Powerful Poinsettias

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Powerful Poinsettias

Producing poinsettias in today’s competitive market requires a great commitment of time and energy (and a bit of Tylenol). Taking steps that make your operation more sustainable can greatly increase your overall efficiency, reduce waste and boost your price per finished plant. Preventing nutrient imbalances that lead to deficiencies and toxicities can add a few extra steps at the start of the season, but you will reap the rewards, not to mention avoid the devastating damage a disease can spread throughout your entire greenhouse. Tracking the nutrient and disease status of your crop will improve your efficiency by saving you time on the end. Here are some ways crop tracking can improve your plant quality as well as overall sustainability.

An ounce of prevention is worth a pound of cure; it may be a cliché, but its message is worthwhile. As we come into another poinsettia season, we often are entering into the unknown. Yes, you have grown poinsettias for a number of years, but think back over the last few seasons. Each season throws something new at you. How do we know what to expect from 2008? The best bet is to know what you are starting with.

Establish A Baseline

Start Disease Free. Sampling of different cultivars for root pathogens at the rooted cutting stage is an excellent way to ensure that your cutting is disease free. One of the most common diseases of poinsettias is Pythium root rot and black leg. The cause of the disease is either Pythium aphanidermatum or Pythium ultimum. Disease often starts during rooting of the cuttings and can rapidly spread throughout the crop in ebb and flood systems (Figure 1). These fungi are soil-borne and spread via irrigation water, use of contaminated potting media, pots and rooted cuttings. Roots appear water-soaked are mushy and disintegrate when handled. A soft, watery rot anywhere on the cutting stem is also a sign of Pythium infection.

In the cutting phase, symptoms are easily confused with other cutting diseases, including Rhizoctonia stem rot and Erwinia blight. Erwinia is usually easy to distinguish since cuttings fall apart and completely collapse into a slimy mass within a couple of days. The characteristic rotten fishy odor is usually present when Erwinia has caused the disease. There are no reliable, clear-cut differences in symptoms from Rhizoctonia or Pythium. Since control strategies differ significantly, you must obtain a diagnosis to choose the correct path.

Water Quality. An initial water analysis at the beginning of your production cycle is crucial in producing a powerful poinsettia. At least two weeks prior to potting up the poinsettia crop, send in a water sample for analysis. Collect the sample from the source of water by running the water a while until fresh and then collecting a gallon or so in a large, clean plastic container. Dip the sample bottle into the water and cap it off. This method of collection eliminates trapping bubbles from a hose or faucet. Including any air might influence the analysis.

Water quality has a direct influence on the plant media and the availability of nutrients in that media. Since poinsettias are long-term crops, the grower must supply all necessary nutrients for optimum plant growth. Selection of fertilizer must be made with water quality, plant needs, stage of growth and interaction with growing medium in mind. Table 1 shows the water quality guidelines specific for poinsettia production.

Media. Poinsettia media must be well-drained and well-aerated. It must also hold enough water and plant nutrients to facilitate optimum plant growth. It should be pathogen free and physically stable for the duration of the crop. Un-composted organic components should be avoided as they may utilize available nitrogen and create a deficiency situation for the plant.

Controlling medium pH and EC depends on lime charge, media components, water quality, fertilizer selection, irrigation practices, growing environment and stage of plant growth. It is essential to monitor media pH and EC frequently with both in-house tests and through submission to a horticultural laboratory. In-house testing should be performed on each significant segment of the crop (dark versus light leaf plants, 6-inch versus 8-inch pots, trees versus mini pots, etc.) on a weekly basis. Monitor key crop segments every three weeks to verify in-house testing trends and also to understand the composition of the soluble salts that comprise the EC and affect pH. Sample more frequently if crop conditions or history indicates a need.

Two weeks after potting, media should be sent in for complete analysis. By this time, the lime and other media additives will have had time to react for a truer picture of the root zone chemistry compared to submitting an unused media sample. Unused media should be sampled only if subsequent sampling is done (two weeks later) to note the initial changes that occur.

