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Abstract Potato 1s considered one of the most important export vegetable crop in Egypt. Unfortupate1~ this crop is affected many diseases in the field, storage and during transport. Potato tuber rot caused by different species of Fusarium causes great economie losses during storage, transportation and during the growing season. This study was carried out to : a) determine the causal agents of potato tuber rots, b) to study factors affecting the disease incidence and spread, c) to study parasitic behaviour and chemical changes occurring during this period, and d) disease control methods under laboratory, storage and field conditions. The most important results obtained in this study are summarized as follows : 1. Seven different species of the genus Fusarium, causing potato tuber rots, were isolated and identified. The most prevalent were F. oxysporum and F. sambucinum. The latter is considered as the first record of this species on potatoes in Egypt. Also, l. 1ateritium and l. reticulatum were first recorded on potatoes. - 1’4 2. The most virulent species, causing tuber rots and seed piece decay were F. sambucinum,!. solani and F. avenaceum, while F. reticulatum and F. later.-tium were the least virulent. 3. Potato tuber stem-end was the most sensitive site for Fusarium spp. entrance and infection. While, rose-end and tuber sides were less susceptible. 4. ~. eambucinum was the most virulent causing sprout rot on all varieties Arr~n Banner and Claudia variety were z-ea Lstan t to F. solani, F. avenaceum, F. oxysporum, F. reticulatum and F. latertium, while Arr”an and Grata was resistant to l. semitectwn. On the other hand, Spollnta was the most susceptible variety to all Fusarium spp. 5. During storage infection with F. sambucinum increased gradually at 10-25°C. then decreased at 30°0. o and 35 C. Infection of Alpha and King Edward varieties with F. avenaceum increased at 10-20°0., while, l. solani was more virulent at 30 and 35°0. 6. Potato tuber rots increased consequently till 80 % R.H. and there was no significant differences between 90, 95 and 100 % relative humidity. 7. Slice rot, caused by!. sambucinum, decreased by elongate curing period for all potato varieties. It was found that placing tuber wounded slices in - ”H:;5 - good ventilated areas for 1-4 days gained best results against the infection by F. sambucinum. Potato varieties differed in their sensitivity to infection depending on their ability for wound suberization. Arran Banner and Alpha were resistant, while King Edward and Spounta were susceptible. 8. Degree of maturity has significant effect on potato tuber rots during storage period. Tuber rot increased by increasing tuber maturity. 9. Increasing inspection intervals during storage period increased tuber rots in Nawalla compared with examination one time at the end of storage period. 10. Alpha, Arran Banner and Rosalai varieties were resistant, Claustar, Claudia and Dimont were less resistant, while Grata, Spounta and King Edward were susceptible. On the other hand, Baraka, Clada and Cara were highly susceptible to Fusarium potato tuber rot in Nawalla. 11. Change in growth period affected chemical constituents in tubers tissues Fusarium rotsl a) Increasing growth period caused reduction of total and non-reducing sugars in King Edward variety. Reducing and non-reducing sugars increased susceptibility to tuber rot. Reducing sugars decreased in Alpha tubers by prolonging growth period. It was ·also found that total, reducing and non-reducing sugars increased in tubers inoculated with l·sambucinum except in Alpha tubers 110, 120 and 130 days old when reducing sugars decreased after inoculation. b) Total and free phenols decreased by increasing plant age. Rate of reduction in King Edward was more than in Alpha cultivar. Total and free phenols increased after inoculation with the causal agent. c) Amino acid content was higher in 80 days old tubers than in 90, 100, 110, 120 and 130 days old. Inoculating Alpha tubers increased all amin~acids compared with non-inoculated ones except Alanine, Leucine, and Iso-leucine which were higher in non-inoculated tubers. In King Edward tubers, non-inoculated tubers contained higher amounts of amino acid than in inoculated ones, except Arginine and Aspartic at 80 and 90 days old, Tyrosine at 80 days old and Proline and Valine at all ages. 12. Storage period also affected reaction potato tuber rots : a) Increasing storage period increased sugar contents especially at the end of the storage period. b) Orthodihydroxyphenol contents increased after all storag~ periods after inoculation in both Alpha and King Edward varieties. Rate of accumulation of theiJe phenols was higher in tubers inoculated immediately after harvesting than in tubers inoculated one, two or three months after harvesting. It was also found that Alpha tubers contained higher quantities of orthdihydroxyphenols than King Edward. These phenols decreased by prolonging storage period for both varieties. c) Total, free and conjugated phenols decreased gradually by prolonging storage period. 