Submitted to: Plant Disease
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/8/2005
Publication Date: 12/8/2005
Citation: Secor, G.A., Lee, I., Bottner, K.D., Rivera-Varas, V., Gudmestad, N.C. 2006. First report of a defect of processing potatoes in texas and nebraska associated with a new phytoplasma. Plant Disease. 90:377. Interpretive Summary: Phytoplasmas are very small bacteria that lack a cell wall and that cause several hundred economically important diseases in plants worldwide. Potato purple top and similar diseases have caused tremendous damage to potato tuber production in South America, Mexico, and the US. In 2002 and 2003, a major epidemic of potato purple top occurred in Washington and Oregon, causing great economic damage to the potato industry. A phytoplasma belonging to clover proliferation group was the causal agent. In 2004 and 2005, a new potato disease with symptoms similar to purple top disease occurred in Texas and Nebraska, causing patchy brown discoloration of chips produced from commercial processing potatoes. The defect can be a cause for rejection of contracted potatoes by the processor. We have made an extensive survey of the affected plants and identified a new phytoplasma that is only distantly related to the stolbur phytoplasma. This the first report of this phytoplasma associated with disease and defects of potato, and the first report of this phytoplasma in the US. The information will aid implementation of quarantine regulation and it will help extension workers and plant diagnoticians to determine how to combat the disease.
Technical Abstract: An outbreak of a new potato disease occurred in Texas and Nebraska causing a serious defect in potato chips produced from commercial processing potatoes. The defect consists of patchy brown discoloration of chips and can be a cause for rejection of contracted potatoes by the processor. Infected potato plants exhibit symptoms of the purple top wilt syndrome similar to those of the purple top disease in processing potatoes caused by clover proliferation phytoplasma recently found in Washington and Oregon. Foliar symptoms include stunting, chlorosis, slight purple coloration of new growth, swollen nodes, broken axillary buds, and aerial tubers. Tuber symptoms include mild vascular discoloration and brown flecking of medullary ray. Seed potatoes from affected plants produce hair sprouts. Total nucleic acid was extracted from leaf veinal tissue and stolons of symptomatic plants in the field and from tuber samples exhibiting the defect from commercial storages. Nested polymerase chain reactions (PCR) were performed using phytoplasma-universal 16SrDNA-based primers (P1/P7 and R16F2n/R16R2) to detect the presence of phytoplasmas in these samples. In 2004, 13 foliar samples tested positive for phytoplasmas with PCR. None of the apparently symptomless plants or tubers tested positive. Restriction fragment length polymorphism (RFLP) analysis of the PCR-amplified 16S rDNA using enzymes AluI, MseI, HhaI, BfaI, and Tsp509I indicated that four samples are associated with a phytoplasma belonging to subgroup A (16SrI-A) of the ‘Candidatus Phytoplasma asteris’ (aster yellows phytoplasma) group (16SrI) and nine plant samples were associated with a new phytoplasma related to, but distinct from, the stolbur phytoplasma group (16SrXII). Nucleotide sequence analysis of cloned 16S rDNAs confirmed the results based on RFLP analyses. The new phytoplasma is only distantly related to the stolbur phytoplasma, sharing 96.6% sequence homology. In 2005, 14 defective tuber samples from storage and 16 symptomatic plants from the field tested positive for the same phytoplasma. This the first report of this phytoplasma associated with disease and defects of potato, and the first report of this phytoplasma in the US.