Rhizospheara needle cast was discussed in issue 3. It is the most common needle disease on spruce in Illinois. It discolors second-year and older needles, often resulting in defoliation of all but the newest needles. Damage is usually scattered hot spots but may be more uniform. Spider mites also cause similar discoloration and may cause defoliation of affected needles.
Rhizosphaera, however, develops fungal fruiting bodies (spore containing structures) on infected needles. It might be necessary to place some affected needles in a moisture chamber (plastic bag with moist paper toweling) overnight to encourage growth of fruiting bodies. Look for pinhead sized black structures poking out of the needle through leaf pores. A hand lens is usually required to observe these structures, which occur in rows. They do not easily rub off because they are embedded in the tissue. If you leave the material in a plastic bag for a couple of days, all sorts of superficial molds grow on the needles, making disease identification difficult.
Spider mites occur most frequently in the spring or fall. Pinhead-sized, they are visible with a hand lens. Even if mites are not present, injury may be visible as yellow, speckled needles, often with a gritty surface, possibly with webbing and possibly with clear eggs on the needles. You can determine whether mites are present by using this simple test. Hold a piece of white paper below a branch and strike it sharply. If mites are numerous, they are knocked onto the paper, where they can easily be seen. Slow-moving, green to-gray mites that streak green when squashed are probably plant-feeding mites. Faster-moving mites that streak red when squashed are probably predatory mites that feed on the plant-feeding mites.
Some nutrient stress problems can also affect older needles and can also cause yellowing or mottling. If fruiting bodies of Rhizosphaera are not found, and if mites and mite injury are not apparent, consider nutrient stress. We have seen micronutrient stress on spruce in drought or where the soil pH is very high and micronutrients unavailable. Often, applying a chelated micronutrient spray causes foliage to darken in a couple of weeks. If so, test the soil pH to determine what long-range action needs to be taken to correct the problem.