Use of digital images to differentiate reactions of collections of yellow starthistle (Centaurea solstitialis) to infection by Puccinia jaceae

Yellow starthistle (YST) (Centaurea solstitialis) is an invasive weed in the USA and a target of classical biological control. However, the precise geographical origin(s) of YST in the USA is not known. This study was conducted to learn whether the origin(s) of YST in California, USA could be determined on the basis of digital image analysis (DIA) of YST reactions to isolates of the rust Puccinia jaceae. One trial was conducted with two isolates of P. jaceae and another trial with only one of these isolates. Six different YST collections were tested. The DIA consistently detected and measured rust pustules on YST leaves. Compared to visual assessment, DIA was a fair predictor of the number of rust pustules per leaf. There were no differences between isolates of P. jaceae in reactions of different YST collections in the first trial. Differences in plant reactions between trials were predominately due to more disease incidence and severity in the second trial, which was probably from the greater amount of inoculum. Based on analyses of variance and canonical discriminative analyses, YST from California appeared most closely related to the YST collection from Erzurum, Turkey. In examination of squared Mahalanobis distances, all collections, except the collection from Turkey, were significantly distant from the California collection in both trials. More studies are necessary to determine the precise area(s) of origin (within Turkey and neighboring countries) of YST in the USA. Digital image analysis of rust reactions can be a powerful tool to do this.