Aerial Blight Infections on Vinca and Other Herbaceous Annuals

Aerial blight (also referred to as shoot blight) has been popping up in landscapes and garden centers.  Typically, two different organisms can cause aerial blight symptoms – Rhizoctonia solani (a true fungus) and Phytophthora parasitica (a water mold).  Recent outbreaks of aerial blight in Kentucky have been reported on vinca (Figures 1-3), calibrachoa (Figure 4), and petunia, each caused by the Phytophthora pathogen.  Thus, the following information will be specific to Phytophthora aerial blight.

Fig 1 – Initial infections by the aerial blight pathogen occur on leaves of vinca.

 

Phytophthora species are water mold pathogens.  Briefly, that means that they must have free water to complete their life cycles (produce spores, infect, and cause disease).  During wet weather (frequent rains) or under excessive water or humidity (greenhouse conditions or heavy irrigation), water mold pathogens proliferate.  Clearly, this year’s rainy conditions are favorable for disease. 

Phytophthora aerial blight, like other water mold pathogens, is no different.  It thrives in soils of heavily irrigated or otherwise wet landscapes and in production greenhouses.  Infections occur when soilborne pathogens splash up onto stems and foliage.  Leaves become blighted (rapid collapse) (Figure 1) and then infection spreads to petioles (Fig 2) and stems (Fig 3).  Lesions on lower stems cause girdling and entire branches then fail. 

Fig 2 – Infection spreads from leaves, down petioles, and then to stems.

Once plants are infected, fungicides do not cure disease.  Affected plants should be destroyed as soon as possible to prevent diseased tissue (and the pathogen) from building up in landscape soil and greenhouse floors.  Homeowners should consider improving drainage, mulching to prevent splash, and switching to drip irrigation.  No fungicides are recommended for residential or landscape use.  Commercial growers, on the other hand, should employ a preventative program for healthy plants if nearby plants become infected.  Phosphorus acid fungicides (e.g. Aliette) are effective against water mold pathogens and are recommended for protection against Phytophthora aerial blight. 

Fig 3 – Progression of symptoms to stems.

More information for commercial greenhouse growers can be found at

http://www2.ca.uky.edu/agcollege/plantpathology/ext_files/PPFShtml/PPFS-GH-4.pdf
http://www2.ca.uky.edu/agcollege/plantpathology/ext_files/PPFShtml/PPFS-GH-3.pdf

 

Landscape and homeowner information can be found at
http://www2.ca.uky.edu/agc/pubs/id/id87/id87.pdf

Fig 4 – Advanced symptoms of aerial blight on calibrachoa (million bells).