Biology and Host Pathogen Interaction in Powdery Scab of Potato in Pakistan

Abstract

Powdery scab of potato is caused by Spongospra subterranean f. sp. Subterranean. In Pakistan disease was first reported in 1997 and then in 1998 by Turkestan from isolated pockets of Attock, Okara, Sahiwal, and from certain parts of Kalat division. Later in 1996, CDRI’s scientist confirmed the disease from astal valley in northern area on the basis of microscopy, bioassay and serology. As pathogens presence was confirmed inpotato growing soil and due to its seed and soil born nature, risk of pathogen transmission from contaminated to uncontaminated areas, is there. therefore, soil probing of other potato growing areas of Pakistan was done on priority basis. Majority of the soil samples collected from potato fields of hilly areas including sixteen location (Nomal, Naltar, Thol, Nagar, Nasirabad, Murtazabad, Aliabad, Karimabad, Shiskat, Gulmit, Passu, Khyber, Galapan, ,Orkhon, Sus Tog Gilgit and Hunza valley; seven location (Astak, Chunda, Shigar, Harikun, Khaplu, Surmoo And Saltaro) of Baltistan; 12 locations (Khad Kucha, Mangochar, Kalat, Khanozai, Kan Metherzai, Zarghoon, Kanozai, Zarahd, Chana, Karvi Kuch, Ziarat, Quetta, and Urhak) of Balochistan 6 locations ( Lawari Top, Gram Chashma, Bamboret, Buni , Torkhu and Madaklast) of Chitral; 13 locations ( Bela, Poludran, Batal, Naran, Sahoch, Sohni, Pehli Batakundi, Batakundi, Upper Batakundi, Khor, Jaba, Dabook, Batakundi Farm) of Kaghan and plain areas including seven location ( Losar, Bathar, Morgh, Kaloo Kalan, Hassan Abdal, Khaki Doraha, Bajna-Mansera and Jano Mandi-Shinkiari) of NWFP and 25 locations ( Salarwal, NIBGE Potato Farm, Tahirabad, Raio Bagh, Salara Pindh, Rasool Nagar, Chunu Da Thatta, Nawaz da Khooi, Gogaara bangla, Parokian Da Kot Chak-41, 51-21, Okara, 40-D, Shoba Ram, Mazhar Abad, Hujra Shah, Muqeem, Khoo Dolia Da 46-GD, Adda Khoria Shah 52-GD, Ratti Thibbi 6-R, Jahan Khan Joinah Da 29 SP- Chak, Kalar, Kalak Shah Kako, Khana Marrian Da and Daska of seven districts ( Fasilabad, Jhang, Okara, Sahiwal, Pakpatan, Lahore-Sheikhupura and Sialkot-Gujranwala) of Punjab showed infestation with S. subterranean. Presence of pathogen was confirmed through a sensitive technique of bioassay by using tomato bait plants. Another technique of ELISA was also standardized for detection of pathogen from soil and tubers. To make the ELISA test more economical, polyclonal antisera against spore of balls local isolate od S. subterranean wqas prepared by immunizing a local breed rabbit. To eliminate the non-specific reaction of host tssues, antiserum was cross absorbed with sap of healthy potato tuber. IgG was purified through ammonium sulphate precipitation and its concentration was adjusted to 1mg/ml. Previously very little was known about the pathogen. Therefore the aim of the current project is based on to understand the biology of the pathogens and its interaction with Host, so as to develop the basis of sound strategies for the management of powdery scab in Pakistan. Spongospra subterranean is an obligate parasite and its survives in soil for long period in the form of cystosori or resting spores. These biflagellate unicelled zoospores swim through soil and infect roots and tubers of the host. The infection process of the pathogens was studied in the roots of potato bait plants, where plasmodial and zoosporangia formation were observed after 10 days of incubation and baiting period; infection was as in some other members of plasmodiophorales. Pathogens produce symptoms on roots in the form of galls and powdery pustules on the surface of the tubers containing mass of resting spores. Recent studies under controlled and natural conditions prevailed the typical type of scab symptoms on tubers, spores were isolated from the infected tubers and observed under the light microscope at 40x and electron microscope; they showed the typical honey comb, spongy structure of resting spores (cystosori) of S. subterranean. Disease development was studied by using different potato varieties (Cardial, Desiree, Agria) in infested soil at NARC’s micro-plot. Sab lesion on tubers and galls on roots of Cardial variety were observed during autumn 1997-98. When trial was repeated for reconfirmation at the same mocro plot with fresh infested soil, symptoms were not developed. It was concluded from different finding of two successive years that disease development is associated with suitable climate conditions (cool and heavy rainfall), which differ in these years. Conclusion was also supported by the findings of another trial, conducted at micro plot with infected mother tubers. Effect of irrigation on disease development was studied and found that the continuous irrigation provides favourable condition for disease development as compared to dry-wet condition. This indicated that the risk od f pathogens transmission through contaminated seeds from hilly seed producing areas ( having cool and wet condition) to plain areas (having warm and dry condition) is there but the establishment of pathogen depends merely on availability of suitable climatic condition ( may be extensive irrigation ) during crop season. Spongospra subterranean is an obligate parasite; it grows and multiplies only on its best and can’t be cultured on any media. Due to this reason it was difficult to get the contaminated free inoculum of S. subterranean for laboratory work. Therefore a bioassay technique by using TPs ( true potato seeds ) and potato shoots tips, was standardized to obtained pure inoculum of resting spores (cystosori) of S. subterranean for laboratory experiment like bioassay, serology(ELISA) and PCR(polymerase chain reaction). One of the strategies to manage the powdery scab is to minimize the inoculum level in the soil. For such purpose trap (decoy) crops plays an important role in reducing the soil borne inoculum before planting potatoes. Therefore the knowledge of such crops and the mode of infection in these hosts were gained through bioassay technique. Number of solanaceous crops like maize, oat, sorghum, soybean, tobacco, Datura and pea were found the hosts of S. subterranean. All of them were found as trap crops because after infection of zoospores, only zoosporangia were formed in their roots without formation of resting spores.