Crabapple (ornamental)
Cedar apple rust (fungus)
Cedar apple rust galls, left, seen on juniper in the spring produce the spores that infect
crabapple leaves.
Cedar apple rust lesions on foliage.
Numerous small, yellow spots appear on the upper leaf surfaces, usually during
late April or May. Later, cluster cups are evident on the underside of the leaf
spot. The fungus that causes this disease also produces galls on juniper.
Plant resistant varieties. Although eradicating junipers is not feasible,
removing galls from nearby junipers may be of some benefit. Avoid planting crabapples
adjacent to junipers.Fungicides can provide good control, with proper timing.
Make the first application when gelatinous, orange tendrils are noticed on the
cedar galls, usually in mid-April. Continue the applications on a 7 to 10 day
interval as long as galls are active. Chlorothalonil (for crabapples not intended
as food only), fenarimol, ferbam, mancozeb, maneb, myclobutanil, propiconazole,
thiophanate-methyl or triadimefon are labeled.
Fire blight
(bacterium)
General and serious. New shoots appear as if scorched by fire. Leaves cling
to twigs and have "shepherd's crook" appearance. Plant resistant varieties.
Prune in late winter or late summer or fall, not in spring. In dry weather, cut
diseased branches and twigs 12 to 18 inches below visibly infected area. For commercial
growers, spray streptomycin (100 ppm) in early or fall bloom to inhibit fire blight.
Not recommended for home use.
Powdery mildew
(fungus)
Whitish powdery patches on young leaves, buds, blossoms and twigs. May cause
growth irregularities. Does not usually require control. When serious, follow
spray schedule under apple scab. Use chlorothalonil, fenarimol, mancozeb, myclobutanil,
propiconizole, thiophanate-methyl, wettable sulfur or triadimefon. Resistant varieties
available.
Scab (fungus)
Serious in wet seasons. Dull, olive to black, velvety or scurfy spots on leaves.
Leaves may turn yellow and fall off. May also appear on fruit.
Plant resistant varieties. Collect and remove fallen leaves. To prevent, follow
a regular spray schedule, starting when first leaves appear until petal fall at
six- to seven-day intervals, then 10- to 14-day intervals for remainder of season.
Use chlorothalonil, fenarimol, ferbam, myclobutanil, propiconizole, thiophanate-methyl
or sulfur.
Updated 6/8/09