Rose Woes

Diseased Knock Out Rose received in the Clinic

This week the Plant Disease and Insect Clinic at NCSU received a container rose with both downy mildew Botrytis blight. Both are favored by cool, moist conditions, and both can cause extensive damage in a short period of time under production conditions, via the dispersion of airborne spores. That’s where the similarity ends.

Cankers caused by Botrytis on rose canes are often light in color. This same fungus can cause spotting on rose petals and in the present case grew all over clusters of dead new leaves. It enters plants most easily in senescent tissue or through wounds. Recent cold snaps may have given Botrytis a leg up. On the other hand, downy mildew needs to infect and reproduce on living plant tissue. On rose leaves, it typically produces dark angular spots, though here it was found on green leaves that showed only a very slight mottle. On canes the usual symptom of rose downy mildew is purple or black blotching or spotting.

The downy mildew pathogen was found sporulating on the underside of this fairly healthy-looking leaflet.

Botrytis is sometimes called gray mold. This color is the combination of the black of the tiny thread-like stalks (conidiophores) and the white of the spores (conidia). Under magnification these look like pompoms. The spores of rose downy mildew (Peronospora sparsa) may or may not be visible on an infected plant. They are especially scarce on canes. On leaves they will be produced on the underside only. These structures are slightly smaller and more delicate than those of Botrytis, and the stalks (sporangiophores) are white.  They branch repeatedly giving a more open, tree-like growth. At the tips of the sporangiophore’s branches, spores (sporangia) appear white or tinged grayish blue in mass.

Sporulation of Botrytis on a rose cane. Note the thorn in the upper right for size comparison.

Close-up of Botrytis sporulation. Sometimes a second "pompom" of spores will form in the middle of the conidiophore.

Sporulation of Peronospora sparsa. It’s unusual to see so much on a cane.

Close-up of Peronospora sparsa sporulation. This and the previous three photos courtesy of Matt Bertone.

Prune out any affected canes you find, going well into clean wood. Sanitize shears frequently. Do not let debris or spent flowers accumulate, as Botrytis can reproduce on just about any kind of dead plant material. Keep foliage and stems as dry as possible by proper timing of irrigation and adequate plant spacing to allow good air movement. One reason these fungi were doing so well on this sample may have been that the plants are still in a greenhouse-like environment.

This time of year rose growers should maintain a spray program that includes products against both pathogens. Since the pathogens are unrelated, some products work only on one or the other. See pages 465-467 of Table 10-13 of the NC Ag Chemicals Manual. See also Table 10-14 (pp. 474-476) on relative effectiveness of different products. Follow all label directions. Remember that labels can vary depending on whether the plants are being grown outdoors or in an enclosed structure such as a greenhouse. Be sure to rotate among fungicides in different FRAC groups so as to delay the development of resistant strains. Test any new product on a small number of plants to be sure that there are no adverse effects.

Note: Early indications are that the plant pictured above also has a bacterial disease called Pseudomonas blight. Symptoms of this disease include cankers on stems and the death of new shoots, often following freezing temperatures. It affects not only rose but a wide range of woody hosts. Plants like this one are best discarded. For details, see last year’s blog post.

Jack Frost Does Not Work Alone

Dead canes of a flower carpet rose

As March was going out like a lamb, a nursery submitted four container-grown shrubs to the PDIC: three rose cultivars and a lilac. Very young shoots on these plants were withering and dying. At least in the case of the lilac – and possibly with the roses, too – the new flush of growth had been hit by the last freezes of the spring. While you’d expect the tender shoots to be blasted by the cold, in this case the woody stems were also dying. Bacterial streaming was seen in much of the stem tissue. We don’t see fire blight on rose or lilac, so what was happening?

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.

Although Pseudomonas syringae is named after lilac (Syringa), it is capable of causing cankers and dieback in a wide variety of plants. Besides lilac, we’ve found it on the following woody ornamentals: cherry-laurel, flowering quince, Indian hawthorn, Yoshino cherry, and multiple varieties of rose.  In addition, we’ve recovered it from leaf spots of hydrangea and Japanese holly. Bacterial canker caused by Ps. syringae can be a serious problem in peach orchards, but with woody ornamentals we almost always see it in nursery situations. One exception came in last year, on the twig of a weeping willow from a home landscape. As the weather warms up and cankers become inactive, this disease becomes more difficult to detect. According to the PDIC's records, almost every case of Pseudomonas bacterial canker on woody ornamentals since 2008 was diagnosed between February and May. The bacterium is still present on and within plants during the summer, but what I believe is happening is that the hotter temperatures slow down this particular bacterium at the same time as they're (up to a point) invigorating most plants, thus shutting down the disease process temporarily.

