Q. I live in the Lafayette/Orinda area
and have a problem on the leaves of my pear trees. The leaves have growths that
have appeared on the backs of the leaves, brick-red in color and with what
appears to be tubes descending from them. The top of the leaf is yellow and
rust in color across from the growths. The trees produced poorly.
A. You have fungus called Gymnosporangium fuscum, or Pear Trellis Rust ( Pear Juniper Rust). This disease has been found in British Colombia since the 1960’s, in Washington State since 1997; in 1960 the Agriculture Department diagnosed it in one county in California, Contra Costa, in a 4 square mile area in Lafayette/Orinda. The disease originated in Europe, where it causes significant damage to pears grown in proximity to junipers. The fungus affects the growth and the fruit set of heavily infected pears, causing leaves to abscise (drop off) early, and attacking the ripening fruit. The symptoms on pear leaves are one or more irregular yellow to rust colored splotches appearing on the upper and lower leaf surfaces. Pycnia (small black fungal bodies), and Aecia, (light brown to reddish acorn shaped growths), can be seen on infected leaves, twigs and fruit.
This disease moves between Juniperus Sabina var. tamariscifolia, Tam Junipers, and pears. On the Juniper, telia, spindle-shaped growths appear on the twigs and branches. These produce basidiospores (reproductive structures) that are blown by the wind onto young pear leaves, spreading as far as approximately 300 meters ( ~1000 feet). Infections appears as yellow blotches on the leaf surface within 13-17 days. The ‘growths’ (aecia) appear later in the summer on the back sides of the leaves, producing aeciospores that are then spread by the wind back to the juniper plants, starting the cycle over again. The infected areas on the juniper may not be obvious until the second year, when the telia appear. The cycle goes > Juniper- infection, telia formation, basidiospores > Pear, pycnia on leaves and twigs, aecia, aeciospores> Junipers.
The only realistic method of
control is to remove the junipers surrounding your infected trees. In a
suburban area, this may require talking to many of your neighbors.