Adept Phytotoxicity in Poinsettia

Robert Berghage

In late September, there were reports of poinsettia with severe leaf burn in several greenhouses in Pennsylvania. The symptoms included leaf chlorosis, tip burn, leaf margin burn, and in severe cases unusual brown or tan crystalline material on the leaf in the damaged regions (Figure 1). The symptoms at first glance were very similar to Cycocel toxicity, and since they appeared at about the same time, it seemed likely, that might be the problem. However some of the affected plants had not been treated with Cycocel and damage continued to develop indicating that the problem must be something else. Nutritional testing did not point to a nutrient toxicity. Further investigation suggested that a possible common link might be the application of the biofungicide RootSheild", (BioWorks, Inc) and or the insect growth regulator Adept" (Uniroyal Chemical).

In an effort to determine the cause of the problem, Adept and RootSheild were applied to poinsettia plants in the Horticulture Greenhouses at Penn State. Damage from high rate applications was observed about 4 weeks after treatment. Cultivar differences in the severity of the phytotoxicity were also observed. A larger trial was then initiated to attempt to gain a better understanding of the problem. Poinsettia plants were treated with 0, 1x, 5x and 10x the label rate of Adept. Root shield was not included in this trial. Three cultivars were treated, 'Freedom White', 'Angelica Marble' and 'Sonora'. Plants were grown in a 24C greenhouse for 7 weeks after treatment. They were then harvested and the total leaf area, damaged leaf area, number of leaves with chlorosis, tip burn, marginal burn, and latex eruptions were recorded for each shoot of each plant. No data were collected from the untreated plants since no damage was observed on any of these plants.

At the recommended rate (1oz / 100 gal) there was a slight yellowing of the leaf tips in many of the Adept treated plants that was not observed in untreated plants. This leaf tip yellowing was minor and would probably not have been noticed had these plants not been subjected to close scrutiny. At higher rates, (5 and 10x) Adept caused significant damage to the poinsettia cultivars tested. The amount of damage increased with increasing Adept application rate. The leaf area afflicted increased from near 0 when the recommended rate was applied to 1.1 - 3.2% at 5x and 1.4 - 5.0% at 10x. The total damaged area of the leaf was more or less the same for each of the cultivars, but the type and severity of the damage differed from one cultivar to the next. Sonora was most susceptible to tip burn with 65% of the leaves displaying that symptom after 5 and 10x Adept treatments. Sonora also had more leaves with latex eruptions than either Freedom White or Angelica Marble. Latex eruptions were an interesting symptom observed in the afflicted tissue of the most severely damaged plants. Cells in the damaged area apparently burst, resulting in the formation of a small bead of latex. These latex beads dried forming small brown or tan crystals attached to the necrotic regions of the leaf. Latex eruptions seem to be diagnostic of damage caused by Adept.

Leaves expanding during or shortly after the time of treatment were most affected. Damage also varied from shoot to shoot on any given plant. Leaves on some shoots were severely damaged, while leaves on other shoots on the same plant were undamaged or showed only minor symptoms. In general, the most damaged shoots were the large dominant ones. The least damaged shoots were the smaller sheltered ones in the middle of the plant. Some plants had damage on leaves of only a few shoots, while most of the leaves on the plant were undamaged. The distribution of damage seemed to reflect uptake and transpiration. The larger dominant shoots were more damaged because of higher transpiration. In plants where only one or two of the dominant shoots were affected it seems likely that there was an uneven application or uneven root uptake of the drench. This is similar to uneven distribution of plant nutrients observed when different parts of the root system are supplied nutrients and others are not. When nutrients are supplied to only one part of the root system the concentration of the nutrient in the shoot tissue supplied by that root is increased while nutrients in other shoots are not. It is likely that Adept (or most likely its breakdown products) as a drench applied, transpiration stream distributed substance would likewise end up in specific shoot tissues if the concentration were higher in the corresponding portion of the root zone. This could come about as the result of uneven application or distribution of the drench in the medium and likely explains much of the damage observed in commercial greenhouses. Unless great care is taken in applying the drench, it will mostly saturate the zone directly beneath where it is poured onto the medium surface. If the proper amount of active ingredient for treating the whole pot ends up in only part of the medium the result is a higher than recommended concentration in that zone. Water channels and contiguous macropores in the medium and uneven distribution of the root system can further compound this. It has been suggested that the addition of RootShield may have contributed to the problem in some commercial greenhouse ranges. Although that suggestion was not tested in this study, it remains a clear possibility. RootShield has been reported to increase root growth and activity in treated plants. Increased root activity might lead to increased uptake and enhanced phytotoxicity.