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

Like many bacteria, Pseudomonas syringae is able to live and multiply on plant surfaces. This is known as its epiphytic (“on the plant”) phase. In the recent case, the bacteria were almost surely present before the spring flush occurred, and so were able to strike quickly. These bacteria enter plants following injury, in particular frost damage. What’s more, the bacterial cells actually promote freeze damage through a process known as ice nucleation. How this works is succinctly expressed by Sinclair and Lyon:

"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.

*Chemist’s Corner:

Colonies of fluorescent pseudomonads photographed under UV light.

The fluorescent pigments of Pseudomonas species are seen by shining a long-wave UV lamp on the cultures. These compounds belong to a class of chemicals called siderophores. If you know Latin, you might think that siderophore means “star bearer”, but in this case the root is the Greek word for “iron”. (A big thanks to Roland Wilbur Brown’s 1956 book Composition of Scientific Words for setting me straight.) Iron is an essential element for microbial growth, and siderophores have the important task of scrounging precious iron from the bacteria’s environment.

Rose rosette hits close to home.

Several roses in this bed are showing symptoms of rose rosette.

Those of you living in the Raleigh, North Carolina area may have read the article in the News and Observer on Saturday August 25, 2012 about the removal of several rose bushes from the Raleigh Rose Garden and from a traffic circle on Hillsborough Street. The reason: they had been diagnosed with rose rosette. This disease has been known in North America for decades, but it seems that it has become more common in our area over the last two years. The author of the N&O piece, Bruce Siceloff, did a good job of gathering and presenting the pertinent facts about this disease. Let me review some of them here and expand on what he provided.

Witches' broom and leaf deformation

Symptoms can vary depending on the variety of rose involved and may include elongated flexible shoots, proliferation of shoots leading to a “witches-broom” appearance, excessive development of thorns (soft or not), leaf deformation, retention of juvenile red coloration in shoots, flower abnormalities, decreased cold hardiness, and plant death. There is a rather elaborate molecular test that can be used to confirm the presence of the virus that causes rose rosette, but we do not currently offer that service at the NCSU Plant Disease and Insect Clinic. If you see the “hyperthorniness”, then you can be confident in the diagnosis, but some cases are not clear-cut. Not all symptoms may be present in any given plant. Shoot proliferation and leaf deformation can also be caused by accidental exposure to low doses of the herbicide glyphosate (Roundup), so if you observe this symptom do some sleuthing to see if drift might have occurred.

The shoot on the left retained its red color.

Rose rosette was only recently proven to be caused by a virus, but it has been long known to be transmitted by the microscopic eriophyid mite Phyllocoptes fructiphilus. These are not the same as the more familiar spider mites. Small size makes up for their lack of wings, and these mites can be carried about on air currents. I’ll leave it to my entomology colleagues to comment on whether mite control is of any benefit. There is no chemical control for plant virus diseases. Since viruses become systemic in their hosts, pruning may not be effective. Removal of infected plants is the safer bet. You should bag them before digging, to reduce the chance that the mites will scatter on the wind and take the virus to nearby plants. Remove enough of the roots so that the infected plant does not re-sprout. Also remove any nearby weedy multiflora roses that may be serving as a reservoir of the virus. Fragments of small roots left in the soil should pose no risk. I could find no studies proving the spread of Rose rosette virus through natural root grafts, although this has been demonstrated for other rose viruses. For this reason and because of the mite vectors, planting rose bushes next to one another should be considered a risky behavior. Of course propagating from infected plants or grafting onto infected rootstocks is a no-no.

Research has shown that the incubation period for rose rosette can vary from 17 days to 9 months. Incubation period simply means the length of time it takes for a plant to show symptoms once it has been infected. We don’t have set recommendations about quarantining plants you get via purchase or trade, but some period of isolation and observation may be a good idea.

Extreme thorniness and flexibility are often seen in canes with rose rosette.

Is there a bright side to this story? It’s cold comfort to rose growers that this disease does not affect other kinds of plants. More encouraging is that some rose species are resistant. According to the second edition of Sinclair and Lyon’s excellent book, Diseases of Trees and Shrubs (2005, Cornell University Press), resistant species include the native Rosa setigera and Rosa carolina. No doubt some of these will be exploited in breeding programs trying to bring resistance into garden roses. Until then, vigilance and a shovel are our best tools against this serious problem.