PATHOLOGICAL STUDIES ON POTATO TUBER ROTS

Abstract

Dry and white rots incited by Fusarium solani and Sclerotinia sclerotiorum are responsible to cause the major postharvest losses in potato tubers cv. 'Spunta' during cold storage. Isolation trials from potato tubers showing rot symptoms collected from local markets yielded forty one fungal isolates belonging to six genera. The isolated fungi were purified and identified as Fusarium solani (nine isolates), F. semitectum (seven isolates), Sclerotinia sclerotiorum (six isolates), Alternaria sp. (six isolates), F. oxysporum, (five isolates), Rhizoctonia solani (three isolates), Rhizopus stolonifer (three isolates), and Penicillium sp. (two isolates). Pathogenicity tests showed that F. solani and S. sclerotiorum were the most pathogenic fungi. The inhibitory effect of some mineral salts (calcium carbonate, calcium chloride, sodium chloride and maghnesium chloride ) on the linear growth of F. solani and S. sclerotiorum in vitro showed that the tested mineral salts resulted in different degrees of inhibition to the mycelial growth of the two tested fungi compared with control treatment. This inhibition was gradually increased by increasing the concentration of the tested salts. In addition, calcium chloride completely inhibited the linear growth of Fusarium solani and Sclerotinia sclerotiorum, Calcium chloride was the best in this concern, resulted in 100% inhibition to the mycelial growth of the two tested pathogens at all the tested concentrations. The other mineral salts gave intermediate inhibition.The inhibitory effect of organic acids (boric, oxalic and salicylic acids) on the linear growth of F. solani and S. sclerotiorumin vitro showed that the tested organic acids resulted in different degrees of inhibition to the mycelial growth of the two tested fungi compared with control treatment. This inhibition was gradually increased by increasing the concentration of the tested organic acids. In addition, oxalic acid resulted in 100% inhibition to the mycelial growth of the two tested pathogens at 0.4 % concentration. The other organic acids gave intermediate inhibition. The effect of camphor oil (Eucalyptus glabulus), black seed (Nigella sativa), garlic and spearmint oils against F.solani and S. sclerotiorum, in vitro and in vivo on tubers during cold storage was determined .Growth of Fusarium solani, and Sclerotinia sclerotiorum was completely inhibited by the application of spearmint oil concentrations at 0.3 and 0.4%, in vitro. In vivo results showed also that Nigella sativa preparation was the most effective as it completely suppressed disease severity% caused by Fusarium solani and Sclerotinia sclerotiorum (0.0%) followed by camphor oil during cold storage. Essential oils maintained highest tubers quality, decreased sprouting and reduced weight losses. Generally, treatment with salts, organic acids and essential plant oils gave effective control for the two pathogens and reduced the loss in fresh weight of potato tubers in addition to reducing the sprouting of the tubers during cold storage at 13°C for two months.