Released: March 18, 2005

Apple, Crabapple Disease Showing up Early in Kansas

MANHATTAN, Kan. – Cedar-apple rust is showing up early this year in southern Kansas.

“Any susceptible crabapple and apple trees that are already starting to leaf out need to be protected. Apple tree owners in other parts of the state need to be on watch,” said Ward Upham, Master Gardener program coordinator for Kansas State University Research and Extension.

The place to look for the disease now is on cedars and junipers, including the wild and domesticated eastern redcedars growing across the state. In typical years, cedar-apple rust and a closely related disease, cedar-hawthorn rust, are active on these evergreens between April Fool’s Day and Memorial Day, Upham said.

“The rust diseases rarely harm the evergreens, but they do make them look a little peculiar – as if they’ve produced orange, almost exotic-looking flowers,” he said. “Each of those orange structures soon releases millions of spores. Those spores are what infect the apple and hawthorn trees.”

The rust fungi can cause considerable damage to their deciduous tree hosts, Upham warned. They can cause premature defoliation – weakening the tree, making it susceptible to other diseases, and reducing next year’s fruit set and yield. They also can cause cracked, distorted, pitted or lost fruit.

“Since apples are the state’s No. 1 fruit-producing trees and crabapples are among our top ornamental trees, an atypically early spore season could be really destructive – particularly since this year’s unusually wet weather will foster the growth of molds and fungi,” he said.

The outbreak actually started two years ago, when infections on apple and hawthorn trees sent out spores that infected cedars and junipers. For a year, the disease grew on the evergreens – at first looking much like little reddish-brown bumps. By the end of this winter, however, the bumps had become galls from 0.5 to 2 inches wide. “Grown-up” galls look a bit like wrinkled balls until they swell in spring and produce multiple gelatin-like tendrils that are a particularly bright orange during rainy weather.

The cedar-apple rust galls will dry and fall from the evergreens this summer. The cedar-hawthorn rust galls may last for several years, Upham said.

“Two things can increase the odds the diseases will readily spread when the galls expand and release their fungal spores,” the horticulturist said. “The first is relatively cool temperatures – in the 50- to 75-degree range. The other is leaf wetness that lasts longer than four to six hours.

“Rust lesions will start appearing on the deciduous tree hosts one to three weeks after infection.”

On apple, crabapple and hawthorn trees, those lesions will be small yellow-orange spots on upper leaf surfaces. The spots will gradually enlarge and turn orange. Then small, black fungus “fruit” will form in the center of each lesion and may ooze orange “gelatin” during wet weather. Finally, an orange cup-like structure with small tube-like projections will develop under the lesion on the leaf’s bottom surface.

“If a leaf has numerous lesions, however, it probably will drop before that,” Upham said.

Controlling the rust diseases by removing any junipers from within a 1/2- to 2-mile radius of apple orchards is possible, because that disrupts the rust fungi’s life cycle.

“But that’s an impossible task in Kansas because of our large native population of eastern redcedars and widespread use of junipers in landscape and windbreak plantings,” the horticulturist said. “Of course, that’s not to say Kansas homeowners wouldn’t do well to avoid planting apples, flowering crabs or hawthorns adjacent to junipers.”

The most effective fungicides for controlling rust – including Banner, Systhane, Rubigan and Bayleton, applied at 14- to 21-day intervals – are only available to commercial applicators. Upham said that homeowners have three fairly effective choices, however: triadimefon (sold as Green Light Fung Away), propiconazole (Fertilome Liquid Systemic Fungicide) or myclobutanil (the same active as in Systhane, but sold as Immunox). Among the homeowner products, only the myclobutanil is labeled for fruiting apples.

“You have to apply fungicide to the leaves of apples, crabapples and hawthorns as long as the evergreen trees’ cedar galls remain jelly-like and active,” he said. “The only way to avoid this problem is to plant only apple and crabapple varieties that are resistant and to skip using hawthorns in areas where cedar-quince rust, in particular, has been a problem.”

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K-State Research and Extension is a short name for the Kansas State University Agricultural Experiment Station and Cooperative Extension Service, a program designed to generate and distribute useful knowledge for the well-being of Kansans. Supported by county, state, federal and private funds, the program has county Extension offices, experiment fields, area Extension offices and regional research centers statewide. Its headquarters is on the K-State campus, Manhattan.

Story by:
Kathleen Ward
kward@oznet.ksu.edu
K-State Research& Extension News