NUMBER 1191 |
April 14, 2009 |
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SHADE TREES & ORNAMENTALS |
TOBACCO |
Cedar Rust Diseases are Active Now |
Cedar apple rust. The cedar-apple rust fungus (G. juniperi-virginianae) forms light brown to reddish or chocolate brown galls in the leaf axils of infected Juniperus species. These galls are usually rounded and range from pea-sized to 2 inches in diameter. As galls mature, the flesh becomes corky and the surface becomes pitted with circular depressions. In spring, following rainy periods, slimy, yellow-orange tendrils, or “spore horns,” up to 2 inches long swell and protrude from these depressions (Figure 1, Figure 2). A gall may produce many spore horns, which cause it to resemble orange-colored blossoms from a distance. Severely rusted Juniperus can be very conspicuous. Cedar-hawthorn rust. Galls produced by cedar-hawthorn rust (G. globosum) are similar in appearance but are smaller and more irregular in shape and do not develop the regular arrangement of circular depressions. Spore horns, too, are shorter, generally fewer in number, and wedge- or club-shaped (Figure 3). Because of its smaller size, cedar-hawthorn rust is less conspicuous than cedar-apple rust. Cedar-quince rust. Cedar-quince rust (G. clavipes) does not form rounded galls but instead forms perennial, spindle-shaped swellings on the twigs, on which a gelatinous, orange-brown mass of spores is borne in the spring (Figure 2 and Figure 4). Disease cycle. All three fungi have similar life cycles. From dormant galls and swellings, orange spore “horns” or spore masses begin to appear in spring about the time that flowering crabapples are in bloom and apple buds are in the pink to early bloom stage. These spore horns or spore masses are actually columns of fungal spores (teliospores), each of which can germinate under moist conditions to form four new spores (basidiospores). Basidiospores are then carried to apple, crabapple, hawthorn and other susceptible trees by wind currents where they can germinate and cause infection during relatively short periods of wetness (about six or seven hours for moderate levels of infection when temperatures are in the 50s and 60s). The springtime period of infection usually ends about 30 days after apples bloom when the fungus no longer produces basidiospores on cedars and the majority of alternate host leaf, shoot or fruit tissues have aged to the point where they are no longer susceptible. Although cedar rusts can cause unsightly growths on Juniperus, they do not usually cause serious damage to these plants. However rust diseases can cause serious fruit losses on apples and weaken and kill shoots of crabapples and hawthorns. Infected fruits can drop prematurely or have a reduced commercial value if they remain on the tree through harvest. Leaf infections often result in premature leaf loss, reducing the size and quality of the fruit crop, weakening the tree, and reducing bloom the following year. Hawthorn and crabapple twigs infected by the cedar-quince fungus can become swollen and die. Cedar rust disease management efforts are usually aimed at reducing damage to apples, crabapples, and hawthorns.
Figure legends:
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By Julie Beale and Paul Bachi |
In the landscape, more cases of Rhizosphaera needle cast have been seen on spruce. Landscape evergreens such as holly, boxwood and magnolia are showing symptoms of winter drying.
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By Patricia Lucas | |||||||||||||||||||||
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NOTE: Trade names are used to simplify the information presented in this newsletter. No endorsement by the Cooperative Extension Service is intended, nor is criticism implied of similar products that are not named.