Marssonina leaf spot disease is a disease of apple trees caused by the fungus Diplocarpon mali. Infection causes the leaves of the affected host plant to begin to wilt and fall off - in very severe infestations, this can lead to complete defoliation even before harvest. The increased leaf drop has negative effects on fruit formation and the frost hardiness of the tree.
After rainy periods, dark gray to black spots about 1-2 mm in size form on the upper leaf surface of infested host plants. These are sometimes surrounded by a purple border and may also merge or run in branching black lines.
In late summer, round, black fungal camps (acervuli) form on the affected leaves. As a result, the leaves turn yellow and fall off. Heavily infested trees are almost completely defoliated before harvest. Symptoms on the fruit (individual dark spots) are possible, but have not yet been observed in Austria.
Propagation and transmission
During dormancy, fruiting bodies (apothecia) of the fungus form in the fallen leaves, leading to new infections in spring. As soon as the spores (ascospores) formed in them mature in spring, they can be ejected and infect the young leaves. As a result, fruiting bodies of the fungus (acervuli) form on the leaves. The spores (conidia) they contain can be spread by wind and rain. This can result in the shedding of several generations of the fungus per year.
In our latitudes, the Marssonina leaf spot disease occurs mainly in organically or extensively managed plantations and in private gardens. Heavy infestation leads to premature defoliation of the trees, which has negative effects on fruit formation and frost hardiness of the tree.
Prevention and control
- Removal of fallen leaves or promotion of their decomposition - this eliminates the source material for infections in spring.
- Good aeration of the tree - favors rapid drying after precipitation and thus counteracts further spread.
- Use of plant protection products (see list of plant protection products approved in Austria) after flowering until the end of August.
Last updated: 01.09.2023