Wednesday, September 16, 2009

Plant Pest: Bacterial Leaf Spot on Ivy

The most common English Ivy (Hedera helix) disease in the South is Bacterial Leaf Spot. This disease, caused by the bacterium Xanthomonas campestris pv. hederae, causes large, irregularly-shaped lesions on the leaves.

As the Latin name indicates, this bacterium only infects Ivy. The abbreviation in the name stands for "pathovar" (the pathology equivalent of "cultivar") and the word hederae tells you that it infects Hedera.

An easy way to identify this disease: Hold a leaf up to the light. If there is a yellow halo around each dark-brown spot, the Ivy is likely infected with Bacterial Leaf Spot.

Why is there a yellow halo? As the bacteria progresses, it uses enzymes to break down the leaf cells. The yellow area is where the bacteria is active.
Bacterial Leaf Spot will be worse in environmental conditions where the leaves stay wet for extended periods of time. Xanthomonas spreads from leaf to leaf via water-splashing or direct leaf contact.

The best way to manage this disease is to keep the leaves dry. That means no overhead irrigation, if possible. Grow in an area with good air circulation and periodically clean up debris and dead leaves.

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