13. Sugar content decreased by increasing storage temperature and inoculation with the causal agent in both Alpha and King Edward. On the other hand, orthodihydroxyphenols decreased by increasing storage temperature and rate of infection increased in both cultivars. OrLhodihydroxyphenol content was higher in Alpha tubers than in King Edward. 14. Orthodihydroxyphenol contents increased by prolonging curing period in all varieties. It reached maximum in Alpha and Arran Banner varieties, while Clada and Spounta contained the least amounts of the phenols. 15. Inoculated tubers of Alpha variety exhibited higher auxin activities than in King Edward variety at all Rf (a ) values except at Rf (a) 0.7 and 1.0. Inculation decreased auxins activity in Alpha var. at all Rf (8) except Rf 0.6, 0.7 and 0.9. On the other hand, King Edward showed less activities except at 0.1, 0.1 and 1.0. - 168 - Concerning gibberellins, inoc.King Edward gained more activity than Alpha var.except at Rf 0.6. lated tubers, King Edward tubers In inocuwith F. ~bucinum showed higher activity at all Rf values except at Rf (s) 0.6 and, while Alpha was less active after inoculation at 0.1, 0.3 and o. 5 Rf value. 16. Pectinase and cellulase enzymes increased gradually by increasing incubation temperature of cultures till 20°0. then it decreased at 30 and 35°0. for all Fusarium species except F. solani was which polygalacturonase enzymes activity increased at 20. 25 and 30 °c. then decreased at 35 0c. However. o activity of theseenzymes increased at 35 c. when l. oxysporum was used. Concerning!. semi tectum enzymes activity increased by increasing incubation o 0 temperature till 20 c. then decreased at 25 C •• incr- ° eased again at 30 c. then decreased at 35 0C. 17. l. sambucinum showed higher activity in producing pectinase and cellulase enzymes when inoculated tubers were incubated at different temperatures. Activity enzyme increased in inoculated tubers by increasing incubation temperature till 20°C. then decreased at 25, )0 and 35 °C. In non-inoculated tubers. pectinmethylesterase increased till 20°C. then decreased at while polygalacturonase and cellulase increased till o 0 25 c. then decreased at 30 and 35 C. The same results were obtained in Alpha and King ~dward varieties. In general, activity of these enzymes was somewhat higher in King Edward variety than in Alpha inoculated and non-inoculated tubers. 18. Effect of different relative humidities on cellulase and pectinase enzymes in the host inoculated with three Fusarium species it was found that pectinmethylesterase increased by increasing relative humidity till 90 %. This occurred when ~. sambucinum and F. avenaceum were used in inoculation. This enzymes increased till 80 % R.H. when F. solani was used. It was also found that polygalacturonase increased till 95 % R.H. when ”inoculating with F. sambucinum. In case of F. solani and E.avenaceum this enzyme increased till 90 % R.H. But cellulase enzyme increased gradually till 100 % R.H. in the three Fusarium tested. Similar results were obtained in both Alpha and King ~dward varieties but the enzymes activity were higher in King Edward than in Alpha tubers. 19. Phenoloxidase enzyme activity increased in cultural filtrate of l. sambucinum than the other two species, F. solani and !.. avenaceum. While, enzyme activity was nearly similar in the mycelium of the three species. 20. Peroxidase enz~]e activity in culture filterate of F. sambucinum was higher than in F. solani and l. avenaceum. Concerning the mycelium, the enzyme activity was similar in the cultural filtrate of the three species. 21. Rate of increase of phenoloxidase activity was higher in Alpha tubers than in other tested varieties. Spounta and Clada were the least ones. Peroxidase activity reached its maximum in Claudia variety after 2 minutes, While it reached its highest activity in Arran Banner after 3-4 minutes and after 5 minutes in Alpha variety. 22. It was found that Tecto (thiabendazole), as a systemic fungicide, reduced rots of potato tubers artificially inoculated with F. sambucinum, using least concentration of active ingredients. Using high concentrations of this product did not cause phytotoxicity on tuber germination. On the other hand, high concentrations of chlorotha1onil or Vitavax/ Captan caused toxicity on tuber germination. 23. Thiabendazole (Tecto) can pentrate the tuber periderm at distance of 16 and 20 Mm. inside the tuber tissues. However, this fungicide prevented spore germination of l. sambucinum upto 98 % at s110es ~ . to 5 depth after 6 days of tuber treatment with 100 ppm. 24. Pectolytic and cellulolytic activities were inhibited by using Tecto, Benlate, Vitavax/Captan and Chlorothalonil at 1, 25, 50 and 100 ppm was the most effective in reducing enz~nes activity. Enzymes activity reducing reached the maximum at the time of adding fungicides to the medium. F. sambucinum cultures were used in this experiment. 25. Using a mixture of Tecto or VitavaxlCaptan (fungicide) and 10 % Sevin (insecticide) dusting at 1 : 1 (1~25 kg/ton tubers) reduced potato tuber rots. 26. Using VitavaxfThiram, Orthocide-8J, Terrazol, Terrachlor and Agrimycin 500, increased average percent of tuber storage rots when adding thoroughly after digging more than when using after two weeks of harvesting. Vitavax/Captan and Benlate fungicides decreased tuber rots in the field under artificial and natural conditions. |