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The tar spot complex of maize, induced by the fungi Phyllachora maydis and Monographella maydis , is a disease of economic importance in Mexico and Central America because it causes severe yield losses. Given the growing importance, standardized methods are required to quantify disease that allow for reproducible epidemiological methods. The evaluation considered ten evaluators and 50 plants with different levels of severity, determined by the Image Tool 3.
The precision and accuracy was determined by a simple linear regression analysis between actual and estimated severity. High levels of accuracy were observed; r 2 was comprised between the range of 0.
Key words: : Phyllachora maydis , Monographella maydis , Zea mays. The incidence of TSC causes severe yield losses and deteriorates the quality of the fodder; furthermore, it has the potential to entirely destroy parcels of land Pereyda et al.
In the field, it has been observed that the maize is first infected by P. Subsequently, a necrotic diameter is formed around each one of the lesions of P.
From all the organisms associated with M. Under favorable conditions, the foliage of the plant may completely dry out in less than eight days due to the fusion of the lesions of the two pathogenic organisms and due to the possible production of a toxin Hock et al.
Given the increasing importance of the Tar Spot Complex, standardized methods for the quantification of the disease are required, methods that allow the conduction of accurate, precise, and reproducible epidemiological studies; understanding 'accuracy' as the proximity that an estimated value has to the real value, 'precision' as the variation or repeatability associated with an estimation, and 'reproducibility' as the absence of change in the estimations when several evaluators quantify the same characteristics Nascimiento et al.
Among the methods of measurement for the intensity of a disease, the use of diagrammatic logarithmic scales is the most widely used. These scales consist in the representation of a series of plants or parts of plants that show the symptoms of a disease in varying degrees of severity Nascimiento et al. Despite the increasing importance of TSC in maize and the resulting need to carry out epidemiological or control studies, there is no standardized quantification method of the disease that provides easily reproducible results among investigators or institutions; therefore, the objective of this investigation was to design and validate a diagrammatic logarithmic severity scale for the maize - Phyllachora maydis and Monographella maydis - pathosystem that allows the evaluation of the damage caused by the disease in fields, analyzing the accuracy, precision and reproducible values generated with its use.
Design of the diagrammatic logarithmic scale. The methodology proposed by Mora et al. The total foliar area and the total diseased area of each digitalized image was determined through the Image Tool 3. The logarithmic scale was generated through the 2-Log V1. Validation of the diagrammatic scale. A sequence of 50 photographs of leaves and maize plants with different TSC severity levels was used, previously determining the affected area of each one of them.
The severity of each image was evaluated by ten evaluators with the help of the scale, five of them with experience in the quantification of disease severity and five without direct experience in the phytopathological area.
The accuracy and precision of each evaluator was determined through a simple analysis of linear regression as described by Nutter et al. The accuracy of the tests was estimated with the coefficient of determination r 2 of the same linear regression and with the absolute deviation of the error. The accuracy, represented by the degree of proximity of the estimated values to the real values Nutter et al. The values of the b 0 intercept were closer to zero for the first group.
The value of the b 0 intercept was positive in almost all cases, which indicates that there is a tendency to overestimate the severity of the disease; this tendency was greater for the evaluators without experience. Nascimiento et al. According to Sherwood et al. The overestimation of the severity levels using logarithmic scales has been common in several pathosystem.
This has been reported, among others, by Barbosa et al. The precision reproducibility or variation associated with an estimation , estimated by the coefficient of determination r 2 and by the variability of the absolute errors Nutter et al.
The precision levels observed indicate that the first contact with the scale is appropriate for successful implementation in the evaluation of TSC. In regards to this, Michereff et al. The precision values observed in the evaluators without experience are acceptable; prior training could have a positive influence on the quality of the evaluations. This has been demonstrated for other pathosystems: Barbosa et al. The same author indicates that there was no significant improvement for the experienced evaluators.
The reproducibility of the scale was high using the same scale for the assessment of the material, given different evaluators estimated similar severity percentages.
The linear regression of the severities estimated by the evaluators produced coefficients of determination that varied within the range of 65 to 91 Table 1. Therefore, the use of the diagrammatic scale for the evaluation of TSC generates a high concurrence with the real severity values, reflected in a high precision among the evaluators.
Hock et al. This was due to the complicated quantification system and to the elevated number of classes with which it was developed; furthermore, it considered the symptoms caused by both infectious agents separately. In this study, the symptoms of P. Finally, the use of standardized systems for the quantification of maize diseases, and those of any other crop, proves to be a powerful tool as it allows the perfect comparison of experiments carried out by different institutions and investigators.
The diagrammatic logarithmic scale for the TSC proposed in this study represents a standardized quantification method for the disease that will help obtain easily reproducible results, due to its high levels of accuracy and precision among several evaluators, therefore, it can be used as supporting material for different kinds of studies related to the quantification of the severity of TSC.
The authors express their appreciation to all evaluators of the Colegio de Postgraduados that participated in the validation stages of the logarithmic scale.
Sections -. Agrociencia 43 5 Revista Fitotecnia Mexicana 36 1 Tests of standard diagrams for field use in assessing the tarspot disease complex of maize Zea mays Tropical Pest Management, Tests of standard diagrams for field use in assessing the tarspot disease complex of maize Zea mays. Tropical Pest Management Summa Phytopathologica Mora et al. Nutter et al. Disease assessment terms and concepts. Plant Disease Table 1 Estimate of the interception parameters b0 , slope b1 , and coefficient of determination r2 of the simple linear regression analysis calculated between the real and estimated severity of TSC, carried out by 10 evaluators with and without experience in the quantification of diseases.
Sherwood et al. Illusions in visual assessment of Stagonospora leaf spot of orchardgrass. Phytopathology 73 2 : Elaboration and validation of diagrammatic key to evaluate white rust severity in Chrysanthemum Summa Phytopathologica, Elaboration and validation of diagrammatic key to evaluate white rust severity in Chrysanthemum. Summa Phytopathologica 34 4 Development and validation of diagrammatic key for Cercospora leaf spot of sweet pepper Summa Phytopathologica, Development and validation of diagrammatic key for Cercospora leaf spot of sweet pepper.
Summa Phytopathologica 32 3 Barbosa et al. Share this article Bookmarks.
Revista mexicana de fitopatología
More significant damage to leaves and yield is caused by the fungus Monographella maydis whose infec More significant damage to leaves and yield is caused by the fungus Monographella maydis whose infection follows that of the tar-spot fungus, at least where studied in Mexico Hock et al. The source of initial inoculum for both fungi is not determined. The disease they cause occurs in the cooler and higher elevations of Mexico, and Central and South America, and the West Indies, so their ability to spread over land through other climatic zones may be limited. Not known to be seedborne or to infect other species, P.
Infeccion del maiz por el "complejo mancha de asfalto" 
Sequential development of pathogens in the maize tarspot disease complex
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