Downy Mildew on Sweet Woodruff (Galium odoratum)

Downy mildew (caused by a species of the oomycete, Peronospora) on the herb/groundcover Sweet Woodruff (Galium odoratum) was found on a recent sample. At first glance the symptoms suggested iron deficiency (based on yellow foliage and other indications by the grower). However, a closer look shows leaf dieback and gray lesions which indicate a pathogen may be involved. Flipping the leaves over (see photos below) reveals the characteristic fuzzy growth of downy mildew.

Downy mildew loves cool, wet conditions! Management should involve reducing leaf wetness (which is required for the spores to germinate). There has been no research done on fungicide control on Galium spp., so proceed with caution if choosing to use a fungicide. Test in small areas to ensure that phytotoxicity isn't going to be an issue. The Chase Research Gardens, Inc. out of California have done quite a bit of work on downy mildew fungicide control in ornamentals and some results can be found here.

Cucumber Mosaic Virus (CMV) on Houstonia

It has been interested to see the wide range of symptoms that can be caused by virus. We received plugs of Houstonia caerulea into the diagnostic lab last week with leaf spots and dieback on the flower stalks. The spots themselves had a orange brown color. The root systems looked great-white, healthy looking, not pot-bound.
We incubated a few plugs in a moist chamber and plated the leaf spots onto CMA and PDA. No fungal growth and no sporulation ever occurred. There was also never any bacterial growth or streaming noted. Running out of possibilities , I decided to test with an AgDia combo strip for CMV, INSV, TSWV and TMV. The Houstonia tested postive for Cucumber Mosaic Virus (CMV)! I reran the test to confirm and again it was positive. I haven't been able to find a record of Houstonia being a host for CMV, yet (I'll admit I haven't done a concise search). The symptoms are interesting-with bright orange speckles on the leaf tissue and stems. (Click on any photo for a larger version.)

Close-up of foliage-note orange speckling.

This is the base of the stem, which also showed the orange speckling.

CMV is spread by aphids but also by mechanical transmission-meaning that workers can spread the virus when taking cuttings or otherwise moving infected plant sap.

Possible Phytotoxicity on Cordyline

These are Cordyline australis 'Southern Splendor' from a finishing operation. The main symptoms are the distorted growth and fusing together of the 'middle-aged' growth . The new growth is unaffected while the oldest growth shows tip burn. The plants were heavily infested with two-spotted spider mites (Tetranychus urticae) with obvious symptoms of yellowing and stippling present. The question from the grower was, "Is the distortion caused by the mite injury or is some other factor involved?"

Mite infestations are not known to cause such extreme symptoms so we needed to look elsewhere. The most likely cause is phytotoxicity caused by an pesticide application. The grower is looking into their rotation of miticides as mites have been a constant problem in this crop. Since they are a finishing operation (meaning they received the plants a few months earlier to grow to the final selling size) the grower may have difficulty tracking down what pesticides were applied before they took possession.

These symptoms have been seen by diagnosticians in other states, but a definitive cause of the problem has yet to be identified.

Tip burn was seen on the oldest leaves. Cordylines can be very sensitive to fluoride levels in irrigation water which can manifest as tip burn, but fluoride is unlikely to cause the other symptoms.

Two-spotted spider mites and associated injury (yellowing and stippling of older leaves) was observed.

Vinca minor-CMV (Cucumber Mosaic Virus)

This sample is Vinca minor from a propagation operation. The sample tested positive for CMV (Cucumber Mosaic Virus) using an Agdia immunostrip test.
CMV has a very wide host range and affects both vegetables and ornamental plants. Aphids are the primary source of infection but infection by mechanical transmission (taking cuttings, etc) is also possible. Infected plants should be destroyed to avoid spreading the virus within a greenhouse.

Aster-TSWV (Tomato spotted wilt virus)

This sample is Aster x Frikartii 'Monch' from a propagation operation showing signs of virus infection. The sample tested positive for TSWV (Tomato Spotted Wilt Virus) using an Agdia immunostrip test. TSWV & INSV (Impatiens Necrotic Spot Virus) are two related and very common viruses in greenhouse production. Cornell University has an excellent fact sheet with more information on these viruses.

Abiotic problem with Lavender plug production

A propagator brought in a flat of lavender plugs (Lavandula x intermedia cv. Grosso) showing dieback and plant death. The grower has an intense fungicide rotation for Pythium so it was unlikely to be the cause. Soil pH and salt levels were within range for plug production. When the plants were removed and washed, no roots were seen on the affected plants. This indicates a rooting problem, not a disease. Several plants in the flat were able to root and mature althought they weren't at 100% (see above. Note:this photo was taken after holding in the lab for a week so the plants have deteriorated somewhat). It is possible that fertilizer levels were a problem earlier or growth temperatures were not in the range that lavender prefers. At this point, the exact cause of this problem is unknown.

