Evaluation of disease assessment methods in quinoa for their ability to predict yield loss caused by downy mildew

Seven disease assessment methods to measure severity of downy mildew (Peronospora farinosa f.sp. chenopodii) were tested in eight cultivars of quinoa (Chenopodium quinoa) in two field experiments in Peru, one at low (Lima) and one at high (Huancayo) disease pressure. Two levels of downy mildew were established by means of fungicide treatments. Area under disease progress curve (AUDPC) values were calculated for each method and correlated to the grain yield. In addition to the severity measurements, spectral reflectance was measured throughout the growth season in Huancayo and area under reflectance curve (AURC) values calculated and correlated to yield. The disease level was ten-fold higher in Huancayo compared to Lima. AUDPC based on the Three-leaf method (mean disease severity of three leaves per plant selected randomly from the lower, middle and upper part of the plant), showed the highest negative correlation to yield (r = -0.736) and is regarded as the best method to predict yield loss caused by downy mildew. In addition, this method proved to be the quickest and easiest compared to the other six methods included in the study, which were based on assessments of disease severity of the entire plant or parts of the plant. Yield losses recorded in the tested cultivars ranged from 33% to 99% under the high-disease conditions. The reflectance method was superior to all other methods by showing a higher correlation between AURC and yield (r = 0.853), which is explained by its ability to measure pathogen-induced defoliation. Remote sensing is an excellent tool to compare the effect of treatments on disease severity within a cultivar. However, the method does not allow for comparison of disease severity among cultivars as the spectral reflectance is determined by size, shape and colour of the plant. The Three-leaf method provides an easy, cheap and readily applicable tool for researchers and plant breeders to study downy mildew of quinoa, characterise germplasm for resistance and compare results across regions and years. (C) 2003 Elsevier Ltd. All rights reserved.