Citrus is one of the most popular fruits in many tropical and sub-tropical countries. Numerous pathogens attack citrus fruits in the field or during harvesting and marketing causing considerable losses. A survey of citrus fruit diseases caused by Alternaria citri was carried out in three districts in Ismaillia Governorate during two years 2016 and 2017. The disease was found in all inspected citrus orchards i.e. Abo-swear, Al-Kassasin and Al-salhia districts during two seasons. The percentage of naturally infected citrus cultivations varied in the different locations as well as in the two seasons. During these two years, percentages of the natural infection ranged from 1% to 11% in citrus cultivations. On the other hand, the highest incidence of the disease was observed in Abo-swear citrus orchards which reached 11% during 2017. Alternaria citri may infect a plant through wounds on the plant surface or through the end of the stem of the fruit at the location called a stylet where natural openings exist.  The internal decay causes the fruits to ripen and drop prematurely. Alternaria rot is a fungal disease that affects mainly navel oranges and lemons. Fruit infected with A. citri change color prematurely. Pathogenicity tests with A. citri were tested on mature citrus fruits. The pathogen recorded the higher infection in navel orange (Length and Width) at Abo-Swear district, followed by summer orange, grapefruits, blood orange, balady orange, succary orange, murrcot, and mandarin. However, lime fruits recorded the lower fruit rot. Studding the host range of A. citri indicate that the higher fruit rot was observed in pear fruits followed by yellow apple, apricot, gawafa , red apple and kiwi fruits .Testing the effect of three fungicides, Rizolex 50% WP, Topsin –M and Nasrzim 50 showed that   in Rizolex 50% WP was the superior one inhibited the linear growth of A. citri at the lower tested concentrations (100 ppm).  Antagonistic study using different bioagents against A. citri in Petri dishes indicate that interaction between Alternaria citri and B. subtilis 48 hrs after incubation showing that the first phase of antagonism occurred as contact followed by abnormal shape of Alternaria conidial spores in the second phase. Mode of action of this antagonism may be include lysis enzymes led to cell disruption.