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.

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).

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.

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