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Asian soybean rust (Phakopsora pachyrhizi): quantification for disease progress evaluation in the field.

Presenter: Ing. Agr. Angela Norma Formento, Factores Bióticos y Protección Vegetal. INTA EEA Paraná
Coauthor(s): J. de Souza 1, T. Boca2, and A. del C. Ridao3. 1Fact. Bióticos y Prot. Veg. INTA EEA Paraná (Entre Ríos), 2Inst. de Clima y Agua. CIRN INTA Castelar (Buenos Aires), 3Patologa Vegetal. Fac. de Cs. Agrop. Univ. Nac. de Mar del Plata, Balcarce (Buenos Aires). Argentina

Field quantification of Asian Soybean Rust (ASR) is difficult when accurate and nondestructive measurements of disease progress is desired. In addition, the diagnosis by symptoms is often complicated due to the appearance of other foliar diseases and insect damage. Disease diagrams are extensively used to assess diseases, but this method underestimates low levels of ASR, as those present in early stages of epidemics. In this study, central leaflets of soybean (Glycine max(L.) Merr.) with different ASR levels were collected and direct uredinia (Malupa sojae (P. Hennings) Ono, Butirica & Hennings) were quantified on the abaxial surface of the leaflets. A frame of 1cm was placed at four equidistant leaflet positions and uredinia within the four areas were counted to estimate the average disease measurement. This average value and the leaflet area were used to estimate uredinia per leaflet (i.e., density). Linear regression analysis using actual number of uredinia and estimated number of uredinia (n=45) was carried out. The coefficient of determination (R2) of 0.97 indicated a strong association between both variables. In order to measure ASR, we propose that the direct uredinia counts could be made when densities do not exceed 100 uredinia per leaflet. When densities exceed 100 uredinia per leaflet, the ASR measurement could be estimated by the average of 1 cm2 area method at four leaflet equidistant positions.

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