الفهرس 
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Abstract
Aquaculture remains one of the fastest-growing food-producing sectors and is set to play a key role in meeting the rising demand for fishery products that consider one of the cheapest sources of protein, Nile tilapia is the most economically important Freshwater Fish in Abbassa Farms and the most popular in Egypt.
A total number of 430 fish O. niloticus were obtained from Abbassa fish farms during the period from September 2014 to August 2015. These fishes were subjected to clinical, P.M, mycological and histopathological examinations.
The isolated fungi from infected fishes were (Saprolegnia spp. Cladosporium sp., chrysosporium sp., Scopulariopsisbrevicaulis, Aspergillusversicolor, Aspergillusochraceus, Aspergillus niger, Aspergillus glaucus, Aspergillus wentii, Rhodotorulla sp.,Penicillium sp., Syncephalastrum sp.,Aspergillus tamarii., Aspergillus flavus, Trichoderma sp., Acremonium sp., Rhizopus sp., Fusariumsp., Phoma sp.,Absidia sp., Aspergilluscandidus, Alternaria sp., Aspergillus terreus, Aspergillus fumigatus, Paecilomyces sp.).
The total prevalence of the total fungal infection was (30.4%) among the examined O.niloticus fish.
The prevalence rates of different fungi in infected fishes, were (18.3%), (12.2%), (10.6%), (9.9%), (9.1%) for Penicillium sp., A, flavus, A. tamarii, Rhodotorula sp., A. niger respectively followed by (7.6%) for both of A. versicolor sp. and Trichoderma sp. and Saprolegnia sp. followed by (6.8%) for both of Cladosporium sp. and Rhizopus sp. followed by (6.1%), (5.3%) for Syncephalastrum sp., Fusarium sp. respectively, followed by (4.5%) for both of chrysosporium sp., A. wentii, Absidia sp., A. terreus and Saprolegnia sp. Followed by (3.8%), (3%) and (2.29%) for both of A. ochraceus, Phoma sp. and Scopulariopsis brevicaulis, followed by (1.5%) for both of A. glaucus and Alternaria sp., followed finally by the lowest prevalence rate of infection rate (0.7%) for both of Acremonium sp., A. candidus, A. fumigatus and Paecilomyces farinosus.
The seasonal prevalence of fungal infection showed that, Summer season had the highest infection rate among examined fishes (46.6%), while the lowest infection rate was (17.1%) in spring season.
Regarding the prevalence of various fungi in different organs cleared that, the infection percentages were (44%), (26.4%), (25.6%) and (4%) from gills, skin& fin, internal organs, and lesion respectively. But by considering lesion belonging to skin, the ratio would be (44%), (30.4%) and (25.6%) from gills, skin& fin and internal organs respectively.
The clinical signs and postmortem findings in the infected fishes were recorded as follow:
• Fish infected with A. flavus showed scales loss and ulceration of the skin. The post mortem lesion was yellow liver.
• Fish infected with A. niger, showed emaciation and the post mortem lesions were congestion in the internal organs beside enlargement in the spleen and gall bladder.
• The most clinical sign which appeared on infected fishes with Cladosporium sp. was severe erosion and ulceration of the skin and the post mortem lesions were black liver and the gall bladder was enlarged.
• Fish infected with Saprolegnia spp. showed epidermal desquamation, sloughing and erosion especially around caudal and ventral fins. Areas of necrosis and hemorrhages were rarely detected in few cases suffering emaciation.
Sensitivity tests with disc diffusion can be used in fungal infections caused by molds while media incorporated with naturally antifungal plants as Garlic or Clove in different forms (oil, powder and grain). where raw Garlic was efficient in Absidia sp. only but Garlic powder was efficient in Aspergillus flavus, Aspergillus ochraceus,Fusarium sp., Rhizopus sp. and Absidia sp.While in Garlic oil gave negative effect for all tested fungi; but in case of Clove grain was efficient in Fusarium sp., Rhizopus sp. and Absidia sp. Also Clove powder was efficient in Fusarium sp., Rhizopus sp. and Absidia sp., but Clove oil gave negative effect for all tested fungi. Others that are tested against fungal infections with antifungal drugs as Clotrimazole, Nystatin and Fluconazole, were efficient for all tested fungi, the efficiency characterized by forming clear zone means inhibition of fungal growth; Clotrimazole mark the highest efficiency on A. ochraceus; Fluconazole mark the highest efficiency on each of Fusarium and Rhizopus; while Nystatin mark the highest efficiency on Absidia sp.
Most histopathological alterations in infected fishes were changes of the epithelial covering of the secondary lamella and most brachial blood vessels showed congestion in gills, vacular degeneration in liver, depletion in the blood components used in activation of melanomacrophage centers in spleen, condensation of some gromeruli and edema was encountered in Bowman´s capsule in kidney, complete sloughing of the epidermis together with infiltration with melanin covering cell in fins.
Factors influence mycotic infection spread of fishes including surrounding factors like temperature as the lower temperature increases the virulence of the organism, the change in environmental temperature increases the rate of transmission of fungal disease between fishes, and the effect of elevated ammonia and carbon dioxide levels combined with low dissolved oxygen concentration that increase the mortality rate. Over Crowding and bad handling result in significantly high mortalities.