Alternaria

• Alternaria is a collective term for any plant disease that is caused by a species of fungus in the Alternaria genus. Some of the most common pathogenic Alternaria species include Alternaria alternata, A. brassicae, A. Brassicae f.sp. phaseoli, A. brassicicola, A. cucumerina, A. dauci, A. dianthi, A. dianthicola, A. gerberae, A. gomphrenea, A. oleracea and many others. Alternaria diseases occur everywhere in the world and are among the most common plant diseases.
  
  

Host Flowers

• The list of possible Alternaria host plants is a rather lengthy one. With regard to flowers specifically, Alternaria is known to affect sweet William (Dianthus barbatus), carnation (Dianthus caryophyllus), Transvaal daisy (Gerbera jamesonii), globe amaranth (Gomphrena globosa), giant granadilla (Passiflora quadragularis) and many others. The susceptibility of certain types of flowers to Alternaria diseases is also highly region-dependent, so it is often wise to ask a university extension agent as to the susceptibility to Alternaria diseases of the flowers you are planning to grow.
  
  

Common Symptoms

• If you do cultivate a susceptible species of flower, it is vital to know the most common symptoms of Alternaria diseases so as to save the flower before it is too late. The earliest symptoms of Alternaria diseases are usually spots around 1/2 inch in diameter and usually gray, tan or black. The most distinguishing symptom of Alternaria diseases is the tendency for these spots to develop concentric rings around the spots themselves. According to the University of California Cooperative Extension, these concentric rings "are diagnostic for Alternaria" since "few other pathogens ... cause this type of diagnostic expression."
  
  

Treatment Approaches

• The ideal treatment for Alternaria diseases always depends on the plant being infected and on the specific strain of Alternaria that has infected the plant. In general, however, look for disease-resistant flower species and cultivars and ensure, to the best of your abilities, that flower seeds are disease free before sowing them. Crop rotation, weed control and cleanup of plant debris from infected plants has also been shown to help control the disease. For extreme infections, you may need to resort to the use of a chemical fungicide such as captan, fludioxonil, imazalil, iprodione, copper fungicides or other compounds.