Potato Zebra Chip Disease: A Phytopathological Tale

Potato zebra chip (ZC) disease is a relative newcomer to the world of important potato diseases. First reported in Mexico in the 1990s, by 2004-2005 the disease was causing serious economic damage in parts of Texas. ZC is now widespread in the south-western and central United States, Mexico, Central America, and was recently reported in New Zealand. By 2006, there seemed to be an association between ZC and the potato psyllid (Bactericera cockerelli). The exact nature of the relationship, however, has only recently been identified by the discovery of a new Candidatus Liberibacter bacterium that is transmitted to potatoes, tomatoes, and other solanaceous hosts by the potato psyllid. This review examines the history of this disease, the association of ZC with the potato psyllid, the host range, and recent research into the bacterial pathogen. Introduction In the midto late 1990s, potato tubers produced in Mexico when sliced open displayed distinct internal brown discoloration. These tubers subsequently developed unacceptable dark stripes and streaks when processed into potato chips. Foliar symptoms on affected plants included chlorosis, leafrolling, purple discolorations, aerial tubers, and leaf scorch. Due to the characteristic tuber symptoms the name “zebra chip” (ZC) was used to describe the disease and this designation has become well established in the literature. By about 2000, similar symptoms were observed in potatoes grown in the lower Rio Grande valley of Texas. Many growers suffered serious economic losses because the tubers were rejected by processors and entire fields were often abandoned. In about 2008, tomato plants were observed in California with an unknown disease and a novel bacterium was shown to be associated with the symptomatic plants. The California researchers subsequently detected this new bacterium in diseased potatoes. The bacterium was also found to be associated with the potato psyllid (Bactericera cockerelli). A similar set of circumstances was also reported in New Zealand greenhouse tomato and pepper crops in 2008 (14), and in 2007 in psyllid-infested greenhouse-grown tomatoes in Arizona (2). This article examines the history of this disease, symptomatology, hosts, distribution, and the recent association of an insect-transmitted bacterium with the ZC disease of potato and diseases of other solanaceous crops in multiple locations worldwide. 17 March 2010 Plant Health Progress Early Descriptions of the Disease and Start of Research In the mid 1990s, an apparently new affliction of potatoes was observed in Mexico and parts of Central America (6,11,30). Foliar symptoms resembled those caused by phytoplasmas, including upward rolling of the leaflets, purple or yellow discoloration, aerial tubers, leaf scorch, and early senescence (Figs. 1, 2, and 3). The fresh tubers showed brown discoloration when cut (Fig. 4). When affected tubers were sliced and fried for potato chips they showed very dark blotches, stripes, or streaks (Fig. 5). Often these streaks coincided with the medullary rays of the tuber. The presence of these characteristic stripes and streaks led to adoption of the common name “zebra chip.” By 2000, these symptoms were observed in US potato fields, particularly in the Pearsall and lower Rio Grande valley areas of Texas, and by 2004-2005 the disease was causing significant economic damage in these areas (see PDF from the Center for North American Studies link ). Fig. 1. Chlorosis and purpling of zebra chip plants at a field trial in Weslaco, TX. Fig. 2. “Purple top” like symptoms of zebra chip infected plant. Fig. 3. Severe chlorosis and scorch of zebra chip infected plant. Fig. 4. Browning of freshly cut tuber of cv. FL1867 affected by zebra chip. These tubers were collected from a commercial field in Texas. Fig. 5. Severe zebra chip symptoms in processed chips. 17 March 2010 Plant Health Progress By 2007, there were reports of similar symptoms being observed in potatoes grown in Nebraska, Colorado, Kansas, New Mexico, Arizona, Nevada, and California (21). Also, in 2006-2007, potatoes grown in Honduras were observed that had foliar and tuber symptoms resembling those of zebra chip. Some fields were severely affected and were total losses because of unmarketable tubers (Fig. 6) (26). In the summer of 2005, Crosslin and Munyaneza were contacted by personnel at FritoLay, Inc. who were seeing the symptoms described above in tubers grown in their Texas production areas. Because the foliar symptoms resembled those caused by phytoplasmas, symptomatic shoot, and tuber samples were tested by conventional nested PCR for phytoplasmas (20) and all were negative. At about the same time, samples from Texas were sent to other plant pathology laboratories and similarly tested for phytoplasmas, with the same negative result (31). These results indicated that ZC differed from a similar potato disease, “purple top,” that was associated with a phytoplasma infection