ارزیابی انتقال بیماری های فایتوپلاسمایی از طریق غده های بذری سیب زمینی

Potato purple top and witches’ broom are two destructive phytoplasmal diseases in potato growing areas of the world and Iran. These diseases with symptoms including stunting, purpling, witches’ broom and aerial tubers cause considerable losses in potato farming. The latest surveys have shown that Ca. P. solani and Ca. P. trifolii are prevalent in potato producing regions of Iran. The potato phytoplasmas are transmitted by leafhoppers and seed tubers; however, the rate of phytoplasma transmission through potato seed tubers in different generation has remained unclear. In order to investigate transmission process of potato phytoplasmas through seed tubers, symptomatic potato plants with their daughter tubers were collected from potato growing fields of Isfahan and Chaharmahal-o-bakhtyari. DNA extracted from each plant subjected to nested-PCR reaction with universal phytoplasma primer pairs R16F2n/R16R2. Of the 73 samples tested, a DNA fragment of approximately 1239bp in size was amplified in 47 samples. Concerning to specific identification of phytoplasmas associated with potato, two specific primer pairs were designed for diagnosis of Ca. P. solani and Ca. P. trifolii. In PCR reactions with the specific primer pairs, 35 samples were identified as Ca. P. solani, and three samples were determined to belong to Ca. P. trifolii. Also, nine samples were found mixed infected with both phytoplasmas. All the results obtained were confirmed by PCR-RFLP and nucleotide sequencing analyses. To determine the transmission rate of the phytoplasmas via potato seeds, all daughter tubers of the 47 infected samples were grown in greenhouse. After tuber developments, DNA was extracted from the samples to use in nested-PCR assays for detection of the exact phytoplasmas in the plants. According to the results, 25 were found infected by Ca. P. solani, but none of them amplified the expected fragment for Ca. P. trifolii. Based on the results, the rate of phytoplasma transmission through the seeds was estimated about 22% in second generation of potato plants. High presence of Ca. P. solani in second generation plants raise the notion that Ca. P. solani is more transmissible to progenies. In order to evaluate the rate of potato phytoplasma tuber transmission in third generation, phytoplasma infected tubers of the second generations were planted in greenhouse. The procedure for assessment of disease symptoms and detection of phytoplasma from the plants by nested PCR were done as described previously. According to the results, Ca. P. solani was detected only in five of the 54 samples examined (9%) revealing that the transmission rate of potato phytoplasmas through seed tubers decreases continuously from the first generation to the third; however, this low level of transmission can also be important as a source of phytoplasma inoculums for dissemination of the disease by leafhoppers.