Black spot, which is also called tar spot, is a type of fungus that grows on tree leaves, including fruit tree leaves. Black spot fungus usually does not severely harm trees, but it damages their leaves and looks unattractive. There are a couple of different ways to get rid of the fungus, but it returns easily if nearby trees are infected or if the fungus spores are mixed in with the nearby soil.
As the name suggests, the main symptom of black spot is black spots on the fruit trees' leaves. Cornell University explains that the spots usually start out as light yellow spots in June and become dark by mid or late summer. The University of Illinois explains that the spots start out small and grow larger in damp conditions and as time progresses.
Black spot is caused by a fungus that grows in or on tree leaves, fruits and buds. Even though the fungus shows up on tree leaves, the University of Illinois explains that it can survive through winter when trees are bare by living in fallen leaves.
How It Spreads
According to the University of Illinois, black leaf spot mostly spreads through air currents, being carried by insects, and through splashing rain drops. Therefore, the fungus spreads easily and appears on new trees when nearby trees are infected.
Black spot mould spores prefer cool and moist climates. According to the University of Illinois, the ideal time for this fungus to infect trees is when new leaves and buds are growing during the spring. The fungus needs water to survive.
The National Gardening Association recommends trimming off badly infected parts of trees. During fall or early winter, rake away all of the fallen leaves and infected tree trimmings to reduce the number of mould spores near the fruit trees. Removing the spores will help prevent spores from travelling through the air and infecting new leaves in the spring. Fungicidal sprays also kill black spot, but they are difficult to spray on both sides of all infected leaves on a large fruit tree. Organic fruit tree farmers may not want to use harsh fungicidal sprays, but a solution of baking soda, horticultural oil and water also kills black spot fungus.