RESUMO
A new disease of potatoes, tentatively named zebra chip (ZC) because of the intermittent dark and light symptom pattern in affected tubers which is enhanced by frying, was first found in Mexico in 1994 and in the southwestern United States in 2000. The disease can cause severe economic losses in all market classes of potatoes. The cause of ZC has been elusive, and only recently has been associated with 'Candidatus Liberibacter' sp. Field samples of potato plants were collected from several locations in the United States, Mexico, and Guatemala to determine transmission to potato and tomato by grafting of ZC-infected scions and psyllid feeding. The disease was successfully transmitted, through up to three generations, by sequential top- and side-grafting ZC-infection scions to several potato cultivars and to tomato. The disease was also successfully transmitted to potato and tomato plants in greenhouse experiments by potato psyllids collected from potato plants naturally affected with ZC. Transmission electron microscopic observation of ZC-affected tissues revealed the presence of bacteria-like organisms (BLOs) in the phloem of potato and tomato plants inoculated by grafting and psyllid feeding. The BLOs were morphologically similar in appearance to BLOs associated with other plant diseases. Polymerase chain reaction (PCR) amplified 16S rDNA sequences from samples representing different geographic areas, including the United States, Mexico, and Guatemala, were almost identical to the 16S rDNA of 'Ca. L. solanacearum' previously reported from solanaceous plants in New Zealand and the United States. Two subclades were identified that differed in two single base-pair substitutions. New specific primers along with an innovative rapid PCR were developed. This test allows the detection of the bacteria in less than 90 min. These data confirm the association of 'Ca. L. solanacearum' with potatoes affected by ZC in the United States, Mexico, and Guatemala.
RESUMO
Fusarium dry rot of potato can be caused by several species of Fusarium, but F. sambucinum is considered the primary cause in stored potatoes in North America and Europe (2). Potato tubers of cvs. Shepody and Russet Burbank with severe dry rot were collected from a commercial processing storage facility in central North Dakota during 2003-2004. Pathogen isolations were made from infected tubers on one-half strength acidified potato dextrose agar (APDA). Only F. graminearum was isolated from all rotted tubers used. Identification was based on colony morphology and conidial and perithecial characteristics, which included a carmine coloration of the underside of the agar and white fluffy mycelium on APDA, the presence of black perithecia on carnation leaf agar, and large distinctive macroconidia (1). The identity was confirmed by the Fusarium Research Institute at Pennsylvania State University. Pathogenicity was tested in potato tubers and greenhouse-grown potato plants cv. Atlantic. Nine tubers were wounded by removal of a plug of tissue with a cork borer, 3 mm in diameter and 5 mm deep, and inoculated by placing either 100 µl of a conidial suspension (5 × 104 conidia per ml) from a 7-day-old culture or a mycelial plug, 3 mm in diameter, from a 7-day-old culture in the wound. Nine tubers wounded and treated with either sterile distilled water or one-half strength APDA served as controls. Plant inoculations were performed by cutting a slit in the lower stem with a sterile scalpel and placing a cotton collar saturated with a conidial suspension (5 × 104 conidia per ml) around the wound and held in place with a clothespin. Four plants were inoculated with a conidial suspension, and four plants were treated with sterile distilled water. All tubers inoculated with either Fusarium treatment developed typical potato dry rot symptoms consisting of a brown, dry decay with mycelium lined cavities, and F. graminearum was reisolated from all symptomatic tubers. The control tubers did not develop symptoms. No symptoms developed in any of the greenhouse inoculated plants. Fifteen isolates were tested for sensitivity to thiabendazole, and all were sensitive with EC50 (50% effective concentration) values ranging from 0.8 to 3.7 µl/ml. The results indicate that F. graminearum can cause dry rot of potato, and to our knowledge, this is the first report of F. graminearum as a cause of potato dry rot. These results have epidemiological implications in the persistence, spread, and management of F. graminearum in cereals and potatoes, since potato is often used in rotation with other hosts of F. graminearum, including wheat, barley, and corn. References: (1) P. E. Nelson et al. Pages 118-119 in: Fusarium Species: An Illustrated Manual for Identification. The Pennsylvania State University, University Park and London, 1983. (2) G. A. Secor and B. Salas. Fusarium dry rot and fusarium wilt. Pages 23-25 in: Compendium of Potato Diseases. 2nd ed. W. R. Stevenson, R. Loria, G. D. Franc, and D. P. Weingartner, eds. The American Phytopathological Society, St. Paul, MN, 2001.
