Controlling aggressive bedding plants with PGR plug dips

Matthew Blanchard and Erik Runkle
Horticulture

April 16, 2008 -- Spring bedding plant production is well underway and many crops are being planted now for sales in Michigan later this spring. Controlling stem extension of aggressive annuals continues to be a struggle for some growers, especially for plants grown in mixed containers. In some situations, growers cut back aggressive species that have outgrown their container before the shipping date. Although this strategy is effective, it can be both labor-intensive and costly.

One height control technique that has been used successfully by an increasing number of growers is a plant growth retardant (PGR) “plug dip” or “liner dip.” The concept is simple: the plug tray is soaked in a PGR solution before transplant and the chemical is absorbed by the media. This strategy allows for many plants to be treated at once and can offer several weeks or more of height control depending on the rate, volume and growing environment. PGR plug dips are particularly useful when producing mixed containers that contain species with different growth rates and PGR responses. In mixed containers, aggressive species grow faster than less vigorous ones, thus dominating the container quickly (Figure 1). A plug dip on aggressive plants before transplant can allow the less vigorous plants time to develop in the mixed container and improve crop quality.

Over the past several years, university researchers have performed experiments to determine appropriate application rates and to evaluate factors that may influence PGR plug dip effectiveness. Below are three key factors that can influence the response of a PGR plug dip. Controlling these factors can improve success and reduce the possibility of obtaining inconsistent responses.

1. Application rate.

The application rate influences the amount of PGR active ingredient absorbed by the media and thus, the magnitude of the response. For example, plugs of calibrachoa ‘Callie Dark Blue’ dipped in Sumagic (uniconazole, Valent Professional Products) at 2, 4, or 8 ppm were 37, 41, and 59 percent shorter than nontreated plants when measured four weeks after application (Figure 2). Table 1 provides a list of suggested application rates for bedding plants that we have studied at Michigan State University. These rates are based on growing conditions in Michigan during mid- to late spring. Growers are advised to perform their own trials on a small scale to determine appropriate application rates for their growing conditions and desired response.

Application rate for a plug dip may be comparable to a rate used with a PGR drench application. However, an advantage of a plug dip compared to a drench is that there may be less potential for a flowering delay with a plug dip because the PGR does not contact the stems or leaves.

2. Duration of plug dip.

The duration of the plug dip can affect the volume of solution that is absorbed by the media. We recommend that growers begin with a 30-second dip and adjust the time if necessary. If the media is saturated at the time of application, there may be little difference in response between a 30-second versus a 2-minute dip. However, if the plug is dry at application, a 2-minute dip will absorb more solution than a shorter duration and therefore increase the response.

3. Moisture content of media.

The moisture content of the plug media will affect how much solution is absorbed during the dip. A plug that is dry at the time of application will absorb more of the PGR solution than a saturated plug and therefore produce a greater response. For example, moist plugs of verbena ‘Escapade Bright Eye’ dipped in Piccolo (paclobutrazol, Fine Americas) for 30 seconds were 6.5 inches taller when measured six weeks after application than plugs with dry media at the time of the dip (Figure 3). We recommend that plugs are moderately dry at the time of application. This can be accomplished by irrigating the plug trays during late afternoon on the day prior to the dip and performing the application on the following morning.

In summary, a PGR plug dip is a very effective height control strategy for controlling stem extension in aggressive species. There are several factors to consider when using this technique to maintain consistency between applications. Each greenhouse operation should develop a repeatable application protocol that provides the desired height control. We encourage growers that are not using this PGR application technique to give it a try.

Table 1. Suggested plug dip rates for growers in northern climates. Growers should perform their own trials on a small scale and adjust these rates based upon the desired response.

Species	Paclobutrazol (Bonzi, Piccolo)	Uniconazole (Concise, Sumagic)
Argyranthemum	6 to 8 ppm	3 to 4 ppm
Bacopa	4 to 8 ppm	not tested
Calibrachoa	6 to 8 ppm	2 to 4 ppm
Coleus	6 to 10 ppm	not tested
Nemesia	not tested	2 to 4 ppm
Osteospermum	4 to 8 ppm	not tested
Petunia Cascadias	4 to 6 ppm	1 to 2 ppm
Scaevola	4 to 6 ppm	2 to 4 ppm
Sweet potato vine	1 to 3 ppm	not tested
Verbena	8 to 12 ppm	2 to 4 ppm
Wave petunia	6 to 8 ppm	not tested

Fungicides and black root rot
Mary Hausbeck, Plant Pathology and
Thomas Dudek, MSU Extension

April 24, 2008 -- While root rots caused by Pythium and Rhizoctonia are the most common among greenhouse crops, black root rot is a serious threat to pansies, viola, petunias and vinca. Black root rot is caused by the fungus Thielaviopsis and may also infect cyclamen, poinsettia, primula, impatiens, snapdragon, verbena, phlox, begonia and nicotiana. Plants with black root rot often show symptoms that mimic nutrient deficiencies such as stunting with older leaves shriveling. Leaves may turn yellow and the youngest leaves become stunted and tinged with red. In mild infections, older leaves are yellow-green with the veins retaining their green color. Black root rot may also affect the lower stem on crops such as poinsettia, causing cracks that appear black.

