Contact Dermatitis: This is often the most common cause of penis shaft erythema and itching, resulting from an allergic reaction or irritation to soaps, condoms, lubricants, or other products that come into contact with the skin.

Eczema/Dermatitis: Atopic dermatitis or seborrheic dermatitis can affect the genital area, including the penis shaft, causing erythema and itching.
Psoriasis: Although less common in the genital area, psoriasis can cause erythema and itching on the penis shaft.
Tinea Cruris (Jock Itch): A fungal infection that can spread to the penis shaft, causing erythema and itching.
Balanitis: Inflammation of the glans penis, which can sometimes extend to the shaft, causing erythema and itching, often due to poor hygiene, infections, or irritants.

Sexually Transmitted Infections (STIs): Such as herpes simplex virus (HSV), syphilis, or gonorrhea, which can present with erythema and itching, and are critical to diagnose due to their potential for serious complications and transmission to others.
Lichen Planus: A chronic inflammatory condition that can affect the skin and mucous membranes, including the genital area, and may present with erythema and itching.
Squamous Cell Carcinoma: Although rare, it's crucial to consider in cases of persistent or unexplained erythema and itching, especially in older adults or those with risk factors like HPV infection.

Zoon's Balanitis: A rare condition characterized by a chronic, painless, erythematous plaque on the glans penis, which can extend to the shaft.
Fixed Drug Eruption: A rare condition where certain medications cause a skin reaction, which can include erythema and itching on the penis shaft.
Erythrasma: A bacterial infection more commonly affecting the groin area but can extend to the penis shaft, causing erythema and itching.