الفهرس 
MATERIALS AND METHODSOnly 14 pages are availabe for public view
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Abstract
Root rot of sugar beet caused by Sclerotium rolfsii is one of the most destructive diseases which cause 15-59% reduction in root yield of different cultivars. The present study was carried out to investigate variability among S. rolfsii isolates, varietal resistance, effect of beet root exudates on radial growth of S. rolfsii and production of secondary metabolites. Role of some biotic and abiotic agents to manage sugar beet root-rot disease was, also studied.
Results obtained throughout the present study can be summarized in the following:
1-. Variability of the ten isolates of S. rolfsii in infecting the roots of sugar beet plants was studied in pot experiment. Results showed that all isolates of S. rolfsii were so virulent and caused damping off as well as root-rot to beet plants.
2- In a pot experiment, the ten isolates of S. rolfsii were differed in their aggressiveness against the susceptible sugar beet cultivar Kwamera. Five out of the ten isolates gave the higher degree of infection at seedling stage. Whereas, estimating the disease severity at mature stage showed that isolates SrK1 and SrK4 (that collected from Kafr El-Sheikh and isolate SrB2 which collected from Beni-Suef) caused the higher levels of disease damage to plant roots.
3- Fungal characteristics of the ten isolates of S. rolfsii in the Lab. illustrated that some isolates were found to be faster growers (three from Kafr El-Sheikh and two from Beni-Suef), where they covered the whole PDA surface (9cm) after 72h from incubation at 27-28°C. These isolates were Srk1, Srk2 and SrK5 that isolated from Kafr El-Sheikh and isolates SrB4 and SrB5 from Beni-Suef. Isolates SrK1, SrK3 & Srk4 from Kafr El-Sheikh and SrB4 & SrB5 from Beni-Suef gave the highest numbers of sclerotia/plate compared with the other isolates. Measuring the diameter of the produced sclerotia revealed significant differences between isolates. Diameter of sclerotia produced by isolates SrK1 and SrK3 averaged 2.1 & 2.0, showing the largest diameters compared to the rest isolates.
4- Sugar beet root exudates were collected at 2, 4 & 8 weeks of planting to study their effect on radial growth of isolates of S. rolfsii on PDA. Fungal isolates were differed in their response of the exudates. The fungal growth was varied according to the exudates taken at the three stages of plant growth. Factors of fungal growth was calculated, accordingly, isolate SrB2 was found to be highly stimulated in response of root exudates (growth factor estimated -76.55 % after one day on PDA). Whereas, SrB4 grew positively (growth factor estimated 14.34 % after one day on PDA).
5- The production of secondary metabolites was determined in the culture filtrates obtained after growing different isolates in PDB. Ferulic acid and cinnamic acid were not detected in any of isolates of S. rolfsii. Whereas, indol acetic acid (IAA) and oxalic acid could be detected in filtrates of all isolates. Meanwhile, gallic acid was found in detectable amounts only in isolates SrK1 & SrK4. Also, clorogenic acid was detected, only in SrK5 & SrB3.
6- To screen sugar beet cultivars for susceptibility and resistance to the infection with S. rolfsii, all possible released sugar beet varieties (20 varieties) were planted in artificially infested field of Sakha. Kwamera variety was shown to be highly sensitive to infection with S. rolfsii, giving about 62 and 50 % infection in two successive seasons. In addition to Kwamera, a number of five other cultivars were shown to be susceptible to infection in the second season of evaluation.
7- To study the possibility of controlling the root-rot of sugar beet by using some bio-agents and humic acid, bioassay of three microorganisms was done against the ten isolates of S. rolfsii. These are Trichoderma viride, Bacillus subtilis and Pseudomonas fluorescens. These microorganisms were studied for their efficiency to control root-rot under field conditions.
8- Testing the three microorganisms for their efficiency in inhibiting the radial growth of S. rolfsii on PDA revealed that T. viride was superior in retarding the growth of all isolates compared with the other two organisms. Isolate SrB5 was shown to be highly sensitive to the effect of T. viride. Whereas, SrK5 isolate was efficiently suppressed by B. subtilis. Meanwhile, P. fluorescens had the potent effect, particularly against SrK5 and SrB3 isolates. Trials were carried out to control root-rot by applying bio-agents in greenhouse and field.
9- Under greenhouse conditions, the three bio-agents as well as 50 % humic acid and vitavax thiram were cause improved of seedling emergence (after 15 days of planting). After 150 days of planting, all treatments caused reduction in the root-rot disease incidence. T. viride gave the best effect in controlling the disease followed by B. subtilis and P. fluorescens, and 50 humic acids came after them.
10- In a field trial carried out in disease nursery established at Sakha Experiment Station, the same trend of controlling root-rot was noticed. Where, T. viride followed by B. subtilis and P. fluorescens was shown to have to potency to reduce infection significantly in the two successive seasons.
11-Fresh weight of the produced roots was significantly increased in case of T. viride treatment compared with the other treatments. While, insignificant increase in root dry weight was observed between treatments.
12-Determination of sucrose and total soluble solids (TSS). Sucrose component was found to be increased in roots of both of healthy and diseased roots in all treatments. The same effect on TSS was noticed in both seasons.
13-Infection of sugar beet plant with S. rolfsii caused an increase in the activity of peroxidase (PO), polyphenoloxidase (PPO) and invertase in the produced roots. Controversial results were observed in roots of treatments under study. Treatment with P. fluorescens caused the higher production of PO and PPO. Whereas, T. viride treatment produced high amounts of invertase in healthy and infected roots.
14- Also, the same infection caused an obvious decrease in the contents of nitrogen in the roots. Except the humic acid treatment, all treatment caused an increase in P and K contents. Percentage of Na was decreased in healthy as well as infected roots of all treatments, except that of P. fluorescens, where it caused clear increase of this element in infected roots.