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.

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.