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.