The disease is caused by a fungus, Cladosporium, that grows superficially on leaves, stems, and petioles, causing unsightly spotting but not significantly affecting plant vitality. Symptoms. Nematodes Leaf Blotch (aka Measles) Cladosporium paeoniae. This fungus only attacks Paeonia spp. The underside of each spot is reddish-brown in color. Typical symptoms include glossy purple to brown spots or blotches on the upper surfaces of the leaves. Infection of peony can occur from 50°F to 82°F but the latent period increases as the temperature declines. Leaf Blotch. Cause Graphiopsis chlorocephala (formerly Cladosporium paeoniae), a fungus that can impact all aboveground tissues and survives in stem and leaf debris.It has been reported from Washington and found in Oregon. It goes by many names, but red spot seems most descriptive. The disease may cause slight distortion of the leaves as they continue growth. Peony > Leaves > Spots on leaves. Cladosporium paeoniae: Avoid overhead irrigation. Leaf blotch or measles: Small, reddish spots that can coalesce to form large, irregular purple blotches on leaves and stems. )-Peony Red Spot. Red Spot, Leaf Blotch, or Measles of Peony; July 29, 1998: If you have seen peonies, you have most likely seen this disease. Maintain low humidity. The disease is also known as red spot or measles. Lesions are also formed on stems. Leaf blotch is a result of the fungus Cladosporium paeoniae. The leaves of peony plants sometimes fall victim to a disease called peony leaf blotch, which causes dark or red spots to form on the leaves. The rotted portion of the plant will become covered with a soft brown or blackish mass of spores. On peonies afflicted with botrytis, the young shoots rot off at ground level when they are 5 to 8 inches tall. The leafy shoots wilt suddenly and fall over. The stems often have a water-soaked, cankerous appearance. Diseases that commonly afflict peonies are associated with lack of air movement and cool wet conditions, minimizing these factors may be difficult, but recognizing them can allow them to be addressed. This disease generally appears in spring or early summer when the shoots of affected herbaceous paeonies wilt and turn brown for no apparent reason. Clean up debris at season end. Tiny red to purple spots appear near the veins of the peony leaves. See: Peony (Paeonia spp. Peony wilt is an infection of the leaves and stems of peonies and tree peonies by the fungus Botrytis paeoniae, which is closely related to the grey mould pathogen Botrytis cinerea. 1 of 5. Remove infected plant parts. Peony leaf blotch is caused by the fungus Cladosporium paeoniae. Peonies are relatively disease and pest resistant plants, however there are a number of ailments that gardeners should be aware of. Apply a fungicide to protect plants. Peony leaf blotch is probably responsible for the large, brown spots.