Common Scab of Potato as Affected by Soil Mineral Elements and Tuber Chemical Constituents

During 2009/2010 growing season, a survey was conducted in three regions where potato was intensively cultivated in three different governorates. These were ElNubaria, El-Salheya and El-Sheikh Zaied in El-Behera, ElSharkia and El-Ismailia governorates respectively. The causal common scab isolates of Streptomyces scabies recovered were isolated from potato tubers showing different scab symptoms and these isolates were all monomorphic for the morphological, biochemical and physiological traits tested. However, two types of necrosis were developed in the pathogenicity test with the tested S. scabies. These were phenotype 1 which characterized with little brown flecks, and phenotype 2 which characterized with brown black necrosis. The study revealed that incidence and severity of common scab differed among the three surveyed regions of the three governorates with the highest incidence in El-Nubaria region (60.0 %) followed by El-Salheya (52.5 %) and El-Sheikh Zaied (22.5 %). The same trend was observed for the common scab disease severity with highest severity in El-Nubaria (25.8 %), followed by El-Salheya (22.8 %) and El-Sheikh Zaied (11.4 %). Severity of the scab was found to be correlated with the soil mineral elements and the tuber biochemical constituents. A positive and highly correlation was revealed in the three surveyed regions between soil calcium concentration and the potato scab severity (r = 0.949 – 0.941). The macronutrients, i.e. nitrogen and phosphorus,