Up to the beginning of November, the latest plum varieties are also off the tree. Remove fruits remaining on the tree after shedding of leaves, as these are infected with the brown rot fungus (Monilia fructigena). Infestation already happened months ago and is generally greatly dependent on weather conditions. Thus, the pronounced and frequent change between warmth and moisture in spring is conducive to infestation of plums and damsons with this fungus.
Infestation of the fruits can take place at any time of fruit development. Even from blossoming onwards, when weather conditions are rainy. Damage on the fruits also serve as a point of entry for the fungus. Because, for example, the fruit volume of the damson greatly increases when the colouring begins to change from green to purple, but the skin of the fruit then becomes softer, fruit growth prior to the harvest leads to stretching of the skin of the fruit. Thus micro-cracks and a means of entry for the fungus into the fruit appear.
So much for understanding the infection – but what can be done about it in concrete terms?
Most important of all is tree hygiene: if you do not now clear the remaining, dried up, so-called mummy fruits, they are a major hymenium or spore store for the infestation of your fruits in the coming year. Because the fungal spores also spend the winter on the tree bark and are carried on the wind again from anywhere, as it were, you can also try to reduce the Monilia infestation in the coming year with appropriate thinning out of the crown of the tree. The greatly thinned crown of the damson tree means that leaves and also fruits can dry off faster after precipitation. Everything which aids fast drying of the tree and fruits also reduces the weather-related intensity of infestation.
In addition, use nitrogen fertilisation of the tree sparingly and only dispense the quantity actually required. Nitrogen fertilisation reduced to the necessary degree also reduces the frequency of infestation with Monilia.
Tip: With a chemical preparation – a fungicide – the “mummy fruit fungus” Monilia fructigena cannot currently be combated, only Monilia laxa, as occurs on sour cherries and leads there to shoot dieback. Mummy fruits on sour cherry trees (as with those on apple and pear trees), on the other hand, are also infested, like the damson trees, with Monilia fructigena, and you should therefore also remove these ""dry fruits” from the tree.