Photo below shows rooted cutting (left) compared to an unrooted cutting (right) 30 days after sticking. A variety of secondary pathogens were isolated from the decaying stem of the unrooted cutting.

Verbena-Thielaviopsis Root Rot

A recent sample of Verbena cv. Valley Lavender from propogation stock showed stunting, poor root formation, and plant death. Roots were light brown and failed to grow beyond the original plug. The problem was diagnosed as root rot caused by the fungus Thielaviopsis basicola.

The above photos shows chlamydospores in crushed root tissue under magnification - this a characteristic diagnostic sign of T. basicola infection.

For more detailed information on Thielaviopsis root rot and management, check out this article in Greenhouse Product News.

Venus Flytrap-Diagnosis in progress

A very cool sample came into the clinic this week-Venus flytraps!



Venus flytraps (Dionaea muscipula) are native to North & South Carolina (their natural habitat is only within a 100 mi radius of Wilmington, NC). They are, however, a very popular plant and considered easy to grow as long as you treat them right. 'Right' for a Venus flytrap is a humid swamp! Check out this site for detailed info on Venus flytraps (disclaimer: I can't vouch for complete accuracy on this site as I'm not a D. muscipula expert!).

The sample presented with blackened petioles and traps. Many of the older leaves were dying. The plants were being grown commercially and Phytophthora cinnamomi had caused problems in previous years. Leaves and rhizomes were plated for Pythium and Phytophthora.
Diagnosis is pending....

Abiotic stress of Carnation

At first glance we suspected Fusarium wilt (Fusarium oxysporum f. sp. dianthi). However, the characteristic staining in the vascular system was absent. The stem and leaves were plated and only secondary pathogens grew out. This problem is likely a nutrient stress.

Lavender-Rhizoctonia

Looks okay, right? Now look at the base.....
This sample was so fun to work with! Usually samples have an incredible odor (like the last post on Geotrichum soft rot), but this was a lavender sample. Even when decaying, it still smells great! This sample presented as potted cuttings that develop dieback on the lower leaves and petioles. Possible Phytophtora present that I need to confirm. Root, leaf, and stem samples were plated and a section of the plant is incubating to induce sporulation. My money is on Phytophthora...results forthcoming!

Well, I lost that bet! The plates showed Rhizoctonia growing from the stems and leaf lesions. What I thought was a Phytophthora turned out to a be a very very young asexual fungus. I'm (slowly) learning to have a little more patience with samples and let them incubate and grow in culture longer. This was one case of jumping to judgment too soon!

Phyllosticta leaf spot on Impatiens

Description: Three impatiens in pots were submitted showing leaf spots. The leaf spots occurred randomly on the plants and showed concentric rings with or without a dark purple halo.





Possibilities: Here is a nice article (.pdf/pg 3) showing various leaf spots on impatiens.

Tests done: Moist chamber affected leaves overnight at room temp. to induce sporulation


Results: Pycnidia formed within the rings of the lesions. Botrytis was, of course, also present.


Final diagnosis: Phyllosticta leaf spot caused by Phyllosticta spp.

Abiotic stress leading to secondary pathogen invasion on Calibrachoa

Description: Calibrachoa is an ornamental plant related to the petunia but with smaller flowers. It is commonly used in hanging baskets and as a groundcover. It is propogated as cuttings and requires full sun.
Sample showed entire missing seedlings in the liners, leaf spots that began at the margins and tips, and dieback beginning at the base of the plant.

Possibilities: Rhizoctonia root rot, Phytophthora, Pythium

Tests done:
1) moisture chamber-showed extensive Botrytis
2) plating roots and leaves onto acidified water agar (AWA), Phytophthora agar and Pythium agar
3) check soil pH and soluble salts (SS)
4) float leaves in sterile distilled water to see if sporangia develop

Results:
1) moisture chamber-extensive Botrytis (common secondary pathogen on ornamentals)
2) plating roots and leaves onto acidified water agar (AWA), Phytophthora agar and Pythium agar-No Rhizoctonia, mixed secondary infections on other agars
3) check soil pH and soluble salts (SS)-within normal range
4) float leaves in sterile distilled water to see if sporangia develop-negative
5) replated roots and leaves onto Phytophthora agar and AWA-negative

Final diagnosis: This is likely abiotic stress (probably nutrient issues) which affected the lower leaves during transplant allowing secondary pathogens (Botrytis, etc) to invade.