(prepared by Neil C. Gudmestad and Gary A. Secor, North Dakota State Univ.)
A recent disease of potatoes called “zebra chip” (ZC), so named for the characteristic symptoms that develop in potato tubers from infected plants, has been documented by the research group at North Dakota State University (NDSU) to occur in commercial potato production fields. The disease was first documented in potato fields around Saltillo, Mexico in 1994, and was first identified in the USA in the year 2000 in commercial potato fields in the Pearsall, Texas, production area. Since that time ZC has spread to a number of other states, including Nebraska, Colorado, Kansas, New Mexico and California. Zebra chip was sporadically important economically until the 2004 and 2005 growing seasons, when it caused major losses to potato producers and chip processors in some locations.
Zebra chip continues to be economically important in the fresh and processing potato producing areas of Mexico, particularly in the states of Coahuila and Nuevo Leon in northeastern Mexico, where it is called “papa manchada” (stained potato). In 2004, ZC incidence in this region was as high as 80% in some fields. This area borders the winter potato production area in Texas where ZC was found in the USA. The disease is also serious in potato production areas of Guatemala where it is named “papa rayada” (striped potato), and causes a serious problem of market potatoes, subsistence gardens, and processed potatoes. A complete survey to determine the geographical extent of the disease has not been conducted but there is anecdotal evidence that a disease of potato with similar symptoms exists in Eastern Europe and Southern Russia.
Zebra chip-affected potato plants exhibit a range of foliar symptoms that resemble those of the purple top wilt syndrome caused by the clover proliferation phytoplasma carried by potato psyllids (see Insects, Vectors, Potato Psyllids). Foliar symptoms include stunting, chlorosis, swollen nodes causing a “zig-zag” appearance of the upper growth, proliferated axillary buds, aerial tubers, browning of the vascular system in below ground portions of stems, and leaf scorching (Figure 1). Zebra Chip-affected plants are usually scattered throughout the field. In some instances plants affected by ZC appear to coalesce in areas of the field with clusters of collapsing plants that die early (Figure 2).
Below ground plant symptoms include enlarged lenticels of the underground stem, collapsed stolons, and brown discoloration of the vascular ring and necrotic flecking of internal tuber tissues (Figure 3) and occasionally streaking of the medullary ray tissues (Figure 4). These symptoms are similar to, but distinct from, symptoms of net necrosis in potato tubers caused by the phloem-restricted potato leaf roll virus. These symptoms affect the entire tuber from the stem end to the bud end. It is these tuber symptoms that differentiate ZC from all previously described potato diseases. Potato chips made from tubers of affected plants have a severe dark brown streaking defect, hence the name ‘zebra chip’ (Figure 5).
Tubers affected with ZC generally do not sprout, or if they do, produce hair sprouts or weak plants. In some instances, tubers with ZC symptoms produce nearly healthy appearing plants that produce progeny tubers with or without ZC symptoms. It is not known currently if this secondary ‘seed’ spread of ZC is due to a late infection by the pathogen or some other factor, but the role of seed borne infections of ZC needs to be investigated further. Symptoms of ZC have been observed in many cultivars, none appear to be resistant.
Causes and Carriers
Based on recent research (early 2000s), Zebra chip is likely a disease complex involving at least two distinct pathogens and no less than two insect vectors. There are a number of plant diseases in which similar symptoms are caused by two distinctly different pathogens, such as ‘spraing’ of potato in the United Kingdom and marginal necrosis of strawberry and ‘bassess richessess’ in sugar beet in France. While this type of disease complex is unusual, it is not unique to ZC.