Pear rust—Gymnosporangium libocedri
The fungus infects pear, certain other Rosaceae plants, and incense cedar. Its other names include cedar broom rust, incense cedar rust, and Pacific coast pear rust.
Infected pear and other rose family plants develop discolored spots with dry, powdery or rough, reddish, yellowish, or orange spore masses. Groups of tiny, cup-shaped, spore-forming bodies appear as pustules on fruit, green stems, and leaves. On fruiting pears, infections occur primarily on the fruit.
Gymnosporangium libocedri alternates between incense cedar and apple, hawthorn, mountain ash, pear, quince, and serviceberry. Spores are produced on incense cedar in the spring and disperse in air to infect the young tissues of pear and the other rosaceous hosts. Wet weather favors rust disease, so infection and damage is more common on rosaceous hosts when it is rainy in the spring. The fungus grows in infected rosaceous hosts and produces airborne spores that infect incense cedar, usually in late summer or fall.
Flower petals, fruit, green stems, and leaves can become spotted. Infected fruit become distorted. Fruit and leaves may drop prematurely.
Avoid overhead watering. Dispose of infected fruit, leaves, and shoots as soon as symptoms appear. Fungicides applied in the spring can prevent or reduce rust disease. Because Gymnosporangium damage to fruit is uncommon in California, the applications required to prevent pear rust may not be warranted. For more information on the disease in conifers, see cedar, cypress, and juniper rusts.
Cup-shaped fruiting bodies
Powdery spores in leaf spots
Orange, spore-forming bodies