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Abstract This work was aimed to study the f ac tors influencing the severity of root rot disease of peanut and method of control. The causal organisms were isolated from different lOCalities in A.R.E., identified and the patnogenicity was determined. The susceptability of different peanut varieties to the disease was evaluated. The effect of organic and inorganic manuring and ~hizobi~ inoculation on plant growth, disease severity and nittogen fixation was determined. The effect of different systemic and non-systemic fungicides on growth of pathogens, plant growth, nitrogen fixation and disease severity was investigated. Moreover, the effect of these fungicides on the soil and rhizoshpere microflora Was studied. The results might be summarized as follows: 1. The fungi lihizoctonia sOlani (Kuhn), Sclerotium rolfsii, Rhizoctoni.a bataticola (Taub.) Butl., F-usarium 0x’y’sporum(Sohlech tendahl) and .EgsaritynBolani (Martius) were pathogenic to peanut causing root rot disease. li. solgni was superior in this respect than §. rolfsii while the other fungi were of low importance. 2. The local tested varieties were susceptible to infeotion”with root rot disease organisms but Giza (3) va.riety was less suceptible to infection as compar ed with the other tested varieties. 3. Soil infestation with lie solan~ increased root rot disease and decreased the numbers, dry weight, size of nodules and total nitrogen content and dry weight of plants. 4. Inoculation with Rhizobium c~re resulted in a decrease in the percentage of disease peanut plants. It increased the numbers, dry weight and size of nodules, also dry weight of plants and total nitrogen content pet plant. Organic manure gave the same results. 5. It Was found that nitrogen fertilization increased the inQidence of disease, numbers, dry weight, size of nodules, dry weight of plants and total nitrogen content .. 6. Soil infestation with g,. solani and .2• .t:0lfsii alone or combined increased the percentage of pre-, postemergence damping-off and root rot disease. However, it decreased the nitrogen content. It also reduced the number, fresh and dry weight of nodules and fresh and dry weight of roots and shoots. 7. Inoculation with Rhizobiu-m.-. in soil infested with the pathogens resulted in a decrease in the pre-, poatemergence damping-off and root rot disease percentages. While it increased the numbers, fresh and dry weight of nodules. It also increased the fresh and dry weight of plants and total nitrogen content. 8. Counts of total microbial flora in the soil were lower than in the rhizosphere. Rhizosphere/Soil ratio (R/S ratio) was high in the vegetative growth period. Fungicidal treatment decreased the total microbial flora density. 9. Actinomycetes counts were low at the beginning of the experiment but increased as the plant grew up. ActinomYcetes counts in the soil were higher than those obtained in the rhizosphere except in the case of the un-inoculated soil treated with fungicides where higher count was observed. R!S ratios for this grouP were negative and also quite law. Fungicides differ~d in their effectiveness on actinomycetes eouat. 10. The count of fungi was higher in the control than in rhizosphere soil treated with fungicides especiallY in the vegetative stage. At the flowering and maturity stages a low count of fungi was observed. 11. The fungicides Bavistin and vitavaxlCaptan were the most effective fungicideS in inhibiting the growth of lie solani while vitavaxlCaptan and vitavaxlThiram were the most effective in inh1biting tne growth of -S. .rolfeii. The other fungicides inhibited the growth of the pathogens at varying degrees. 12. Sclerotial formation of R-. -801-an-i waS completely inhibited at low concentrations of Benlate and Brassicol. Sclerotia of -S. -ro-lfsii were not formed at 5 ppm of Vitavax/ Captan and 10 ppm of Vitavax/Thiram. 13. Treating seeds with fungicides reduced the percentage of pre- and post-emergence damping-off and root rot disease in the field. The most effective fungicides were found to be Benl.a te , Vitavax/Th1ram and Vitavax/Captant respectively. 14. Tre ating soil with fungic ides reduced the percentages of pre-, post-emergence damping-off and root rot disease. Brassicol-75 and Orthocide-75 were the most effective. |