FIR-FERN RUSTS ( Uredinopsis sp., Milesina sp. )
Several species of rust fungi may be found infecting a variety of fir, and some of the most important rust species are in the genera Uredinopsis sp. and Milesina sp. and are known as fir-fern rust. These rusts are characterized by producing white spores while the other common rust genera that infect fir produce yellow or orange-yellow spores. Many species of fungi causing fir-fern rust have more than one fir host, although each may have a specific fern host. Several species are perennial in the host, growing for several years in the needles before killing them. Others infect and kill current year's needles within weeks or months of infection.
Fig. 2. The yellow needles on this fir were infected with fir-fern rust and will eventually be cast from the tree.
The fir-fern rusts caused by Uredinopsis sp. and Milesina sp. are characterized by cylindrical or tongue-shaped, white fruiting bodies called "aecia" which may form on yellow or even green needles. The blister-like aecia break through the epidermis of the needle, and shortly thereafter, burst open and begin to release spores that will infect ferns.
Depending upon the species of the rust, the fungus may overwinter either in some (varied) form on fern or as mycelium in living fir needles. Where a rust species does not kill fir needles the first season, it may become perennial in the needles and twigs. Each year thereafter, the pathogen forms aeciospores on the newest needles until the twigs eventually die. Where the fungus does not overwinter in the fir needles, spores are produced in the spring and carried by the wind back to newly emerging fir needles.
Fungicides are available for treating the Uredinopsis sp. of rusts. To control fir-fern rusts, remove ferns in areas near trees by mowing them or using a registered herbicide on them. Alternatively, or in conjunction with fern eradication, use a registered fungicide to protect the trees.
Fig. 2. A close-up of infected fir needles.
Fig. 3. The white, blister-like fruiting bodies shown here on the underside of these fir needles are the aecia. Spores produced here will infect the alternate host.
Fig. 4. A close-up of the aecia. Note the spores are white, not yellow or orange.
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