Zebra chip disease appears to have arisen in north-central Mexico, starting in the mid-1990s and appeared in Texas around 2000. Since then, zebra chip disease has affected potatoes in Arizona, California, Kansas, Nebraska, and New Mexico. Recently, this disease has now appeared in New Zealand. Zebra chip disease is vectored by an insect called the ‘potato (tomato) psyllid’ Bactericera cockerelli (Sulc.). This insect overwinters in Mexico and south Texas, migrating north in the spring and summer where it infests plants along the way. Favored hosts include members of the nightshade family, but most importantly potatoes, tomatoes and peppers.
The putative pathogen responsible for causing zebra chip disease has been identified as “Candidatus Liberibacter”. A very similar pathogen causes the devastating citrus greening disease (or Huanglongbing), and is also vectored by a psyllid. Psyllid control in potatoes is sporadic but often ineffective in preventing significant losses to this disease. Therefore, researchers in many States (and several countries) are working together to better understand this disease and why it has suddenly become a significant concern to potato producers.
Here at Amarillo, researchers are directing their attention at understanding the epidemiology of zebra chip disease, that is, how incidence of this disease in potato fields changes over time and space. We are examining spatial patterns of the disease, how the disease progresses within fields as well as within individual plants, the role of infected seed tubers as sources of infection, temporal changes in pathogen titer levels within plants and effects of temperature, and within-field movement of the potato psyllid.
Initial studies of the spatial pattern zebra chip disease have given us insights into how the disease is distributed in potato fields. It appears that infective psyllids are landing at random locations within the fields and then making short range movements to nearby plants and infecting them. Eventually, large clusters of diseased plants appear as local psyllid populations grow and spread from initial focal locations. We are finding that potato psyllids are capable of considerable range of within-field movement. Initial studies indicate that movement of individual psyllids > 10 m is not uncommon, showing that potentially many plants can be infected by individual psyllids. Although pathogenic psyllids spread disease, feeding by nonpathogenic psyllids significantly reduces marketable yields as well.
Although most seed tubers arising from infected plants will fail to germinate, a substantial portion nevertheless can produce plants that contain the zebra chip pathogen. One concern is that these plants may be sources of infection should nonpathogenic psyllids feed on them, acquire the pathogen, and then spread the pathogen to nearby healthy plants. Initial field surveys indicate that this is not likely; however, laboratory and greenhouse studies are underway to assess this.