Several conifer species can be infected by Rhizosphaera Needlecast, but this disease is most commonly found on Blue Spruce trees. Healthy Blue Spruce trees will retain their needles for 5 – 7 years. Rhizosphaera Needlecast causes premature needle loss, sometimes to the point where only the current years needles remain on a branch. While Rhizosphaera Needlecast will rarely kill a tree it can disfigure it enough that it loses it appeal in the landscape.
The fungal spores of Rhizosphaera Needlecast are spread to newly forming needles in the spring by wind or rain. Stressed trees are more likely to be infected and damaged by this tree disease. Rhizosphaera Needlecast can infect any size spruce tree, but trees 15 – 20 years old, or older, are more commonly infected.
TREES INFECTED BY RHIZOSPHAERA NEEDLECAST
Colorado Blue Spruce is the landscape tree most commonly infected by Rhizosphaera Needlecast. Engelmann Spruce, White Spruce, and Norway Spruce are occasionally infected. Other conifer species, such as Austrian Pine, are rarely affected by this disease.
SIGNS & SYMPTOMS OF RHIZOSPHAERA NEEDLECAST
Newly forming needles are infected in the spring, but the symptoms of the disease may not be seen for 12 – 18 months following infection. Infected needles often turn purple or brown in the late fall, or the following spring, and begin to drop from the tree. The disease usually begins on the lower half of the tree and gradually progresses to higher branches in subsequent years.
Healthy spruce needles have rows of white stomata on the underside of the needles. On spruce needles infected with Rhizosphaera black fruiting bodies can be seen growing from the rows of stomata. These fruiting bodies will produce the spores that will infect needles the following spring.
HOW TO CONTROL RHIZOSPHAERA NEEDLECAST
Most of the spores that infect the new needles in the spring come from the fallen needles under the tree. Raking and removing the fallen needles will reduce the amount of innoculum in the vicinity of the tree. If the needles cannot be raked up then they can be covered with mulch to prevent them from blowing or splashing onto the new growth.
Cultural Control of Rhizosphaera Needlecast
Keep trees healthy with proper watering, mulching and fertilizing.
Overhead watering should be avoided. Use a soaker hose or drip irrigation to water susceptible trees.
Maintain a 2 – 4 inch mulch layer that extends to the drip line of the tree. Woodchips, pine needles, bark mulch, or composted leaves are the preferred materials to use for mulching trees. Mulch should not be in direct contact with the tree trunk.
Fertilize with a complete tree fertilizer. A complete tree fertilizer will contain nitrogen, phosphorus, and potassium. A soil test can be used to determine the most beneficial nutrient ratios.
Fungicide Control of Rhizosphaera Needlecast
Timely fungicide treatments for Rhizosphaera Needlecast will reduce the level of infection on new needles. Two to three treatments per year, beginning when the needles are half elongated, is usually sufficient. Two years of treatments are vital because the disease spores may not be produced for 12 – 18 months after the initial infection. Annual treatments may be considered for specimen trees or where any level of infection is unacceptable.