Consistency in sampling is key to understanding and interpreting analysis results. Take a sample from the root zone area, excluding the top crust of media where salts are most likely to accumulate. Discussion abounds regarding exact technique of sampling media. Again, decide on a technique and be consistent about sampling at the same interval following irrigation with fertilizer. Taking the sample 24 to 48 hours after fertilization is recommended. (Figure 2)

When Problems Occur

Cultural effects and control of disease. There has been quite a bit of research done on controlling Pythium infection with focus on cultural conditions that promote Pythium root rot as a special focus. Disease has been found worse at pH above 5.5. Keeping the pH lower will help reduce severity. Disease increased 100 percent with a pH change from 4.5 to 6.8.

Disease severity increases as fertilizer application increases. Plants potted in peat-vermiculite appeared more sensitive to high soluble salts than those in other media. The moisture-holding capacity of the potting medium is also critical. Moisture-holding capacity above 70 percent seriously increased Pythium ultimum. Use of highly decomposed peat (dark) results in worse Pythium root rot (P. ultimum) compared to medium or light peat that is not greatly decomposed. 

Chemical And Biological Control. Rootshield (also called PlantShield–Trichoderma harzinanum) worked better than SoilGard (Gliocladium virens) but was less than Banrot (thiophanate methyl and etridiazole) when used as a drench at 12 oz./100 gal. Poinsettia cuttings in root cubes soaked in Subdue (metalaxyl) had slightly reduced root development but not as much as other fungicides tested. Trials on poinsettias with Pythium root rot showed that Heritage (azoxystrobin) and RootShield did not control the disease. Subdue was effective even under high disease pressure.

Common Nutrient Deficiencies

Molybdenum. Molybdenum (Mo) is needed by poinsettias in greater amounts than in other crops. It should be intentionally added on a CLF basis. Mild deficiency affects nitrogen utilization in the plant and progresses to cause yellowing in young leaves with upward rolling and edge burn. Leaf tissue may not expand properly in some cultivars due to distortion. Excessive amounts of Mo cause little or no plant damage.
Molybdenum may be difficult to detect at low levels in root media, but if a grower is consistently adding Mo, tissue analysis of most recently mature leaves may show very high levels (>15 ppm) as the element is readily taken up at proper pH levels.

Calcium. Calcium (Ca) is primarily moved into the plant via mass water flow rather than ionic root action. For this reason, deficiencies are not only a result of low concentration in root media, but can also result if environmental conditions restrict water flow through the plant (high humidity, stress, low light/temperature). Maintaining good air flow around plants via horizontal air flow enhances Ca uptake. Calcium recommendations vary, but most experts agree that supplying 100 to 150 ppm Ca through media and fertilizer application is sufficient. Alternating or co-injection of calcium nitrate, or 15-0-15 type fertilizers, with complete fertilizers is recommended. Or, N-P-K-Ca-Mg fertilizers like 15-5-15, 13-2-13 or 17-4-17 can be used.

Foliar Applications of Ca. Calcium is also absorbed through foliar application. Some growers use foliar applications of calcium nitrate or calcium chloride at between 300 and 400 ppm Ca on a weekly basis beginning when new breaks are in active growth phase and environmental conditions restrict uptake via the roots. Many growers begin foliar applications of Ca on a weekly basis at first sign of bract color. This is done to prevent bract edge necrosis.

Taking preventative steps to avoid nutrient imbalances and the devastating effects of disease will lead to a strong and successful poinsettia crop. Early analysis by an established horticultural nutrient or diagnostic lab gives you the information to be confident that you are starting to build yourself a high-quality crop of powerful poinsettias.

Dr. Cari Peters is vice president at J.R. Peters. You can e-mail her at caripeters@jrpeters.com.

A.R. Chase is president and plant pathologist at Chase Horticultural Research. Chase can be reached at archase@chaseresearch.com.

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