Hosts
Only geraniums

Symptoms
Bacterial blight can devastate geraniums grown in the greenhouse and in the garden.  Symptoms may vary with the cultivar and environmental conditions.  Leaves often wilt and turn brown rapidly. V-shaped browning that are widest at the leaf margin and narrowing at the center of the leaf are common.  Often, small, darker brown, water-soaked spots form inside the larger discolored area on the leaf.

Look-alike symptoms
Drought injury, bacterial wilt, blackleg, Botrytis blight, Rhizoctonia stem rot, and Pelargonium flower break virus all may look like bacterial blight.

Favorable conditions
Infected plants may seem healthy during cool, dry conditions (below 68 F).  Symptoms often do not become apparent until the weather warms. Wet conditions and temperatures between 70 and 80 F favor severe disease. High levels of nitrogen and phosphorous and low levels of calcium also promote disease symptoms.

Disease spread
The bacterium is absent from most greenhouses and gardens in Montana. It can be brought in via infected plants or seeds and become a serious problem on geraniums grown in this same location for many years afterwards.

Bacterial blight can enter plants whenever they are wounded. Infected cuttings and plants do not always show symptoms.   The bacterium can reproduce very quickly and spread through water, pruning shears, plants in close contact, hands, and similar means. Whiteflies and other insects also can spread the disease.

Survival
The bacterium can survive in soil and decaying geranium tissue for long periods of time. This pathogen is quite aggressive and virtually every susceptible geranium planted in contaminated soil is likely to become diseased. This can be a real problem in commercial greenhouses gardens.

Control
The best controls include avoiding introducing the disease to an area and using stringent santitation if the disease is found. There are no effective chemical controls and even the most tolerant varieties harbor the disease without showing symptoms and therefore can spread it easily to other geraniums.

Resistant cultivars
Lady Washington and Martha Washington (Pelargonium domesticum), Madame Layal and Marie Vogel (P. x domesticum), P. 'Torento', Peppermint (P. tomentosum) and Countess of Scarborough (P. scarboroviae)

References
Plant Disease Alert, February, 25, 1997, No. 97-2, Kansas State University Cooperative Extension Service.

Taylor, R.J. et al.  Leaf spot and stem rot (bacterial blight) of Geranium, North Dakota State University Extension Service Bulletin OO-739.

Pelargonium flower break virus of geranium