Spruce needle cast

Spruce needle cast is a fungal disease that causes discoloration and eventual loss of needles. Initial symptoms of needle cast disease include a mild yellowing of current-season needles, and usually show first on lower, crowded branches. Symptoms tend to progress outward from the inside and upward into previously healthy branches and often an entire tree will ultimately be impacted (Figure 1). Severely infected trees lose the ability to retain needles for their normal five- to six-year life span and only first-year needles are held at the ends of the branches. Continued cycles of infection will eventually kill the branches completely. Older trees may be partially or totally denuded of healthy, dark green needles. Young trees may die if weather favors the disease for several years in a row.

Biology

The fungal pathogen Rhizosphaera kalkhoffii overwinters in the infected needles that remain on the tree or fall to the ground close by. During wet periods of 48 hours or longer during the spring, rain-splashed spores may spread to current-season new growth. During the summer, continued spread can occur during periods of sprinkler overspray or the activity imposed by pruning and shearing. Season-long spread can occur on trees that are irrigated with overhead sprinklers.

Needle cast disease requires two seasons to complete its cycle. Infection of new needles occurs in spring and early summer of the first year. The fungus penetrates the needles and creates a mild yellowing in these current-season needles. At this stage, the disease is difficult to diagnose correctly. May or June of the second season after infection, the fungus continues to develop in the needles and symptoms become more pronounced. Yellowing progresses, the fungus produces visible fruiting bodies, and by late summer into early fall, the needles turn dark reddish-brown (Figure 2-3). By late fall, most of these discolored needles are cast from the tree, leaving relatively bare branches.

Damage

Diagnostic symptoms and signs of spruce needle cast include:

• Mottled yellowing of current-season needles (alone, this symptom is not diagnostic)
• Development of black fruiting bodies, visible in straight, parallel rows along the needles (emerging from the stomata). This is the most important diagnostic trait and typically becomes visible in the late spring after initial infection.
• Needles turn dark reddish-brown in the fall of the second year after spore dispersal
• Needles are cast from the tree in the late fall of the second season
• Only the current-season needles are retained through the second winter on infected branches. These needles are usually infected with the fungus and will express second-season symptoms the following season.
• Branch die-back begins in lower, shaded branches and slowly spreads outward and upward; eventually affecting much of what was once a healthy tree (Figure 1)

Control

Management and control of all pathogens that cause needle cast demand similar protocols, thus allowing this guide to be applied across all such disease-causing pathogens.

Physical

• Remove visibly infected trees to prevent spread to healthy neighbors

Cultural

• Prune only when foliage is dry (between trees, sterilize pruning equipment using 70% ethanol)
• Remove and destroy dead needles that fall beneath trees
• Select and plant Norway spruce or spruce, fir, or pine species that have resistance to needle cast
• Use only pathogen-free planting stock
• Plant only in sites with deep, rich, fertile soils that will support excellent tree health
• Space trees far enough apart to prevent overcrowding resulting high humidity within the canopy
• Manage to prevent drought and fertility stresses that make trees more susceptible to the disease
• Use irrigation equipment that provides adequate water without routinely wetting the foliage

Chemical

• Pesticide recommendations for control of spruce needle cast can be found at the PNW Plant Disease Management Handbooks website.

Pesticide warning

Always read and follow the instructions printed on the pesticide label. The pesticide recommendations in this University of Idaho webpage do not substitute for instructions on the label. Pesticide laws and labels change frequently and may have changed since this publication was written. Some pesticides may have been withdrawn or had certain uses prohibited. Use pesticides with care. Do not use a pesticide unless the specific plant, animal or other application site is specifically listed on the label. Store pesticides in their original containers and keep them out of the reach of children, pets and livestock.

Trade names — To simplify information, trade names have been used. No endorsement of named products is intended nor is criticism implied of similar products not mentioned.

Groundwater — To protect groundwater, when there is a choice of pesticides, the applicator should use the product least likely to leach.

Steve Love, emeritus professor of crop science
2023