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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.

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.

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.

Prohexadione-Ca is currently developed as an inhibitor of excessive vegetative growth in apple. In addition to the control of shoot growth, distinct effects on the incidence of fireblight are observed that are not due to any bactericidal effect of the compound. Further, the compound often causes a delay of senescence. It is known that prohexadione-Ca acts as a structural mimic of 2-oxoglutarate thereby inhibiting dioxygenases, which catalyze distinct steps in gibberellin (GA) biosynthesis. As a result, levels of growth-active GAs are lowered and, hence, longitudinal shoot growth is reduced. Prohexadione-Ca also induces changes in flavonoid metabolism, which may also be related to the inhibition of dioxygenases involved in this pathway. Preliminary evidence is available that this effect is related to a lowered susceptibility of apple tissue towards infection with fireblight. ACC oxidase, another dioxygenase, which catalyzes the conversion of aminocyclopropane-carboxylic acid into ethylene, is also affected by prohexadione-Ca and related compounds. Lowered levels of ethylene may account for a delay of senescence.

Prohexadione-Ca (calcium 3-oxido-4-propionyl-5-oxo-3-cyclohexene carboxy-late), an inhibitor of distinct deoxygenase which catalyses the last steps of GA metabolism and the hydroxylation of flavanones, has been found to control vegetative and cropping performance and to counteract some pathogen infections (scab and fireblight) in apple and pear. To further evaluate these effects, prohexadione-Ca was tested under northern Italian climatic conditions on two pear varieties of differing vigour (William/Kirkensaller, Abbé Fétel/Beurré, and Hardy/Quince C). Prohexadione-Ca was applied at different concentrations and application schedules (50 ppm x 4 times and 125 x 3 and x 4 times) starting from 5-cm shoot length up to the onset of secondary bloom. Data were collected on vegetative, cropping, fruit quality parameters and incidence of Erwinia amylovora in field conditions. Prohexadione-Ca controlled vegetative growth on both cultivars, although the effects were related to the differing vigor of the cultivar-stock combinations. Cropping performance was also affected: on the vigorous William/Kirkensaller, yield was increased by the concentration while average fruit weight was unaffected; on Abbé Fétel/Quince C, prohexadione-Ca was ineffective on yield/tree but significantly increased average fruit weight.

Growth retardant could represent an interesting tool complementary to the cultural practices to maintain vegetative/reproductive equilibrium. Prohexadione-Ca, a new bioregulator, showed to be quite effective on pomefruit although little information is available on the residual effect on apple. The experiment was carried out in Emilia-Romagna region in 1999 and 2000 on a seven-year-old Fuji/Mark 9 orchard. Prohexadione-Ca was applied as a single or multiple sprays at 125, 175 and 250 ppm; sprays were applied at 5 mm and 20 mm shoot length. Treatments were repeated for two years on the same trees. Results were compared with untreated control trees and with treated only in 1999. In both years prohexadione-Ca reduced shoot length. No residual effect was found in the trees that received the chemical application only in the first year. As far as the reproductive parameters were concerned little positive influence was noticed in both years on average fruit weight although trees sprayed tended to yield more than control. Prohexadione-Ca positively affected return bloom

To further investigate the efficacy of prohexadione-Ca on scab control, experiments were carried out on Golden Delicious seedlings and scions grown under greenhouse conditions. The efficacy of prohexadione-Ca against scab was compared with the one of other types of growth retardant. Furthermore, the time required to build up resistance was determined. Four growth retardants (chlormequat chloride, paclobutrazol, prohexadione-Ca, and trinexapac-ethyl) were applied at different concentrations and timings prior to inoculation with Venturia inaequalis. The results showed differential effects of the tested chemicals as related to time of application and to concentration. Chlormequat chloride was unable to control scab under all the tested experimental conditions. Paclobutrazol showed an initial, but transient effect, which is most likely due to its fungicidal properties. Prohexadione-Ca and trinexapac ethyl led to reduced scab incidence starting approximately 15 days after application. It appears likely that the latter two compounds, which are structurally related and have a similar mode of action, are inducing a physiological form of resistance, which is independent of morphological or anatomical effects. Accompanying experiments with prohexadione-Ca on older apple plants also pointed out that some three weeks are required to allow the treated trees to build up resistance against scab.