Use of Area Under the Disease Progress Curve to Assess Resistance to Late Blight in Potato Germplasm, The

American Journal of Potato Research, Mar/Apr 2004 by Haynes, Kathleen G, Weingartner, D Peter

ABSTRACT

The multiple evaluation of potato cultivars and breeding selections (clones) for disease during the season can be costly and may not be necessary for accurate assessments of disease resistance or susceptiblity. For diseases whose progression can be described by sigmoid curves, an estimate of the area under the disease progress curve from two data points may provide as much information as from repeated assessments. Twenty-three clones were planted in a randomized complete block design in Hastings, FL, in 1997 and evaluated for percent diseased foliage 14 times during a 31-day period after late blight was first noticed in the plots. The area under the disease progress curve (AUDPC) was calculated for all 14 assessments. The AUDPC was also calculated for nine sub-sets of the original data set. There was a high correlation between all the methods in the calculation of the AUDPC. The AUDPC calculated based on two dates (representing the beginning of the epidemic and the time until two of the clones were dead from late blight) was as informative as the AUDPC calculated on the entire data set. The AUDPC could be calculated based on any two dates from this time frame as long as one date was shortly after the epidemic started and the other date was as the epidemic was reaching its peak. Much Information was lost if AUDPC was calculated based on dates involved only in the early part of the epidemic. A considerable savings in time and effort can be realized by only a few assessments.

RESUMEN

La evaluacion multiple de cultivares de papa y selecciones en proceso de mejoramiento (clones) durante la Epoca de cultivo puede ser muy costosa y tal vez no necesaria para evaluar resistencia o susceptibilidad a las enfermedades. Para enfermedades cuyo progreso puede ser representado por medio de curvas sigmoidales, el estimado del area debajo de la curva de progreso de la enfermedad en base a dos evaluaciones puede proporcionar tanta informacion como la que se obtiene de evaluaciones multiples. Se sembraron 23 clones en un diseno de bloque completamente al azar en Hastings, Florida en 1997 y se evalue el porcentaje de follaje infectado por 14 veces durante un periode de 31 dias despues de que el tizon tardio fuera observado por primera vez en las parcelas. Para las 14 evaluaciones se calcule el area debajo de la curva de progreso de la enfermedad (AUPDC). El AUDPC tambien fue calculado para nueve sub-conjuntos del conjunto de datos original. Como resultado, se observe que habia una correlacion muy estrecha entre todos los metodos empleados para el calculo del AUDPC. El AUDPC calculado en base a dos fechas (representando el inicio de la epidemia y el tiempo en el que murieron dos de los clones por causa del tizon tardio), fue tan informativo como el AUDPC calculado con todo el conjunto de datos. El AUDPC puede ser calculado en base a cualquiera de dos fechas dentro del periode evaluado siempre y cuando una de las fechas sea tan pronto se inicie la epidemia y la otra cuando la epidemia este alcanzando el maximo. Mucha de la informacion se perdio cuando el AUDPC fue calculado en base a las fechas proximas al inicio de la epidemia. Se puede ahorrar considerable tiempo y esfuerzo cuando se hacen pocas evaluaciones.

Accepted for publication 10 December 2003.

ADDITIONAL KEY WORDS: Phytophthora infestans, Solanum tuberosum, disease resistance, disease trials.

INTRODUCTION

Late blight, caused by Phytophthora infestans Mont, de Bary, is a devastating disease of potato (Stevenson et al. 2001) and the focus of intensive breeding efforts worldwide for resistance, molecular and epidemiological studies, and studies on the biological effectiveness of potential fungicides. Late blight spreads rapidly in susceptible potato clones, and when assessing resistance in the field, multiple disease ratings are often made during the epidemic and the data used to calculate area under the disease progress curve (AUDPC) (Shaner and Finney 1977). The AUDPC expresses the dynamics of an epidemic as a single value and different epidemics can be compared by normalizing the different AUDPCs of each epidemic by calculating the relative area under the disease progress curve (RAUDPC) (Fry 1978; Pscheidt and Stevenson 1986). Most U.S. researchers calculate one or both of these values when assessing late blight resistance in potato cultivars.

Often, however, due to constraints of time, finances, and logistics, it is not possible to make a sufficient number of disease assessments to calculate AUDPC. Also, both the AUDPC and RAUDPC can give misleading epidemiological data because a short fast epidemic can have the same value as a long slow epidemic.

It would be useful, especially for breeding projects, if fewer observations than are typically needed to determine the AUDPC could be used to reliably assess differences in late blight resistance between potato clones. Recent theoretical research based on stem rust of wheat epidemics (Jeger and Viljanen-Rollinson 2001) suggests that for sigmoid disease progress curves an estimate of the AUDPC based on two data points from distinct stages of the epidemic provides as much information as from multiple measurements.