Characterization and Epidemiological Significance of Potato Plants Grown from Seed Tubers Affected by Zebra Chip Disease

Tuberborne diseases of potato, such as Potato virus Y and Potato leafroll virus, present significant problems for potato seed growers and producers alike. An emerging disease of potato, called “Zebra Chip” or “Zebra Complex” (ZC), has recently appeared in the United States and elsewhere and is causing significant losses to individual growers and processors. First identified in northeastern Mexico in 1994 and south Texas in 2000, ZC has been reported from at least eight states within the United States, and was recently reported from New Zealand (7). Plants affected by ZC exhibit a broad range of symptoms that are similar to potato purple top (9–11): stunting, chlorosis, leaf scorching, swollen internodes near apical portions, axillary bud and aerial tuber proliferation, necrosis of vascular system, and early death. The name “Zebra Chip” refers to the characteristic brown discoloration of the vascular ring and medullary ray tissues within the tubers that is enhanced when tubers are sliced and fried into chips. Although the pathogen causing ZC has defied efforts at identification, recent research strongly suggests that the fastidious bacterium ‘Candidatus Liberibacter solanacearum’ is involved in causing ZC (6– 8). It has been demonstrated that ‘Ca. L. solanacearum’ is vectored by the potato psyllid, Bactericera cockerelli Sulc. (Heteroptera: Psyllidae), (10,11) and this insect is widespread within the western and central United States, migrating northward annually during the spring and summer from breeding areas in the southwestern United States and northern Mexico. However, evidence is accumulating that not all B. cockerelli harbor ‘Ca. L. solanacearum’ (15,16). One factor that must be considered in the spread of ZC is the potential role of tuberborne (=seedborne) inoculum. Potato crops are maintained by vegetative propagation of “seed” tubers, a process by which tuberborne diseases present in the propagative material can be systematically transmitted to sprouts and progeny tubers (2,17). Previously, ZC was not reported from potato seed production areas within the United States. However, Wen et al. (20) confirmed the presence of ‘Ca. L. solanacearum’ in certified seed potato tubers produced from Wyoming in 2007 and Nebraska in 2008. In 2008, plants exhibiting aerial symptoms of tuberborne ZC (Fig. 1A) were identified in several commercial potato fields at two widely separated locations in the Texas Panhandle. In contrast to diseased plants that were infected by bacterioliferous adult B. cockerelli (Fig. 1B), plants with tuberborne disease are stunted, often less than 15 cm in height, and the foliage displays a marked ‘witches broom’ appearance; however, a small percentage are apparently nearly asymptomatic (N. C. Gudmestad, personal observations). In addition, symptoms of B. cockerelliinduced ZC symptoms do not normally appear until after flowering, whereas diseased tuberborne plants appear soon after emergence. This source of the infection in early-infected plants is suspected to be the seed tubers themselves and not the result of feeding by ‘Ca. L. solanacearum’– infectious B. cockerelli. It is clear that ZC is spreading to new potato production areas, especially in the western United States, and has potential to develop wherever B. cockerelli is found. Because B. cockerelli has been implicated as the vector associated with ZC, there is concern that noninfectious, abacterioliferous B. cockerelli could acquire the pathogen from ZC-affected tuberborne plants and transmit it to nearby healthy plants, especially in areas where ZC has not been reported previously (20). It has been suggested that ZC-affected tubers rarely sprout (10,15), despite a paucity of data to substantiate that claim. Thus, there is a need to characterize the growth and morphology of plants produced by ZCsymptomatic seed tubers and determine the viability of affected tubers. The objectives of this study were to (i) determine the germination success of ZC-affected tubers and characterize the morphology and growth patterns of plants growing from ZC-affected tubers, (ii) characterize chip discoloration in progeny tubers of plants produced by ZC-symptomatic and ZCasymptomatic seed tubers, (iii) determine ABSTRACT Henne, D. C., Workneh, F., Wen, A., Price, J. A., Pasche, J. S., Gudmestad, N. C., and Rush, C. M. 2010. Characterization and epidemiological significance of potato plants grown from seed tubers affected by Zebra Chip disease. Plant Dis. 94:659-665.