The geranium is a popular garden plant that can be grown in a greenhouse or planted outdoors in a garden. The plant is susceptible to disease, including the emergence of yellow spots on the leaves.
Yellow spots on geraniums are caused by a fungus called Puccinia Pelargonii-zonalis. The disease is often referred to as geranium rust and has been a threat to geraniums in the United States since the late 1960s.
Geranium rust results in yellow spots both on the upper and lower sides of leaves. If left untreated, the spots increase in diameter and evolve into brown spots surrounded by rings. Leaves that are heavily affected turn completely yellow and fall from the plant prematurely.
You can prevent geranium rust by watering the plants directly in the soil and avoiding getting water on the leaves. Ensure good air circulation by allowing some distance in between plants. Remove all infected leaves daily. If you choose to apply a fungicide, do so at 7- to 10-day intervals at the first sign of yellow spots.