Seasonal transitions: a semi-constant state in the realm of floriculture, but here in Santa Cruz, production is definitely on the wane as our version of winter gradually sets in. We have had our first light frosts of the season, but nothing like the killing frosts that most of you experience in other flower growing regions. On the production side, here in mid December we still have marigolds, tithonia, bells of Ireland, mignonette, nigella, larkspur, calendula and stock blooming outside. Overall, flower quality and yield is generally low at this time of year. Stock and mignonette, however, seem to be at their happiest overwintering here in Santa Cruz.

In late spring and early summer, we are lucky if we get 15” tall mignonette and 20” stock. Both plants grow well but with increasing temperatures and day-length, they simply do not produce long and strong stems. Winter is a different story completely. Perhaps because they are stretching for the low-angled sunlight and because they grow relatively slowly, we get long and strong mignonette and stock stems at this time of year. The mignonette typically will be 20-24” between now and late March and, with reduced pollinator flight, it holds on the plant much longer than in the main season. Our overwintered stock is also a dramatic improvement over summer-grown stems. October and November succession plantings yield vibrant, fragrant, stout and long-holding stems that regularly exceed 24” and like their mignonette counterpart, they hold in the field well for a week or more once they begin to open.

On a more challenging note, my entire collection of dahlia tubers, 600 row feet of huge clumps, has been dug and is now feeding some very lucky pigs. Perhaps this is going to be the latest culinary rage: pasture-raised, organic pigs finished on dahlia tubers and oak acorns, coming to the finest restaurants near you soon. Sadly, we seem to have invited some dahlia mosaic virus to come join our plantings two years ago and since that time the virus has spread to virtually every plant. Like most viruses, dahlia mosaic virus has three principal means of introduction onto your farm. The first and most likely is from planting out already infected tubers or cuttings. Another common vector is pest insects such as aphids and thrips. Finally, tissue injury is a common mechanism for the spread of DMV. Tissue injury can easily occur via mechanical weed cultivation, hand cultivation, staking, netting, and of course, during harvest. Harvest injury, which happens every time we cut stems, is inevitable, but the spread of DMV during harvest is a matter of human agency.

Whenever we harvest from infected plants, we contaminate our tools with the virus. If we then harvest from “clean” plants with “dirty” tools, we become the vector by introducing DMV to the exposed tissue and vascular system of the previously uninfected plants. Essentially, we are doing the exact same thing as rasping and sucking insects like aphids and thrips as they move indiscriminately from plant to plant in a quest for food. The only real difference is that we may be able to prevent the spread of DMV by first harvesting our clean plants and then moving to the infected plants, followed by a thorough cleaning of our harvesting equipment with dish soap or rubbing alcohol.

However, from experience, I can tell you that this is a flawed strategy. Once you have DMV, short of destroying your plants or feeding them to a friend’s pigs, there is no way to eradicate the virus. Really, it is just a matter of time before you or your crew inadvertently spread the virus or insects do the job for you. Thus, early recognition, proper identification and roguing of contaminated plants are essential to prevent the spread of this disease.

Like most viruses, DMV symptoms appear as spotting, mottling, vein clearing, necrosis, chlorosis, leaf distortions, shortened internodes, stunted growth and reduced flower production. Dahlia cultivars show some variation in their expression of symptoms and some varieties can be asymptomatic, beyond reduced plant vigor and stem production.

You can use online photos to identify infected plants. I would start with two by Dr. Hanu Pappu, a plant pathologist from Washington State University: http://plantpath.wsu.edu/pdf/Binder11.pdf and http://www.dahlia.org/uploads/bulletin_articles/ADS-DMV_Symptoms_Slides.pdf. If you see any symptoms similar to those in the photos, immediately send tissue samples to your cooperative extension plant pathology lab and ask them to test specifically for dahlia mosaic virus. Until the lab confirms or eliminates the possibility of DMV, institute a very deliberate harvest pattern, harvesting potentially infected plants only after you have harvested plants that show no signs of DMV.

While dahlias are susceptible to several other viruses, DMV seems to have the greatest potential to impact plant productivity and thus, your bottom line. As a result of my lack of vigilance and early intervention, I am spending nearly $1000 to restart our dahlia program, replacing 600 row feet of giant tuber clusters with 1-2 tubers per hole for the 2012 season.

Christof Bernau

UCSC Center for Agroecology

Christof Bernau UCSC Center for Agroecology Contact at [email protected]