Rhizosphaera needle cast (Rhizosphaera kalkhoffii) is a foliar disease of spruce trees. It is most common in trees growing outside of their native range. Rhizosphaera starts on the inner and lower growth and progresses upward through the tree. It can take up to 15 months for the needles to show visible symptoms after the initial infection. Young trees may be killed by this disease, but usually branches die off after 3-4 consecutive years of defoliation, causing trees to look disfigured.
Symptoms usually begin on the inner and lowest portions of the tree and progress upward.
Severely infected trees may hold only the current year's needles; whereas, a healthy spruce will retain needles 5 to 7 years.
Two year old needles turn yellowish brown to purple in mid-summer.
Lower branch dieback.
Tiny fruiting bodies (pycnidia) protrude through the stomata of the infected needles. Under a hand lens, these stomata appear as fuzzy black spots instead of their usual healthy white color.
A healthy spruce will retain its needles 5 to 7 years, whereas a spruce severely infected with Rhizosphaera needle cast may hold only the current year's needles.
Rhizosphaera needle cast infects needles on the lower branches first and gradually progresses up the tree.
This pattern holds true for most needle diseases on conifers and is the result of more favorable conditions for disease development near the ground.
Under epidemic conditions, lower branches may be killed by this fungus.
Although needles on new growth become infected in May and June, symptoms are not visible until late fall or the following spring, when infected needles turn purple to brown and begin to drop.
Tiny fruiting bodies of the Rhizosphaera fungus protrude through the stomata of the infected needles.
During wet weather in late spring, spores are released from these fruiting bodies and are rain splashed onto newly developing needles where infection occurs and the disease cycle is repeated.
Early identification of Rhizosphaera can prevent major damage to individual trees and prevent the spread to nearby trees. Protecting new growth as it emerges is very important. For best effectiveness, fungicides should be applied when the emerging needles are half elongated (1/2 to 2 inches in length). Heavily infected trees may require several years of fungicide applications. Cambistat™ has not shown a direct effect in reducing Rhizosphaera symptoms in trials, but it should be considered to improve the health and vigor of spruce trees. Multiple trials with Cambistat™ have shown a reduction in cytospora canker on Colorado blue spruce.