Bacterial Leaf Spot: Acidovorax in the Greenhouse

It’s been a busy season for bacterial leaf spots on greenhouse ornamentals. Last week, we blogged about Xanthomonas leaf spot of English ivy. This week, we will take a look at leaf spots caused by Acidovorax sp.

Bacterial spot lesions on hydrangea (Photo: Mike Munster)

Over the past month or so, we have received two petunia samples, a verbena sample, and a geranium sample from 4 different greenhouses across the state. All of these samples had leaf or stem spots. The leaf spots had a water-soaked appearance and plants in advanced stages of infection were collapsing. After examining the spots under a microscope, we concluded a bacterium was the problem.

Water-soaked lesions (Photo: Mike Munster)

Stem lesions (Photo: Mike Munster)

Genera of bacteria that frequently cause leaf spot diseases in floricultural crops include Xanthomonas, Pseudomonas, and Acidovorax. Unlike many fungi, which can be identified by their unique spore types, bacteria are difficult to distinguish based on their appearance. Most bacteria look the same as they ooze out a leaf. It is important to know which species you have so you know whether or not the bacterium present is pathogenic or if it is a secondary saprophyte causing little damage. 

To identify the bacteria responsible for causing the leaf spots in these cases, we had to culture them and then perform additional assays. These assays identify bacteria based on their ability to metabolize different kinds of nutrients and compare the results with a database. Results identified the culprit as an Acidovorax sp. We then took this bacteria and inoculated a healthy leaf to make sure the same symptoms were produced.  

Inoculated leaf (Photo: Mike Munster)

Bacteria belonging to the genus Acidovorax have been isolated from geranium (Pelargonium hortorum) and petunia (Petunia hybrida) since the late 1980’s. The geranium and petunia strains appear very closely related to each other, but different from other strains of Acidovorax, possibly indicating they belong to a new, unidentified species of Acidovorax.

Management recommendations for Acidovorax are similar to those we discussed last week for Xanthomonas. Always start with disease free stock or cuttings. Plants with these symptoms should be removed and destroyed, including the planting mix. Healthy-looking plants adjacent to the diseased plants should be quarantined if possible. Acidovorax is found on petunias, verbena, geranium, hydrangea, and lantana. Greenhouse operators should be sure to inspect all plants in this group if Acidovorax is found.

Leaf spot symptoms on verbena (Photo: Mike Munster)

Leaf spot symptoms on petunia (Photo: Mike Munster)

Keep leaf/stem wetness periods to a minimum once plants come out of propagation, and avoid any kind of physical injury or wounding to the plants, which will aid entry by the bacteria. They can be spread from plant-to-plant by hand, insects, water splash, and even as aerosols. Avoid handling plants when wet, and frequently sanitize knives or other tools used on the plants.

Thoroughly clean and then sanitize pots, flats, surfaces, and tools with steam or chemical disinfectants. There are no effective chemical controls for application directly to the plants, but during propagation copper products can be used to hold back the bacteria somewhat.