HYG  Pest newsletterInsectsHorticulturePlant DiseasesWeedsSearch
{short description of image}

Issue Index

Past Issues

Rhizosphaera Needle Cast of Spruce

This fungal disease of spruce will cause “holes” in trees. The holes are not in the trunk—they are areas of the tree canopy from which needles have dropped. The needles have been cast from the tree; thus, the name “needle cast.” Because spruce trees are not able to form new needles where the old ones dropped, the holes in the canopy are with the tree for its entire life. The disease will not kill the tree, but it can cause enough aesthetic damage that you might wish the tree had died. For this reason, Rhizosphaera needle cast has become a disease for which preventive fungicides have been recommended. As long as entire branches are not killed, the new growth at the branch tips can be protected from infection. As tips continue to grow, the new growth may mask the defoliation that has occurred on older needles near the center of the tree.

This fungal disease is a particular problem on Colorado blue spruce, but it may also infect other spruce species as well as some pines. If you have had a problem with this disease in the past, now is the time to take action. How do you know you have had problems? The disease will cause first-year needles to turn brown to purple in the fall. (Keep in mind, however, that many environmental stress factors will cause these same symptoms.) On trees with Rhizosphaera needle cast, the affected needles may stay attached until the next summer or fall.

Rhizosphaera needle cast may kill twigs if infection recurs from year to year. Often the newest needles appear as green, healthy tufts at the end of defoliated branches. When infected needles are moist, the fungal pathogen will form pinhead-sized fruiting structures (pycnidia) in neat rows on the needles. To test for pycnidia, place affected needles in a plastic bag with a wet paper towel; if present, pycnidia should develop in one or two days. These pycnidia actually protrude from the needle surface and are readily visible, especially with a hand lens.

As with most fungal diseases, infection occurs in warm, wet weather. With Rhizosphaera needle cast, spores are released from spring until fall. Good air circulation will help prevent infection, so prune surrounding plants to attain better air movement, or consider removing crowded plants. Fungicides are effective as a preventive control but must be applied when needles are half elongated and again when fully expanded. Keep an eye on the new growth on your spruce and make the first application when new needles are half grown. Chemical options are given on page 106 of the Illinois Commercial Landscape and Turfgrass Pest Management Handbook, 1998–1999. Homeowner chemical options are listed on page 35 of the Illinois Homeowners’ Guide to Pest Management.

Author: Nancy Pataky


College Links