 5/19/2015
Publication URL:
Citation: Magarey, R.D., Hong, S., Fourie, P.H., Holtz, T., Fowler, G., Takehuchi, Y., Christie, D., Miles, A., Schutte, T., Gottwald, T.R. 2015. Prediction of Phyllosticta citricarpa using an hourly infection model and validation with prevalence data from South Africa and Australia. Crop Protection. 75(2015):104-114.

Interpretive Summary: Citrus black spot, caused by a fungal pathogen can be a severe disease causing losses in both yield and quality. The presence of the disease in Florida as well as Brazil and South Africa has resulted in citrus fruit trade barriers especially with European markets. The concern is that fruit could convey the pathogen to areas not currently infected and the resulting epidemic could cause yield and quality losses in these new locations. This paper examines the mathematical model to predict the production of spores produced by the disease causing fungal pathogen under various climatic conditions. Using weather data from citrus producing locations around the world the model can be used to predict climate suitability for the disease to reproduce and eventually establish in new areas. For example the model demonstrates that fruit from infected countries would not cause the establishment of the disease in the majority of places in Europe where citrus is grown. This is because the climate in European citrus growing areas in insufficient for fungal spore production and infection. The powerful new model/tool is intended for use by regulatory agencies in exporting and importing countries to assess the risk of importing fruit from infected countries.

Technical Abstract: A simple 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 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 89% 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. 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. This finding also suggests that if citrus fruits are a pathway for introduction of P. citricarpa then only a few isolated locations in the extreme south of Europe are likely to have a low to marginal risk.