|Dead canes of a flower carpet rose|
The grower suspected Pseudomonas blight. He was right.
A bacterium and its victims
Bacteria were cultured from the stem tissue of the affected plants. Since only Pseudomonas species were of interest, only colonies fluorescent* on a special agar medium were chosen for further work-up. Unfortunately there are a lot of nonpathogenic (non-disease-causing) Pseudomonas species in this world, so it took a little time to sift through the isolates and confirm the diagnosis as Pseudomonas syringae.
|Wilting new shoot of a container-grown lilac.|
Note: We occasionally find Ps. syringae causing leaf spots on ornamentals in the greenhouse, and there are variants – called pathovars – that cause certain very specific problems such as bacterial speck of tomato and angular leaf spot of cucurbits.
How Pseudomonas syringae does its dirty work
|Blighted shoots and a Pseudomonas stem canker on rose|
"Ice-nucleating strains of P. syringae and certain other bacteria can trigger ice formation in plant tissues cooled to between -2 and -5ºC [28 to 23 ºF] but not acclimated to low temperature. Ice then disrupts cells, causing symptoms of frost damage. In the absence of an ice-nucleating factor, frost-sensitive plants may tolerate brief cooling to these temperatures because water in their tissues remains in liquid form, supercooled." (Diseases of Trees and Shrubs, 2nd Ed. 2005. p.368)
For more information, see this review by Gurian-Sherman and Lindow. You might also check out this laboratory video of ice nucleation by bacteria added to supercooled water.
As if this ice-nucleation trick were not enough, Pseudomonas syringae also produces a toxin that damages plant cells.
How to reduce your losses
The most important way to minimize damage to woody plants from Pseudomonas syringae is to limit the stressors that predispose plants to infection. Stress factors include pruning injury and frost injury. Bacterial canker of stone fruits caused by Pseudomonas syringae can be reduced by pruning in the early summer, instead of the fall or winter. Sanitize shears or knives frequently, and avoid working the plants when wet. Don't overfertilize plants, especially when they need to harden off for the winter. Protect plants during cold snaps. Don't allow plants to undergo stress from too much or too little water. Keep foliage and stems as dry as possible by changing irrigation methods or reducing overhead irrigation, which favors and spreads the bacteria. If you’ve already had this problem, Ps. syringae is probably present as epiphytic populations on the surfaces of much of your nursery stock and even the surrounding weeds. There are few chemical options that hold any promise, at least not enough to make a recommendation.
As I write this, winter is getting ready to take one last shot at North Carolina, with freeze warnings up for the western half of the state. It's another opportunity for Pseudomonas syringae, too.
|Colonies of fluorescent pseudomonads photographed under UV light.|