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).