Possible Phytotoxicity on Cordyline

These are Cordyline australis 'Southern Splendor' from a finishing operation. The main symptoms are the distorted growth and fusing together of the 'middle-aged' growth . The new growth is unaffected while the oldest growth shows tip burn. The plants were heavily infested with two-spotted spider mites (Tetranychus urticae) with obvious symptoms of yellowing and stippling present. The question from the grower was, "Is the distortion caused by the mite injury or is some other factor involved?"

Mite infestations are not known to cause such extreme symptoms so we needed to look elsewhere. The most likely cause is phytotoxicity caused by an pesticide application. The grower is looking into their rotation of miticides as mites have been a constant problem in this crop. Since they are a finishing operation (meaning they received the plants a few months earlier to grow to the final selling size) the grower may have difficulty tracking down what pesticides were applied before they took possession.

These symptoms have been seen by diagnosticians in other states, but a definitive cause of the problem has yet to be identified.

Tip burn was seen on the oldest leaves. Cordylines can be very sensitive to fluoride levels in irrigation water which can manifest as tip burn, but fluoride is unlikely to cause the other symptoms.

Two-spotted spider mites and associated injury (yellowing and stippling of older leaves) was observed.

Vinca minor-CMV (Cucumber Mosaic Virus)

This sample is Vinca minor from a propagation operation. The sample tested positive for CMV (Cucumber Mosaic Virus) using an Agdia immunostrip test.
CMV has a very wide host range and affects both vegetables and ornamental plants. Aphids are the primary source of infection but infection by mechanical transmission (taking cuttings, etc) is also possible. Infected plants should be destroyed to avoid spreading the virus within a greenhouse.

Aster-TSWV (Tomato spotted wilt virus)

This sample is Aster x Frikartii 'Monch' from a propagation operation showing signs of virus infection. The sample tested positive for TSWV (Tomato Spotted Wilt Virus) using an Agdia immunostrip test. TSWV & INSV (Impatiens Necrotic Spot Virus) are two related and very common viruses in greenhouse production. Cornell University has an excellent fact sheet with more information on these viruses.

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.

Pythium root rots of bean and watermelon seedlings

Today was a day of root rots! Two samples of pythium root rot but both came in with complaints of foliage problems on field transplants (browning and death of bean leaves & wilting of watermelon). Luckily, the submitters had included the full root system and soil so we were able to quickly make an accurate diagnosis. Pythium agar was used to confirm the presence of Pythium spp. Note the characteristic stem lesions.
Pythium root rot on snap bean transplants:

And Pythium root rot on watermelon transplants:

What's your plant's problem?

Did you know that most U.S. states have a clinic that will diagnose your sick plant for a small fee? The American Phytopathological Society maintains a list of all university clinics. Click here to find one in your state!