Susceptibility of Different Potato Plant Growth Stages to Purple Top Disease

Since 2002, potato growers in the Columbia Basin of Washington and Oregon have experienced outbreaks of potato purple top disease that have caused significant yield losses and reductions in tuber quality. It was determined that the beet leafhopper-transmitted virescence agent (BLTVA) phytoplasma is the causal agent of the disease in the Columbia Basin and that this pathogen is transmitted by the beet leafhopper. Little is known about the impact of purple top disease on potato in the Pacific Northwest and effective management strategies for the disease are lacking. Trials were conducted in 2006, 2007, and 2008 under laboratory and field conditions to assess susceptibility of different plant growth stages of selected potato cultivars to purple top. Ranger Russet and Umatilla Russet plants of different plant growth stages were exposed to BLTVA-infective leafhoppers in the laboratory and transferred to outdoor field cages. In a second study, Ranger Russet, Umatilla Russet, Russet Burbank, and Russet Norkotah plants were exposed to naturally occurring BLTVA-infective leafhoppers in the field by uncovering portions of caged rows of potatoes at desired intervals beginning at plant emergence. Plants were monitored for purple top symptoms and tested for BLTVA by PCR to confirm infection. Purple top foliar symptoms were observed in all the tested cultivars under both laboratory and field conditions. Results from both the laboratory and field experiments indicated that younger plants were more susceptible to purple top than older ones. In the laboratory trial, disease incidence was 87.5, 70, and 18.4% for Umatilla Russet, at 10 days, 25 days, and 50 days after plant emergence, respectively. Similarly, 65%, 52.3%, and 14.3% of the Ranger Russet plants became infected with purple top after being exposed to beet leafhoppers at 10 days, 25 days, and 50 days after emergence, respectively. In the field trial, disease incidence was relatively high in potato plants exposed to leafhoppers during the first 5 weeks to 6 weeks after plant emergence and the infection declined thereafter. Statistical analysis of laboratory and field collected data indicated that there was a strong correlation between purple top infection and plant growth stage in all potato cultivars tested. Information from the present study will help potato growers prevent damage caused by purple top disease by appropriately protecting susceptible plant growth stages against the beet leafhopper.