Diagnosing rust on geraniums can be a challenge for growers. Rust begins with yellow spots on the underside of the leaves and can rapidly progress with brown fungal spores called pustules in the center of each spot. The pustules are raised up off the leaf surface and feel rough to the touch. As the pustules grow, they produce a distinctive concentric ring. The areas on the upper leaves above these spots will also turn yellow, then eventually brown. If left untreated, infected leaves turn yellow and dry, and drop prematurely, resulting in defoliation of the plants.
Combinations of cultural and chemical control are required to control geranium rust. Splashing water and handling are the main factors in spreading this disease. Avoid splashing water and minimize handling of infected crops. Likewise, avoid carrying over geranium stock and never bring geraniums into the greenhouse from outside, as these are also common avenues of infection.
As soon as an outbreak is detected, remove all obviously infected leaves from the plant and carefully take them out of the greenhouse in sealed bags. Immediately treat all of the geraniums with a fungicide such as Camelot O, Compass, Daconil, Disarm O, Eagle, Heritage, Spectro, Strike, Pageant Intrinsic, Phyton, Protect or Terraguard. Repeat this sequence on a weekly basis, rotating modes of action (MOAs) to avoid resistance, until pustules are no longer appearing.
Or Is It Edema?
Edema occurs predominately during cool, damp and low-light conditions. Some growers have dealt with the high cost of fuel by turning back the thermostat, which increases the relative humidity that contributes to edema. Edema results from an imbalance in osmotic pressure inside the plant cell. If this pressure inside the geranium plant is great enough, it can cause the cells to rupture, creating scab-like wounds. Ivy geraniums, for example, typically display brown, corky lesions on the undersides of the leaves. In severe cases, the leaves turn yellow and may fall off.
Preventing edema requires the management of environmental conditions. Do what you can to reduce the relative humidity and keep the air moving with HAF fans. Running the soil dry can aggravate edema, so strive for even soil moisture. Maintaining adequate fertilizer keeps cell walls more flexible. Improved ventilation, along with warmer, sunnier weather as spring progresses, often reduces the occurrence of new symptoms.