Biology and Management of Black Scurf of Potato

Abstract

Potato (Solanum tuberosum L.), a member of family Solanaceae, is fourth major crop of Pakistan and rates fifth in the world. Three crops of potato are cultivated in a year, in Pakistan, during spring and autumn in plains and summer planting in the upland in eight different potato production agroecological zones. These zones provide excellent conditions for potato cultivation. This crop of high nutritional value still has great potential to enhance its production to save foreign exchange through export and also gets self-sufficiency. Potato is vulnerable to attack of more than 18 diseases caused by fungi, bacteria, viruses and nematodes. Among various diseases of potato caused by fungi, Rhizoctonia canker, commonly called black scurf caused by Rhizoctonia solani Kuhn has been a serious problem in all the potato production agro-ecological zones of Pakistan. Therefore\ the present investigations were initiated to study the "Biology and Management of Black Scurf of Potato" with the following objectives: • to study the incidence and distribution of black scurf disease in potato production agro-ecological zones of Pakistan. • to study the genetic variability of R. solani1 isolates. uring 1997-2000, 525 potato tuber samples were collected from 176 sites scattered in 110 locations of eight potato production agro-ecological zones of Pakistan. It was observed that black scurf is a common disease in all the eight potato production agro-ecological zones of Pakistan with the highest disease prevalence of 99.50% in potato production zone 2, the major potato production zone, comprising of Sahiwal, Okara, Pakpatan, Sialkot, Pasrur and Jhang areas of Punjab followed by 96.66% in zone 1 (Khairpur, Satharja, Tharushah, X xi Ghotki and Shikarpur areas of Sindh province) and 95.23% in zone 8 (Quetta, Mastung, Kalat, Qila Saifullah, Pishin and Ziarat areas of Balochistan province). The lowest disease prevalence of 59.19% was found in zone 7 (Ganche, Baltistan, Gilgit and Chitral areas of Northern Area). Ten localities in Chitral namely Booni, Mastuj, Laspur, Shahidaas, Chinar, Garam Chashma, Roi, Narcoraite, Kajal and Murdan were found disease free. However, the highest disease incidence and severity of 65.55% and 2.95, respectively, were also found in zone 2 followed by 30.35% incidence in zone 4 (Abbottabad, Mansehra, Malakand Agency and Buner valley areas of NWFP) and 2.34 severity in zone 1 (Sindh area). The minimum incidence and severity of 12.81 % and 1.19, respectively, were found in zone 7 (Northern Areas). In individual localities, the highest disease incidence of 87% was found at Sunga Hayat (zone 2), whereas, the lowest disease incidence of 0.5% was found in Hazratabad (zone 7). In individual farms, the highest disease incidence (97%) was found in Pakpatan city. In individual localities, a highest disease severity range of 0-4 was found in Jowar (zone 4), Miandam (zone 5), Hunza proper, Gilgit proper and Bughust Seegain (zone 7). The lowest range of 0-1 was observed in Ghazi (zone 3), Kishvera and Dand (zone 5), Mandik, Surma, Pali Saltoro, Chapurson, Sost, Chitral proper, Bamburet (Kalash valley), Hazratabad and Wahat (zone 7). Locality-wise, the maximum disease serevity of 3.6 was found at Punjab Seed Corporation Farm, Sahiwal and Arifwala, whereas, the minimum disease severity of 0.16 was found in Chitral proper. One hundred and two R. solani isolates were recovered from potato tuber samples and all of them were found multinucleate. Their anastomosis group determination revealed that sixty isolates (58.82%) belong to anastomosis group 3 (AG 3) followed by AG 5 twelve (11.76%) and AG 4 eleven (10.78%). None of the isolate was found member of AG 1-1 B. Frequency of occurrence of AG 3 isolates was the highest in all the potato production zones as compared to other AGs. Maximum number of AGs (10) namely AG 1-1A, AG xii 1-1C, AG 2-2, AG 3, AG 4, AG 5, AG 6, AG 7, AG 8 and AG 81 were found in potato production zone 2. During pathogenicity determination, 100% eyes germination inhibition was caused by SL-41 (AG 3) and SL-60 (AG 1-1) isolates, whereas, collectively, isolates of AG 1-1A caused 71% inhibition in eyes germination. Maximum sprout killing (75%) was caused by CL-58 (AG 3) and SW-9 (AG 4) isolates. Anastomosis group wise, isolates of AG 8 caused maximum sprouts killing (38.50%), whereas, AG 6 isolates caused no effects on potato sprouts. Very severe stem girdling was induced by CL-7 and CL-57 (AG 5) isolates and were found highly virulent isolates. Collectively, AG 2-2 isolates caused severe stem girdling (virulent isolates). Isolates of AG 1-1A, AG 2-1, AG 6 and AG 8 were found avirulent isolates. Isolate CL-7 (AG 5) induced 75% stem canker and exhibited virulent reaction. As an anastomosis group, isolates of AG 1-1A, AG 2-1, AG 3, AG 4, AG 6, AG 7 and AG 8 caused no stem canker. Isolates CL- 57, SW-1, CL-48 (AG 5), CL-53 (AG 4) and. CL-55 (AG 7) caused 25% stolon canker disease index (SCDI). Collectively, isolates of AG 4 and AG 5 caused 30% SCDI, whereas, AG 1-1A, AG 2-1, AG 2-2, AG 6 and AG 8 caused no stolon canker. Hundred percent black scurf disease index (BSDI) was exhibited by isolates CL-21, CL-31, CL-58 (AG 3), JG-7 (AG4), OK-7 and JG-6 (AG 3). Collectively, isolates of AG 4 caused maximum (61%) BSDI, whereas, isolate SW-13 (AG 8) caused no black scurf on tubers of "Desiree" cultivar. Collectively, AG 5 isolates induced all the symptoms of black scurf being studied namely eyes germination inhibition, sprout killing, stem girdling, stem and stolon canker and black scurf. AG 3, AG 4 and AG 5 isolates could not induce stem canker, whereas, AG 2-2 could not develop stolon canker. Isolates of AG 1-1 A and AG 2-1 could not cause stem girdling, stem canker and stolon canker. Isolates of AG 1-1A caused the maximum (71%) inhibition in eyes germination. AG 6 and AG 8 isolates exhibited the least effect on "Desiree" cultivar.