Johnsons's Garden Centers Text File document - copyright 2003
PDF file available at
http://www.johnsonsgarden.com/newsletter/nurseryplants.html

Dreaded "Polka Dot" Disease
March/April 2003
Sallie Strole, Johnson's East Store

    Orange polka dots might be an "in" fashion statement but they are
definitely out of style on plant leaves. If you noticed small
yellowish,fetch then larger orange spots on your hawthorn and crabapple
trees last summer, act soon to prevent another season of this aesthetic
assault on your landscape. The culprit behind the spots is a rust
fungus.

    There are as many kinds of rust as there are plants and each rust is
specific to a particular plant host. Rust fungi are also unique in that
they require alternate hosts. The most common rusts we encounter in the
home landscape are cedar-apple, cedar-quince and cedar-hawthorn rust.
For practical purposes, we lump them all together under cedar-apple
rust. The fungi travel from cedars or junipers to rosaceous plants like
apple, crabapple and hawthorn in the spring and then back to the cedars
in late summer.

    The damage comes in summer when, besides being unsightly, rust can
cause substantial leaf drop. This premature defoliation will weaken
trees, especially after multiple years of infection. Preventative
measures include planting resistant varieties, which is a very realistic
option with crabapples but not so much so with hawthorns. The other
physical defense is to not plant the two hosts within a mile or so; not
a very practical solution as you look around at all the junipers in our
yards.

    Spray controls are aimed at coating the leaves as they come out and
as the fungal spores are being released. In early April, the hard galls
on the junipers turn to orange jelly-like masses that release the
spores. The fungus spreads through wind and rain and lands on hawthorn
and crabapple leaves. To restrict infection spray from April through May
at 7-10 day intervals with ferti-lome Systemic Fungicide or ferti-lome
Broad Spectrum Liquid Fungicide.

    On junipers and cedars, control is rarely necessary as the rust
galls are ugly but not life threatening. However, the timing for
spraying these evergreens is when the fungus is traveling back, usually
in early July. To treat cedars and junipers, spray ferti-lome Systemic
Fungicide or Hi-Yield Bordeaux from early July through August. The galls
take two years to mature so you won't notice much improvement until the
second year on these evergreens.

    Each year's spraying does make a difference on the crabapples and
hawthorns. By spraying early and often, your trees can avoid the
horticultural "faux pas" of the orange polka dots.