The GARDENA gardening expert
Dear Mr. M., the leaf curl disease is a fungi, and the spores of this fungi live on peach tree branches over the winter. As soon as the buds come out, the infection breaks out on the unfolding leaves, ending in the damage symptoms we know spreading across the full leaf.
You are correct that plant strengthening tonics – those which merely support the defence and self-healing mechanisms of the plants – are at their most effective as preventatives. However, plant protection products must, before being launched onto the market, be proven in comprehensive tests to actually be effective as claimed against the harmful organism. Insofar, we can always assume that plant protection agents do indeed combat the fungi. The active substances contained in the preparation can be copper (usually contained in environmentally-friendly preparations), but also fungicidal chemical compounds. As far as the actual effectiveness of the preparation in practice is concerned, it is crucial that the products are applied at the correct time and in the correct concentration (i.e. that the treatment is repeated if necessary). In addition: So that the fungi does not become resistant to the preparation, it is necessary to switch the active ingredient from treatment to treatment (meaning not just the preparation, but also its active ingredient, see the product declaration).
Because the fungi (Taphrina deformans) which causes the leaf curl disease works as it does (see above), you can remove affected branches when pruning your peach tree, and thus minimise the infestation pressure. But the rest of the tree will still be infected with the spores on the remaining branches unless you make (correct) use of fungicides.
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