This disease is caused by a bacterium (Xanthomonas pelargonii) that infects leaves through wounds and natural openings. Once inside, it kills the tissues rapidly. The bacteria spread down the leaf stems into the main stem, killing tissue as they move. They may even spread into the roots. Martha Washington geranium is not affected.

This common disease of geraniums is caused by a fungus (Pythium species) that lives in the soil. Black stem rot is favored by wet, poorly drained soil. The fungus attacks the stems at the soil level, then spreads upward. The stems decay and the foliage wilts, shrivels, and eventually dies. Black stem rot is spread by contaminated soil, transplants, and tools.

Trialeurodes vaporariorum The greenhouse whitefly is a common insect pest of many garden and greenhouse plants. The four-winged adult lays eggs on the undersides of leaves. The pinhead-size larvae are flat, oval, and semitransparent, with white waxy filaments radiating from the body. They feed for about a month before changing to the adult form. Both larvae and adults suck sap from leaves; larvae do more damage because they feed more heavily. Adults and larvae cannot fully digest the sugar in sap, excreting the excess in a fluid called honeydew that often drops onto leaves below. A sooty mold fungus may develop on honeydew, causing leaves to appear black and dirty. In warm-winter areas, the insect can be active year-round, with eggs, larvae, and adults present at once. The whitefly cannot live through freezing winters. Spring reinfestations in cold areas come from migrating whiteflies and from infested greenhouse-grown plants placed in the garden.

This disease is caused by any of several different fungi, also known as water molds, that live in the soil. These fungi thrive in waterlogged, heavy soil. Some of them attack the plant stems at the soil level, while others attack the roots. Infection causes the roots and stems to decay. This results in wilting, then leaf yellowing, and eventually the death of the plant. These fungi are generally spread by infected soil and transplants, contaminated equipment, and splashing or running water.

This plant disease is caused by a fungus (Puccinia pelargonii-zonalis). Wind and splashing water spread rust spores to healthy plants. After a leaf has been wet for 5 hours, the spores germinate, forming a spot and, several days later, a pustule. Rust generally does not kill the plant, but heavily infected plants may drop some leaves. The fungus survives only on living plant tissue and fresh plant debris. Wet conditions favor the development and spread of this disease.