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Spots and discoloration on the leaves of apple trees and premature leaf fall are caused by various pathogens. Most of these are apple scab or leaf spot diseases caused by fungi of the genus Phyllosticta. In recent years, home gardening and organic farming has more often seen a premature leaf fall in which the leaves had similar symptoms. According to investigations by the Bavarian State Research Center for Agriculture, however, the causative agent in these cases was not one of the known domestic pathogens, but the fungus Marssonina coronaria.
After a summer of frequent rainfall, the first spots may form on the leaves as early as July. They later converge and larger leaf areas turn chlorotically yellow. Also striking is an early onset of leaf fall, often in the summer. The fruits remain basically free from attack, but the leaf fall leads to a reduced fruit size and quality. The shelf life of apples is limited. In addition, next year is expected to lower flowering and fruiting.
The symptoms of fungal disease vary from species to variety. The leaves of 'Golden Delicious' show distinct necrotic grains, in 'Boskoop' the leaves are yellow in color and speckled with green dots. Idared, on the other hand, shows few symptoms. Interestingly, the strain 'Topaz' is particularly vulnerable, although it is quite resistant to apple scab, for example.
The apple variety 'Topaz' is considered vulnerable to Marssonina coronaria
The home of Marssonina coronaria is Southeast Asia. The fungus can, like the well-known apple scab, overwinter on a fall foliage and the fungal spores infect the fully developed leaves after the apple blossom. Temperatures over 20 degrees and permanently damp leaves favor the infection - therefore the infestation pressure is particularly high in rainy years. Due to the possible climate change with increasingly humid summers, a further spread is likely, especially in home gardens, organic apple orchards and orchards.
Since the fungus survives on a fall foliage, it is important to remove it
Prevent and fight Marssonia mushrooms
Because the mushroom (Marssonina) overwinters during fall foliage, you should carefully collect it and promote a loose crown structure through regular fruit tree trimming, so that the leaves can dry well during the growing season. Fighting in the home garden with fungicides does not make sense, since the starting point for the hobby gardener is difficult to recognize and repeated spraying would be necessary for a sufficient effect. In conventional fruit growing, the disease is usually combated by the preventive scab treatments.