Geraniums are prized in the home garden for their bold blossoms in hues of red, pink or white and solid to variegated foliage. Versatile plants, geraniums produce leaves in shades of green, gray-green, silver and white. Unless you are growing the cultivar Mrs. Henry Cox that displays variegated leaves in yellow, red and green, yellow leaves signal a problem. Examine your plants regularly to catch issues before damage occurs.
Provide consistent, appropriate care to geraniums as vigorous plants are more likely to ward off and overcome problems like disease when compared to plants with diminished health. Maintain moist, well-drained soil conditions for best growth. Add a 2- to 3-inch layer of organic mulch like wood chips to the area surrounding your geraniums without pushing mulch against stems. Mulch assists in conserving water, stabilizing soil temperatures and keeping weeds from developing. Grow geraniums in areas of the garden that provide a minimum of four hours of daily direct sun. However, plan for shade during the middle of the day in extremely hot climates.
Incorrect irrigation of geraniums results in yellowed leaves. Geraniums are water lovers that, unlike some plants, will not bounce back to health if you allow leaves to wilt. Bacterial blight on geraniums leads to the yellowing of leaves, as well. Caused by the bacterium Xanthomonas campestris pv. pelargonii, this disease infects geraniums through contaminated water and gardening tools like pruning shears or gloves. Look for this disease when temperatures warm to a range of 70 to 80 degrees Fahrenheit when bacteria are most active. Bacteria will remain on plant debris, as well, quickly spreading disease throughout a gardening space.
Improper irrigation begins on geraniums with wilted leaves. Once leaves yellow, they typically drop from the plant and the drought stress leads to stunted growth. When bacterial blight infects geraniums, spots that look saturated with water appear on leaves. These depressed spots grow to a diameter of approximately 1/16 to 1/8 inch, displaying a brown color. Areas of dying plant tissue may also occur between leaf veins, often joining and resulting in larger areas of dead plant tissue or holes. Severe disease that infects the entire plants results in a complete yellowing and death of the plant.
For proper irrigation, thoroughly saturate soil and allow soil to dry before re-watering. Additionally, as a rule of thumb, shiny leaves are not in need of water, but leaves that appear lackluster need water, according to the University of Massachusetts Extension. To control bacterial blight on geraniums, always purchase disease free plants from reliable sources to avoid introducing disease into your garden. Avoid overhead irrigation and use direct methods like drip irrigation instead to keep foliage dry. Remove and destroy affected plant parts to reduce the spread and intensity of disease. Do not attempt to use chemical control, as these methods are ineffective.