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W. Rademacher and T. Bucci

Worldwide, plant growth regulators (PGRs) account for only 3% to 4% of the total sales of plant protection agents. This limited market potential, the rising costs of development and registration, and the demand for high profitability have created major constraints to the introduction of new PGRs. Conversely, PGRs have become an integral part of agricultural and horticultural practices and one might assume that the market is sufficiently lucrative to those companies active in this area. In the past decade, at least seven new PGR products have been introduced. In many cases, reduced requirements for registration have lowered the financial risks relative to expected profits.

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W. Rademacher, G. Stammler and P. Creemers

Many trials have demonstrated that apple and pear trees treated with the plant growth regulator prohexadione-Ca (BAS 125 W) are less susceptible to infestation by the bacterial disease fire blight. In further investigations we have studied the effect of this compound against fungal diseases, concentrating on scab (Venturia inaequalis) in apple. Working with apple seedlings and artificial inoculation under greenhouse and field-like conditions, scab infestation could be reduced by applications of prohexadione-Ca. Whereas this effect was rather marginal if inoculations were made shortly after treatment, highly significant effects were found in the time span of ≈1 to 4 weeks after application. Preliminary results from trials conducted under orchard conditions support these findings. We assume that, similar to the situation with fire blight, changes in phenylpropanoid metabolism are mainly responsible for the reduced scab incidence. It should not be ruled out, however, that anatomical and morphological changes caused by prohexadione-Ca may also contribute to this effect.

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J.R. Evans, C.A. Ishida, C.L. Regusci, R.R. Evans and W. Rademacher

Prohexadione calcium, or BAS-125W, is a plant growth regulator being developed for registration in the United States by the BASF Corporation and Kumiai Chemical Industry Co. Ltd. Prohexadione calcium inhibits the biosynthesis of gibberellin by blocking dioxygenases, which require 2-oxogluterate as a co-substrate. The result is a decrease in cell elongation and reduced vegetative growth. Gibberellin levels are reduced in the plant for 3 to 4 weeks following application of this growth regulator. Prohexadione calcium does not persist in the plant or directly effect vegetative growth the following season. Prohexadione calcium is absorbed by the plant foliage and uptake is generally complete within 8 hr following application. Results indicate that translocation within the plant is predominantly by acropetal movement, while basipetal movement is limited. Preliminary research has shown prohexadione calcium to have a short half-life in the environment; to have no negative effects on non-target organisms; and to offer little risk to users or consumers. Rates of 125 ppm a.i. to 250 ppm a.i. have typically provided effective vegetative control of vigorous apple trees. Generally, as the vegetative vigor of a tree increases; the rate of prohexadione calcium needed for effective vegetative control has to be raised. Vegetative vigor is influence by numerous factors including: fruit load, location, variety, rootstock, age, pruning, and training system. Earlier applications at 2 to 5 inches of new shoot growth have provided more-effective control of vegetative growth than later applications. Due to its short-term effect and lack of persistence, prohexadione calcium can be a flexible tool in developing user specific growth management strategies.

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J.R. Evans, R.R. Evans, C.L. Regusci and W. Rademacher

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G. Costa, C. Andreotti, F. Bucchi, E. Sabatini, C. Bazzi, S. Malaguti and W. Rademacher

As in other countries, pear production in Italy requires an efficient control of excessive shoot growth. Chlormequat chloride, the only available plant growth regulator, does not necessarily represent the ideal solution because it requires high dosages, thus posing the risk of residues in fruit. Fire blight, which has invaded Italy in at the beginning of 1990, is of great concern because `Abate Fetel', one of the predominant cultivars, is highly susceptible and no preparations for control are currently available. To evaluate solutions for both aspects, prohexadione-Ca (BAS 125 11 W) has been tested on pears under growth chamber and orchard conditions. In each case, reductions of shoot growth and, if present, lowered incidences of fire blight could be achieved. Six-year-old trees of `Abate Fetel', located in an area with high fire-blight infestation pressure, were sprayed with prohexadione-Ca four times each at either 50 or 100 ppm of active ingredient at a new shoot length of 6, 12, 33, and 45 cm. The resulting final shoot length was 96% and 75%, respectively, of the control, whereas an average of 15 fire blight incidences could be found in the controls. This number was reduced to 8 and 4, respectively, in the treated trees. Fruit yield and quality was not affected.

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W. Rademacher, J.B. Speakman, G. Krack, M. Scholtissek, R. Wolf, J.R. Evans, S. Roemmelt and D. Treutter

Prohexadione-Ca (BAS 125 W) is currently developed as an inhibitor of excessive vegetative growth in apple. In addition to the control of shoot growth, pronounced effects on the incidence of scab (Venturia inaequalis) and fire blight (Erwinia amylovora) are observed that are not due to any fungicidal or bactericidal effect of the compound. Prohexadione-Ca induces marked changes in the metabolism of phenylpropanoids most likely by inhibiting distinct dioxygenases, such as flavanone 3-hydroxylase, which require 2-oxoglutarate as a co-substrate. The content of flavonoids such as luteoliflavan (which does not normally occur in apple tissue) and eriodyctiol is drastically increased reaching levels in the range of 50 mg per gram of dried young shoot tissue. Simple phenols, the identity of which is still unknown, also undergo intense changes. Since phenylpropanoids have often been found to be involved in defense mechanisms of higher plants, further studies on their role in pathogen resistance in apple are justified from these results.

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J.R. Evans, J.A. Balles, B.A. Brinkman, V.E. Harris, J.D. Helm, K.B. Kirksey, T.E. McKemie, G.G. Thomas and W. Rademacher

Prohexadione-Ca (BAS 12511W or Apogee™ Plant Growth Regulator) acts within a plant by blocking the biosynthesis of growth-active gibberellin. The result is decreased cell and shoot elongation; thus, vegetative growth in apple trees can be reduced. Air blast applications of prohexadione calcium were made in the Spring 1998 in commercial orchards. Application rate was 125 ppm a.i. applied twice beginning at 5 to 12 cm of new shoot growth. Reduction of shoot growth averaged 45% across locations. As a result of reduced vegetative growth, dormant pruning was reduced. In total, significant benefits to the grower included reduced pruning costs in addition to other positive effects such as improved light penetration and enhanced resistance to some pathogens. Research will continue with the effect of prohexadione-Ca on pruning in multiple year studies.