Monday, December 29, 2008

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.

Tuesday, December 23, 2008

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.

Monday, December 15, 2008

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.

Wednesday, November 19, 2008

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.

Tuesday, November 18, 2008

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.

Thursday, May 22, 2008

Pythium root rots of bean and watermelon seedlings

Today was a day of root rots! Two samples of pythium root rot but both came in with complaints of foliage problems on field transplants (browning and death of bean leaves & wilting of watermelon). Luckily, the submitters had included the full root system and soil so we were able to quickly make an accurate diagnosis. Pythium agar was used to confirm the presence of Pythium spp. Note the characteristic stem lesions.
Pythium root rot on snap bean transplants:

And Pythium root rot on watermelon transplants:

Wednesday, May 21, 2008

What's your plant's problem?

Did you know that most U.S. states have a clinic that will diagnose your sick plant for a small fee? The American Phytopathological Society maintains a list of all university clinics. Click here to find one in your state!

Wednesday, May 14, 2008

Fennel-Unknown problem

Another delicious smelling sample- fennel (Foeniculum vulgare). This sample presented with browning on the edges of stems at the bulb. The very oldest leaves are yellowing and dying. The main cause is likely that the plant is pot bound (roots are curling around at the bottom). There was no internal decay in the bulb and the majority of the foliage was fine. The bulb is incubating to double check.Above photo is a little bleached out, note that the Jiffy pot has been pulled away for the photo.

Cut edges are starting to brown as I took the above photo ~10 mins after cutting.

Wednesday, April 30, 2008

Rust on Agastache

Another good smelling herb sample! Agastache sp. (pronounced ah gas TAH kee) is a perennial herb which smells like fruity, minty, licorice. This sample presented with raised rust colored pustules on the upper and lower leaf surfaces. All leaves were affected, but the disease appeared to affect the lower leaves first as many had fallen from the plant. Slightly crushing under a plastic coverslip, a wet mount of the pustules revealed characteristic telia of the rust Puccinia. (The morphology of the telia tells me it is in the genus Puccinia.) Some of the characteristic tails, or stalks, of the telia were broken off but many remained intact (see the last photo).

I need to remember to burn the scale into each picture! The software shows the scale on the screen, but doesn't burn into the image unless you go through an extra step. The leaf pictures were taken under the dissecting scope. I don't like this camera as the resolution isn't that great. The teliospore photos are taken using an Olympus camera and their software, MicroSuite Five. I tried using the merge function on the last photo (big clump of spores) to show the 3D effect, but it appears I need more practice. I saved leaves from this sample and will attempt better photos when time permits.

Saturday, April 26, 2008

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.

Tuesday, April 15, 2008

Lettuce-Powdery Mildew & Rhizoctonia

This was a lettuce sample from an organic hydroponic system. Three heads were submitted-two butter (pictured above) and one that looked like Romaine. There was a lot happening: white powdery growth on the leaves, dieback at the crown, lots of insects crawling around.
Below is a closeup looking across the top of the leaf surface. If you click on the image, you'll see the powdery mildew conidia chains in good detail. This can be seen in the field by rolling the leaf around your finger and looking perpendicular to the leaf with a hand lens. Powdery mildew is caused by the Ascomycete, Erysiphe cichoracearum. Didn't see any cleistothecia on the leaves, but I saved the sample to see they can be induced to form somehow. Here is a closeup of the crown tissue and problem #2. The greenish powdery substance is the growth media, probably an Oasis type media. The arrows are pointing to white mycelium, which turned out to be Rhizoctonia (click photo for bigger image).

Thursday, April 10, 2008


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!

Tuesday, April 1, 2008

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.

Monday, March 31, 2008

Downy Mildew on brassica (cabbage & broccoli) seedlings

Description: Scattered, irregular-shaped, sunken, gray-black lesions on a mixed sample of cabbage and broccoli seedlings. Symptoms ranged from subtle (broccoli) to more severe (cabbage).

Tests done: The diagnostician had already recognized this as downy mildew (based on experience), but I decided to place leaves in a moist chamber to induce sporulation just to double-check.

Final Diagnosis: Downy mildew caused by Peronospora parasitica. Forgot to take a picture of the diagnostic sporangiophores and sporangia! The University of Florida has a good factsheet describing symptoms, although the accompanying photos seem over-exposed.

References used: Compendium of Brassica Diseases

Wednesday, March 12, 2008

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

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.