Two intercropping patterns of onion (ON) with sugar beet (SB) namely (ON ridge: ridge SB) and (side: side "ON+SB ridge: ridge SB alone") were carried out to investigate their effect on severity of four foliar diseases. The target diseases were downy mildew and purple blotch of onion, as well as Cercospora leaf spot and rust of sugar beet under natural infection under field conditions at Damietta Governorate in 2016/17 and 2017/18. Also, the advantage of the two intercropping patterns on yield production of both tested crops, as well as land equivalent ratio (LER) was evaluated. The two intercropping patternssignificantly decreased disease severity of both downy mildew and purple blotch of onion, as well as, Cercospora leaf spot and rust diseases of sugar beet compared to monoculture cultivation of each crop in the two seasons. Intercropping patternsof onion: sugar beet, i.e. (1 ridge ON: 3 ridges SB) and (1ON: 1SB alternately) were the most superior in decreasing disease severity of both onion downy mildew and purple blotch diseases. However, intercropping pattern(3 ridges ON:1 ridge SB) and (1ON: 1SB alternately) were the most effective patterns for decreasing both Cercospora leaf spot and rust diseases of sugar beet in the two growing seasons. On the other hand, intercropping patternsonion (ON) with sugar beet arranged by side to side, i.e. (1 ridge ON+SB: 3 ridges SB alone) and (2 ridges ON+SB: 2 ridges SB alone) were the best patterns for decreasing both onion foliar diseases, meanwhile, intercropping patterns(4 ridges ON+SB: no ridges with SB alone) and (3 ridges ON+SB: 1 ridge SB alone were the most superior ones for reducing severity of  bothCercospora leaf spot and rust diseases during the two seasons. The highest severity of the four tested foliar diseases of the two different intercropping patterns was recorded when either onion or sugar beet was cultivated alone in both seasons. On the other side, most treatments of the two intercropping patterns maximized yield production of onion bulb and sugar beet root yield, and increased land equivalent ratio (LER) compared to each crop cultivated alone with few exceptions. Ridge: ridge pattern of (1 ON ridge:1 SB ridge alternately) was the best one for maximizing either onion bulb or sugar beet root yields. However, the patterns of (side: side) were (3 ridges ON+SB: 1 ridge SB alone) and (1 ridge ON+SB: 3 ridges SB alone) were the best for increasing onion bulb yield and root yield of sugar beet, respectively, in both seasons. The results concluded that onion and sugar beet intercropping could be beneficial in reducing foliar diseases and increasing yield of both crops with a maximized LER and economic return for the farmer compared to each crop cultivated alone.