Prohexadione-Ca in apple trees: Control of shoot growth and reduction of fire blight incidence in blossoms and shoots

Prohexadione-Ca is a new, multi-functional plant bioregulator for use in pome fruits. In apples, applications at a few centimetres of new shoot growth led to clear reductions in vegetative activity in all cultivars tested. Cvs. 'Elstar', 'Golden Delicious', 'Jonagold', 'Rubinette', 'Sampion' and some of their mutants were more responsive than cvs. 'Red Boskoop', 'Braeburn', 'Freedom', and 'Fuji Nagafu 6'. Moderate reductions of shoot growth were almost entirely due to shortening of the internodes. However, more severe reductions in shoot length additionally involved lowered numbers of internodes. Due to less acrotony, a higher number of lateral buds was enabled to grow, which led to improved feathering. Prohexadione-Ca represents a considerable alternative to antibiotics for the control of fire blight (Erwinia amylovora) in pome fruit trees. Indoor experiments with apple trees and artificial inoculation confirm effects both against blossom and shoot infections. After pre-treating trees with prohexadione-Ca, blossom infections could be controlled at the same degree of activity as with regularly applied streptomycin. In trials on shoot infections, prohexadione-Ca-treated trees were much better protected than those treated with streptomycin or kasugamycin. The simultaneous control of excessive shoot growth and shoot infections by fire blight is seen as a major advantage of using prohexadione-Ca in pome fruit trees. However, limitations are still obvious in using prohexadione-Ca against blossom blight. Combinations with other non-antibiotic preparations could lead to a sufficiently reliable control. All in all, prohexadione-Ca will be of great value as a component for a new control strategy against fire blight, which, in many countries, has to be independent of the disputed use of antibiotics.