Coffee cultivation in Sri Lanka was abandoned primarily due to the disease known as "coffee leaf rust" or "Hemileia vastatrix," which is caused by a fungus. Therefore, the correct option is Option 3: leaf rust.

Coffee leaf rust is a devastating disease that affects coffee plants, specifically the leaves. It is caused by the fungal pathogen Hemileia vastatrix and is highly destructive to coffee crops. The fungus infects the leaves, causing yellow-orange powdery spores to appear on the underside of the leaves. These spores reduce the plant`s ability to photosynthesize and ultimately lead to defoliation.

The outbreak of coffee leaf rust in Sri Lanka, formerly known as Ceylon, occurred in the late 19th century and had severe consequences for the coffee industry. At that time, Sri Lanka was one of the leading coffee producers in the world. However, the disease rapidly spread throughout the coffee plantations, causing significant damage and decreasing yields.

Coffee leaf rust is highly contagious and can spread easily from infected plants to healthy ones, especially under favorable environmental conditions. The warm and humid climate in Sri Lanka provided an ideal environment for the fungus to thrive, leading to the rapid spread of the disease across the coffee-growing regions of the country.

The impact of coffee leaf rust on Sri Lanka`s coffee industry was devastating. Plantations were decimated, and coffee production declined significantly. To combat the disease, many coffee growers attempted to use various fungicides and control measures, but the efforts were largely unsuccessful. As a result, farmers had no choice but to abandon coffee cultivation and seek alternative crops.

The decline of coffee cultivation in Sri Lanka due to leaf rust paved the way for the emergence of tea plantations. Recognizing the need for an alternative crop, the British colonial government introduced tea plants, primarily the Camellia sinensis variety, which flourished in the country`s climate and soil conditions. This transition from coffee to tea proved to be a pivotal moment in Sri Lanka`s agricultural history, as the country eventually became renowned for its high-quality Ceylon tea.

In conclusion, the disease that caused the abandonment of coffee cultivation in Sri Lanka was coffee leaf rust (Hemileia vastatrix), making Option 3: leaf rust the correct answer. The devastating impact of this disease on the coffee industry led to the decline of coffee plantations and the subsequent rise of tea production in Sri Lanka.