Detached-Leaf Bioassay for Evaluating Taro Resistance to Phytophthora colocasiae

Taro leaf blight disease, caused by Phytophthora colocasiae, is a major limiting factor in taro production worldwide. P. colocasiae is an aerial pathogen similar to P. infestans, causal agent of potato late blight disease, but occurs in warmer climates. In the year-round subsistence cropping systems of the Pacific Islands, resistant cultivars are essential. Breeding lines from Southeast Asia and Oceania were tested in American Samoa for resistance to taro leaf blight using a detached-leaf bioassay and field trials. Mean lesion diameters from bioassays were highly correlated with field estimates of the number of healthy leaves per plant and yield (corm weight). However, the bioassay did not adequately assess infection efficiency. Additional experiments revealed that attached leaves had smaller lesion diameters than detached leaves incubated in closed containers, but both were very highly correlated. Taro resistance increased with plant age and the second-oldest leaf was more resistant than the third-oldest leaf. The bioassay was a fast, space-saving, effective method of screening taro lines for post-penetration resistance to P. colocasiae. It also provided an easily standardized method of evaluating host-pathogen interactions under controlled conditions.