Survey of Frequency of some Pathogens Associated with Potato Purple Top Disease

Several pathogens, including fungal, viral, and prokaryotic agents, can cause disease in potatoes. Purple top disease associates with the formation of air glands, stubborn, leaf rolling, in potato fields is visible and several pathogens has reported reported to be the cause of this disease. Potato leaf roll virus, purple top wilt Phytoplasma and Rhizoctonia solani fungi are the most important agents. Plants in response to pathogen infection, show a wide range of symptoms. Symptoms of purple or purple leaves usually indicate contamination of the host plant with phytoplasmic agents.

Symptoms of purple top wilt, stubborn, yellowing and leaf rolling, witches broom, big bud and formation of aerial tubers were observed in potato fields of Chaharmal Va Bakhtiari, Hamedan and Fars provinces during 2006-2011. Some samples also showed symptoms of root rot and stem canker, Wilting accompanied by stubborn, color change of the yellow leaves or purple, leaf rolling and formation of aerial tubers at the base of the plant near the surface of the soil or in the lateral buds of the base of the petioles and of course, it was different in different cultivars. Symptoms of disease in the Marfona variety, mostly include the formation of aerial tubers and yellow leaves, and in the most of plants stolon canker was also seen. In Agria, Diamont, Maradona, Sante, Raja, Navita, and Kozima varieties, symptoms of purple top accompanied formation of aerial tubers in some plants were stubborn the witches broom and wilt. In some other cultivars, symptoms of witches broom, rosette and hairy of the roots were observed. Symptoms of disease also weeds in the margins of potato fields. Symptoms of witches broom, phyllody, stubborn and rosette in Dandelion (Taraxacum officinalis), Salsify (Tragopogon graminifolius), Black night shade (Solanum nigrum), Wild raddish (Raphanus raphanistrum), hare's ear mustard (Conringia orientalis) and witches broom and stubborn in alfalfa were observed. Scales of symptoms varied in different regions depending on the region and sampling time. Symptoms of purple top and formation of aerial tubers at the end of the season. Yellowing symptoms and the formation of aerial tubers in the Navita cultivar in the late growing season has reached to 40% in some areas.

460 samples tested by ELISA against PLRV, PCR (using P1/P7 followed R16F2n/R16R2 universal primers) for presence of phytoplasma and cultured on PDA for isolation of fungi caused root rot and canker. Investigation showed presence of phytoplasmas, fungi and PLRV with abundance of 136, 245 and 61(28.7, 53.25 and 13.2 %) respectively. Most of the symptoms caused by phytoplasmas and Rhizoctonia have a symptom of purple top and aerial tubers. Association of PLRV in some cases produced symptoms of leaf roll and purpling. Aerial tuber formation and purple top symptoms in different potato cultivars were seen in various ratio, while fungal and phytoplasmas agent were main casual agents in Marfona and Navita cultivars, respectively. From the samples of potato with roots and stems of rot and canker symptoms, fungi Rhizoctonia solani, Fusarium solani, F.oxysporom and Colletotrichum sp. isolated that 32.9% of the infection was related to Rhizoctonia and 20.3% was related to other fungi. Inoculation of these fungi in greenhouse conditions caused potato plants to cause plant death and drying out, and no symptoms of purple top or aerial tuber formation were observed. It seems that occurrence of such symptoms is possible only in farm conditions. Plants that showed purple top, stubborn, and rolling of the end leaves were infected with PLRV, the symptoms of the virus are varied in many varieties. In some plants, disease is accompanied by yellowing and degeneration and in some leaf roll and purple top were observed. In no case, the symptoms of the aerial tuber formation in plants that were infected with the virus were not observed. In transfer with dodder and grafting to tomatoes and potatoes, samples with stem canker and root rot did not cause any symptoms in healthy plants. But those examples were no symptoms of purple top and aerial tuber in the plant that did not show any symptoms of stolon and rooting of root canker and in the ELISA test, they did not respond positively to PLRV, tomatoes and potatoes were transplanted in greenhouse conditions with dodder and grafting. Potato plants have shown graft symptoms of yellow, stubborn, leaf roll and purple top in greenhouse conditions, but the formation of the aerial tuber was not observed in greenhouse conditions. After dormancy the tubers from plants with symptoms of purple top showed only a small number of these tuber showing symptoms of hairy sprouting. This condition can only be infected in some tubers from an infected plant due to the occurrence of these symptoms but the selection of plants in terms of symptoms and the uncertainty of infection with phytoplasma can be a reason. These two pathogen group caused purple top and aerial tuber formation in other potato cultivars in different region with various ratio. The PCR products of R16F2n/R16R2 from phytoplasma detected in potato, alfalfa and weeds samples were used in restriction fragment length polymorphism, RFLP, by digestion with AluI, HaeIII, HhaI, RsaI, KpnI, MseI and Hae III restriction enzymes. Results indicated presences of two distinct phytoplasma group in potato samples (16SrI-B and 16SrXII-A) and 16SrXII-A phytoplasma from Alfalfa samples, 16SrI-C from salsify (Tragopogon graminifolius) and 16SrVI-A from here's ear cabbage (Conringia orientalis). Contamination of different agents were seen also in some samples. This is the first report of presence of phytoplasma group 16SrI-B and 16SrXII-A in potato in Iran.

Despite the similarities between the symptoms of purple top and the formation of air glands in potato fields, the disease is caused by phytoplasmic agents, PLRV, and rhizoctonia fungi, in its management, the cause of the disease should be identified.