Sanitation is the best preventive measure against black root rot. Once this fungus is established in a crop or in a greenhouse, an effective fungicide program is needed. Based on studies conducted in my lab, we recommend fungicides that have thiophanate-methyl as the primary active ingredient (Cleary’s 3336 F is an example) be used frequently at the high labeled rate. A good rotational product is Terraguard 50W since it has a different mode of action and has shown to be effective in my studies against black root rot. Also Medallion is another product to control this disease.

Choosing an effective fungicide to control black root rot is critical because a misstep early in the disease epidemic may result in an unsalable crop. If the crop is treated for Pythium root rot when black root rot caused by Thielaviopsis is really the problem, not only will time and money have been wasted but the disease will have a head start in causing damage to the crop before it can be halted with the correct fungicide.

Dr. Hausbeck's lab conducted two trials to look at how Cleary’s 3336 F and Terraguard 50W compare with new fungicides and biocontrol agents in controlling black root rot. We used pansy and vinca crops because of their frequent problems with black root rot. We evaluated plant height and death as a measure of whether the fungicide provided protection. In our pansy study, only drenches of Terraguard 50W or Cleary’s 3336 F limited plant death to 25 percent or less, compared to plant death of over 70 percent when no fungicide was used. Also, drenches of Terraguard 50W or Cleary’s 3336 F prevented the severe stunting that occurred when fungicides were not used. In our black root rot trial with vinca, few plants died overall even without fungicide treatment. We did, however, see differences in the ability of the fungicides to prevent plant stunting caused by black root rot. Drenches of Cleary’s 3336 F or Terraguard 50W or Medallion provided plant protection better than all other fungicide drenches tested in this trial.

Sometimes new isn’t always better and for black root rot the standard program of Cleary’s 3336 F rotated with Terraguard 50W is the way to go. Since this was our first look at some of the products included in these studies, it is possible that higher rates and shorter intervals may be helpful in improving black root rot control. Until further testing is done, stick with these proven fungicides to ensure the best control.

Here are some additional cultural practices that growers should keep in mind if they have had a positive identification of Thielaviopsis black root rot in their greenhouse crops:
First, do not reuse old plug trays. We suggest you can disinfest them with a 10 percent sodium hypochlorite solution or some other sanitizer. This disease has a persistent spore stage due to a specialized cell wall that the spore produces, which allows it to persist for several years on the floor, bench or on the plug tray. We recommend buying new trays every year.

Second, if a certain location in your range is where black root rot was found in 2008, rotate away from any black root rot susceptible crops in succeeding years. This is similar to crop rotation used by farmers in outdoor growing systems. We have noticed black root rot problems right to the "foot print" where the infested flats had the problem a previous year when the pansies, as an example, were put down there the next season. Remember, the spore can survive for several years.

And finally, we have recommended the removal and replacement of ground weed mat from greenhouses where the flats were set on if black root rot was diagnosed. One cannot adequately sanitize the ground weed mat to insure a clean start, so start with new fabric on the floor. If you combine these suggestions with a preventative fungicide program as described earlier, you can reduce the risk of reintroducing black root rot into your greenhouses.

Geraniums and magnesium deficiency

Dean Krauskopf, MSUE educator

I’m seeing a lot of geraniums with yellowing lower leaves that have dark green veins. These leaves may also be reddish on the edges. These symptoms indicate a possible magnesium (Mg) deficiency. There are two reasons Mg deficiency can occur: very low magnesium in the media and interactions with potassium (K) and calcium (Ca). If Mg is less than 4 percent of the total soluble salts [ppm Mg/(soluble salts x 700)], deficiency symptoms and reduced growth results because high levels of Ca and K can block Mg uptake. If your media test shows less than 4 percent Mg, one application of ½ pound Epsom salts per 100 gallons of water or switching to a fertilizer containing Mg usually corrects the problem. Make sure the media is moist and soluble salts are low before applying Epsom salts since the application will increase soluble salts significantly.