occurring in Texas and Nebraska at about the same time (29). Numerous additional tests for Tomato spotted wilt virus, Tobacco rattle virus, Alfalfa mosaic virus, Potato leafroll virus, Potato mop top virus, Potato virus Y, Xylella fastidiosa, and Serratia marescens were all negative (25). In conjunction with the molecular tests described above, in the fall of 2005 and winter of 2006 symptomatic potato shoots were tip-grafted onto various potato cultivars to determine if there was some infectious agent that could be transmitted to healthy potato. Approximately 4 weeks after grafting, most of the recipient plants began to display symptoms similar to the original plants (Fig. 7) and tubers harvested from the grafted plants showed the characteristic discoloration (5) (Fig. 8). Similar grafting results were obtained by other laboratories (31). These results strongly suggested that a pathogen was involved in the ZC disease, although none had been identified. Fig. 6. Potato field in Honduras during 2009 that is severely affected by zebra chip disease. This field experienced 100% infection early in the season and was a total loss. Fig. 7. Zebra chip symptoms in potato cv. “Atlantic” 60 days after grafting with a zebra chip symptomatic shoot. Fig. 8. Zebra chip symptoms in freshly cut tuber from plant grafted with ZC symptomatic shoot (right). Tuber from a plant grafted with a healthy potato shoot is on the left. 17 March 2010 Plant Health Progress Association of the Potato Psyllid with the Disease Since the symptoms of ZC resembled those of a phytoplasma infection, potential vector insects were collected from affected fields in south Texas beginning in 2005-2006. However, few leafhoppers, the primary vectors of phytoplasmas, were observed. The predominant insect collected at these sites was identified as the potato-tomato psyllid, Bactericera cockerelli (Sulc) (10). The potato psyllid has piercing-sucking mouthparts and the adult is about the same size as a winged aphid (Fig. 9) (Additional photographs of the potato psyllid can be viewed at InsectImages.org.) Many psyllids were tested by PCR for phytoplasmas, usually in groups of five, and only a small percentage of these were positive, providing evidence that the psyllid was not an important vector of phytoplasmas (20,21). Studies were subsequently undertaken where psyllids were introduced into cages containing potato plants and plants were observed for production of ZC symptoms. Most of the infested plants developed ZC foliar symptoms and the tubers also showed the characteristic symptoms (20,21). Later, more extensive cage trials with psyllids derived from numerous sources gave similar results (Fig. 10). However, infestation with psyllids from some sources did not result in ZC-symptomatic plants (18). This result suggested that some populations of psyllids could be free of the as-yet unidentified pathogen. Some “yellows” type symptoms, however, were produced in plants that were infested with populations of psyllids that did not result in production of ZC symptoms. This could be postulated to be due to psyllid feeding itself, as the presence of a toxin associated with the psyllid has been proposed for many decades (3,7,27). Also, psyllid feeding by itself might cause some necrotic symptoms in tubers (28,33). Identification of the Putative Causal Agent, Geographic Distribution, and Additional Hosts In January of 2008, scientists at the Plant Health and Environment Laboratory, MAF Biosecurity New Zealand, began an investigation to determine the etiology of a new disease of glasshouse-grown tomato (Solanum lycopersicum) and pepper (Capsicum annuum) crops (14), unaware of the connection with ZC. All diseases known to affect tomato and pepper were ruled out, including pathogenic fungi, cultureable bacteria, viruses, viroids, and phytoplasmas. The symptoms looked very similar to those caused by phytoplasmas and therefore attempts to find another kind of bacterium-like organism were pursued. The first breakthrough came in April 2008 when transmission electron microscopy of thin sections of leaf tissue from symptomatic tomato plants revealed the presence of phloem-limited bacteriumlike organisms. Various PCR primers were used in different combinations to amplify putative prokaryotic DNA extracted from both healthy and symptomatic plants. In May 2008, one of the primer combinations produced a unique PCR amplicon from symptomatic plants only. Sequence analysis of this fragment revealed that it shared high identity with bacteria related to the Candidatus Liberibacter species, the causal agents of citrus greening (huanglongbing) disease. On 4 June 2008, MAF Biosecurity issued a press release on this new bacterium. The possible link between the tomato and pepper diseases and ZC became apparent when a New Zealand potato breeder contacted MAF Fig. 9. Adult potato psyllids. Photo courtesy Jeremy Buchman. Fig. 10. Cages used to contain or exclude psyllids, Weslaco, TX, 2008. 17 March 2010 Plant Health Pr