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United States Department of Agriculture

Agricultural Research Service

Research Project: EMERGING DISEASES OF CITRUS, VEGETABLES, AND ORNAMENTALS

Location: Subtropical Plant Pathology Research

Title: Response to the recently published article “Potential distribution of citrus black spot in the United States based on climatic conditions"

Authors
item Graham, James -
item Gottwald, Timothy
item Timmer, Lavern -
item Filho, Armando -
item Van Den Bosch, Frank -
item Irey, Michael -
item Taylor, Earl
item Magarey, Roger -
item Takeuchi, Yu -

Submitted to: European Journal of Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 7, 2014
Publication Date: June 1, 2014
Citation: Graham, J.H., Gottwald, T.R., Timmer, L.W., Filho, A.B., Van Den Bosch, F., Irey, M.S., Taylor, E.L., Magarey, R.D., Takeuchi, Y. 2014. Response to the recently published article “Potential distribution of citrus black spot in the United States based on climatic conditions". European Journal of Plant Pathology. 139(2).

Interpretive Summary: The manuscript is a ‘Letter to the Editor’ of the European Journal of Plant Pathology concerning a recently published manuscript by Er et al. 2013. Our Letter to the Editor manuscript is a rebuttal to the findings of Er et al. The original Er et al. paper discussed a model which predicted the distribution and establishment of citrus black spot in citrus producing areas the United States and has implications for its distribution in Europe as well. It is our contention that the model was based on incomplete and faulty data and an incorrectly parameterized model and therefore invalid. In their paper they interpret their data to indicate that citrus black spot could exist in California as well as in European citrus producing areas. All prior published modeling data and information contradicts this strongly. Because the Er et al. paper was based on an incomplete data set and utilized improperly parameterized climate variables, the output is not only incorrect but has far-reaching implications to worldwide trade of fresh citrus fruit. For instance, the European Food Safety Agency might well utilize the single paper to justify harsher import trade requirements or out and out embargoes against citrus from areas where citrus black spot are presently known to exist as well as areas that the Er et al. paper predict it could exist. This would be highly damaging to international citrus trading partners in the US, Brazil, Argentina, Uruguay, and South Africa. Therefore it was critically necessary to rebut their findings.

Technical Abstract: The publication heretofore referred to as Er et al. (Er et al. 2013) conflicts with worldwide observations that citrus black spot (CBS) caused by Phyllosticta citricarpa occurs exclusively in climates with summer rainfall, i.e., high temperatures accompanied by high moisture levels. Surprisingly, Er et al. predict that dry Mediterranean climatic conditions, i.e., Southern California, where CBS does not exist are favorable for establishment; while conditions in humid, subtropical South Florida where CBS does occur are not favorable. To create this prediction, Er et al. used CLIMEX to model short-term (i.e., seasonal) conditions suitable for infection by setting parameter values that differ between so-called summer and winter conditions. We believe the use of multiple CLIMEX parameter sets for an organism based on seasonality has no justification or precedence. Rather, CLIMEX is used with a single set of parameters for an organism which are displayed as an annual score predicting establishment or as a weekly score to predict population fluctuations during the year. Er et al.’s chosen winter values, to represent P. citricarpa survival in leaf litter, were derived from in vitro experiments on colony growth, and production and germination of pycnidiospores while ignoring feasibility of pseudothecia and ascospore production under the conditions presented. The contradictory results reported by Er et al. are also due to the failure to use parameters in the CLIMEX model based on a combination of field experiments or observations under environmental conditions and published laboratory observations where the pest is known to occur or not to occur, for model validation. This result is clearly incorrect and in contrast to previous models (Paul et al. 2005; Magarey et al. 2011; Yonow et al. 2013). To accept the findings of Er et el. 2013, one must consider CBS to be more likely to establish in a Mediterranean climate (such as California), than a more tropical climate (such as Florida). This is clearly nonsensical and counter to all that is known about the conditions required for CBS to establish. The erroneous application of the CLIMEX technique by Er et al. (2013) makes this output null and void.” If promulgated, these erroneous results could potentially harm entire citrus industries and international trade.

Last Modified: 10/23/2014
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