The difference between a poison and a medicine is the dose. Too much of almost any pesticide can be phytotoxic. The key to safe use of a pesticide is applying enough to do the job at the right time and place. Apply too much, apply unevenly, or under the wrong conditions and non-target plant damage is a likely result. Based on our observations the IGR Adept has a relatively narrow window of safety when used on poinsettia in the fall. Growers should be very careful to follow the label, being sure to apply the chemical evenly to avoid overdosing any portion of the root system.

Summary Adept can cause significant damage in poinsettia when applied at 5x or 10x the label rate. The symptoms of Adept phytotoxicity include leaf chlorosis, tip burn, leaf margin burn, and in severe cases latex eruptions in afflicted tissues. The damage was primarily in tissues with high transpiration and growth rates. Some shoots exhibited more damage than others did, maybe due to greater root activity or uneven drench application.

Figure 2. Percent of total leaf area damaged by application of Adept drenches applied at three rates to three cultivars of poinsettia.

Figure 3. The number of leaves with tip burn in three cultivars of poinsettia plants drenched with 3 rates of Adept

Figure 6. Number of leaves with latex eruptions on three cultivars of poinsettia drenched with 3 rates of Adept.

Landscape & Nursery Trends

Jim Sellmer

Botanical Name	Common Name
Cotoneaster praecox	Early Cotoneaster
Euonymus fortunei "Emerald Gaiety"	Emerald gaiety Euonymus
E. fortunei "Canadale Gold"	Canadale Gold Euonymus
E. fortunei "Sarcoxie"	Sarcoxie Euonymus
E. fortunei "Alata Compacta"	Alata Compacta Euonymus
Forsythia ovata "Northern Gold"	Northern Gold Forsythia
Hibiscus syriacus "Arden" and "Lucy"	Rose of Sharon varieties
Hydrangea paniculata "Grandiflora"	Peegee Hydrangea
Malus "Red Jade"	Red Jade Crabapple
Malus "Sir Lancelot	Sir Lancelot Crabapple
Malus baccata "Gracilis"	Weeping Siberian Crabapple
Malus "Royal Beauty"	Royal Beauty Crabapple
Caragana arborescens "Lorbergii", Pendula, "Walker", & "Globosa"	Weeping and Globe Peashrubs
Catalpa bignonioides "Nana"	Umbrella Catalpa
Syringa chinensis "Saugeana"	Chinese Lilac
Syringa meyeri "Palibin"	Dwarf Korean Lilac
S. vulgaris "Sensation", Mme. Lemoine","Charles Joly", & "Katherine Havemeyer"	French lilac cultivars
Ulmus glabra "Pendula"	Camperdown Elm
Viburnum x Carlcephalum	Fragrant Snowball Viburnum
Viburnum opulus & V. opulus "Roseum"	European Highbush Cranberry & European Snowball Viburnums
Weigela florida "Nana Variegata" & Red Prince	Variegated and Red Prince Weigela

On one hand, this may be a very good method for expanding the market of attractive, flowering, and distinctive shrubs and trees into a patio garden. On the other hand, there are some questions to be asked because each plant has a distinctive growth habit, hardiness, and pruning requirement. For example, Weigela as a shrub may die back to the ground under extreme winter conditions. On a standard, this plant may require winter protection in a similar fashion to tree roses. Similarly, "Northern Gold" forsythia, an upright shrub can grow to 6-8' tall. Grafted on a standard this forsythia may become difficult to manage and prune to maintain form as well as rejuvenation pruning of the older less floriferous canes.

The potential of losing the cultivar on top of the standard should be of concern to purchasers of these plants. Several conditions could lead to the top snapping off. First, the maintenance and environment in which the plant is grown under in the landscape. For instance, the sheer weight of poorly maintained plants under gusty wind conditions or heavy ice and snow accumulation during the winter months. Making the customer aware of the plants maintenance requirements and potential overwintering needs will be important in avoiding this problem. Secondly, graft incompatibility can result in plants growing for years with the graft union between the two species weakening over time and the top finally breaking out of the specimen. Purchasers of these types of plants should ask the dealer what the understock species or genus is and whether there is any compatibility problem between the grafted units. Further, what is the warranty and replacement policy on the plant, in case of graft incompatibility?

All in all this new niche for small area landscape plants may prove very interesting when used properly in the landscape and maintained to assure health and size constraints. Watch for these plants in your local garden centers and nurseries.

If you have, programs that you would like added to the calendar, please forward the information to Jim Sellmer at jcs32@psu.edu. Review the information provided on the site and provide similar information for your program. Thanks!

Checkout the Conference Calendar Website at http://hortweb.cas.psu.edu/ohortex/greenindustrycal.html

If you have, programs that you would like added to the calendar, please forward the information to Jim Sellmer at jcs32@psu.edu. Review the information provided on the site and provide similar information for your program. Thanks!