Pine needle rust

During May and June, several young red pine plantations in east central Minnesota were diagnosed with pine needle rust. Needle rust is most common on young red, jack and Scots pines up to about eight feet in height. It usually does not seriously damage the trees because only older needles are infected. Growth is slowed because the infected needles die. Only when combined with insects or other agents that attack current-year shoots and needles, can tree mortality occur.

Orange aecial blisters of this fungus develop in May and June on previous years needles and release spores that are carried by the wind to the alternate hosts, goldenrods and asters. In August and September orange spores from these alternate hosts are wind-carried back to pine needles. The next spring, rust pustules develop on the infected needles.

Pine needle rustSince this is a rust disease with two host plants, management can focus on either preventing spores from being produced on the asters or preventing spores from infecting the pines. Here are three options:

  • Mowing the plantation, thus destroying the asters, provides good control of this disease.
  • Herbicides, applied in June of July, kill the asters before they produce spores.
  • Fungicides (such as triadimefon or mancozeb) can be applied starting in August to prevent the spores from infecting the pine needles.