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Rhizosphaera Needle Cast of Spruce

by Paige Thrush, Nancy J. Taylor & Francesca Peduto Hand

Introduction

Rhizosphaera needle cast is a foliar disease that occurs worldwide on a diverse range of conifers. The fungal pathogen, Rhizosphaera kalkhoffii, is responsible for needle cast of spruce species. This disease has been detrimental to Christmas tree plantations, especially those in eastern U.S.A., and to spruce trees in nurseries and landscapes outside of their native range. Spruce species vary in their susceptibility to the disease (Table 1), which greatly reduces the vigor, aesthetic quality and market value of trees. Repeated years of infection may result in tree death.

Table 1. Susceptibility of common spruce species to Rhizosphaera needle cast

Rhizosphaera needle cast causes needles to turn brown, or in the case of the Colorado blue spruce, a purplish-brown, and fall from the tree prematurely (Figure 1). Needles of any age and on any part of the tree may be infected, but the disease often starts on 1 year or older needles on the lower branches of the tree, attacking several branches at one time (Figure 2). Lower branches are commonly affected because the foliage is denser and shaded, receiving less air flow and holding moisture for longer periods of time compared to higher branches. If conditions are conducive in the following years, the disease will spread within the tree, from the bottom up and inside out (Figure 3).

Identification

Pycnidia, the fungal fruiting bodies containing spores, begin forming in rows on infected needles during spring when moisture is high. Mature pycnidia appear as brown to black spherical structures that are less than 0.1mm in diameter (Figure 4). These structures form in pore-like openings of the needle called stomata (singular=stoma). The pycnidia will push through the stoma, dislodging the stomatal plug, forming a white waxy cap on top the fruiting body. A hand lens can be used to view the rows of black pycnidia, which may or may not be capped with white plugs.

Infected needles drop between late spring and autumn, 12-15 months after infection. Spruce trees are not able to form new needles where the old ones dropped. As a result, bare areas develop on the tree, reducing its ornamental value and overall health. Branches may begin to die after 3-4 years of early needle loss. In severe cases, the disease can kill entire trees.

Needle cast symptoms can also be caused by pests, such as the pine needle scale or the spruce spider mite, or an abiotic stress to the tree, such as drought. Each of these diseases/disorders has specific and unique symptoms, so make sure to closely inspect the tree before making management decisions.

If uncertain, samples can be sent to a diagnostic laboratory for confirmation. Information on The Ohio State University laboratory can be found on the C. Wayne Ellett Plant and Pest Diagnostic Clinic website.

From top left to bottom right: Figure 1. Purplish-brown blue spruce needles affected by Rhizosphaera needle cast (USDA Forest Service – North Central Research Station, USDA Forest Service, Bugwood.org); Figure 2. Rhizosphaera needle cast symptoms on blue spruce tree (Minnesota Department of Natural Resources, Minnesota Department of Natural Resources, Bugwood.org); Figure 3. Disease will spread within the tree from the bottom up and inside out (Joseph OBrien, USDA Forest Service, Bugwood.org); Figure 4. Pycnidia in rows on needles infected with Rhizosphaera kalkhoffii (Edward L. Barnard, Florida Department of Agriculture and Consumer Services, Bugwood.org).

Pathogen Biology

Rhizosphaera kalkhoffii can be a primary pathogen, infecting and colonizing a host and, resulting in a disease, or it can be a secondary/opportunistic pathogen, colonizing needles that have been previously stressed by another disease or environmental condition. Disease symptoms often develop more rapidly on stressed trees than healthy trees.

The pathogen can overwinter as a vegetative fungal body (mycelium) inside needles and/or pycnidia on the outside of infected needles. Spores formed by the pycnidia are dispersed primarily by water splash beginning in spring and continuing into autumn. Once they land on susceptible healthy needles, spores require about 48 hours of free water on needles with temperatures around 25°C (77°F) to initiate infection. If conditions are not favorable for infection, spores can survive in an inactive state on needles for many weeks.

Infection typically begins on newly forming needles and symptoms will likely progress from areas of the tree that are most favorable to disease development, such as the lower branches, shaded, or wind protected side of the tree, to the areas of the tree that are less favorable. In late summer, diseased needles will show symptoms of yellow mottling. By late winter to early spring, the needles will turn brown to purplish-brown. Pycnidia will begin to develop when infected needles become sufficiently moist. These pycnidia typically mature during spring and release spores in response to moisture. Infected needles will drop prematurely in summer and autumn, 12-15 months after the initial onset of infection.

Favorable Environmental Conditions

  • High humidity and abundant moisture
  • Wet periods of over 48 hours at optimal temperatures around 25°C (77°F)
  • Christmas tree plantations favor the development of kalkhoffii because trees are often closely spaced, decreasing air circulation, which results in foliage staying wet for longer periods of time. In addition, trees on these plantations are usually sheared, promoting denser foliage, which also increases moisture retention. This close spacing allows for spores to be dispersed more easily.

Management Guidelines

Integrated pest management (IPM) is an approach to plant health care and disease and pest control. IPM incorporates a wide range of strategies to prevent, minimize, and/or control abiotic and biotic diseases and pests. These strategies involve monitoring and scouting, learning behavior and life cycle of pests and pathogens, accurately identifying the source of disease, developing threshold levels, employing preventive measures and integrating cultural, mechanical, biological and chemical controls. All management decisions should be carried out based upon the specific requirements of the plant. Analysis of each strategy involves considering the impact on host plants, pests/pathogens, microbial interactions, the environment and society. The costs and benefits of each strategy should be evaluated before making management decisions.

Prevention:

  • Plant selection:
    • Purchase pest and pathogen-free stock from reputable sources.
    • When purchasing plants from a nursery, inspect for signs and symptoms of disease. Avoid purchasing plants that look defoliated, discolored or wilted.
    • If disease pressure is high in the landscape, consider planting Norway spruce, which is less susceptible.
    • If possible, purchase trees grown from local seed sources. These plants will be better adapted to local conditions and more likely to resist disease.
  • Nurseries and plantations:
    • Quarantine incoming plants and inspect for signs and symptoms of disease. Remove and dispose of, or return, all infected plants to the original source.
  • Proper site selection:
    • Avoid planting highly susceptible species in low or shady areas where humidity and moisture remain high for prolonged periods of time.
  • Proper planting practices:
    • Proper plant spacing will minimize moisture retention on the plant surface and will decrease the likelihood of disease development. When planting new trees in the landscape, space them to allow for optimum air circulation and sunlight penetration into the tree’s interior. Plan for the mature size of the tree(s) when spacing plants, or plan to remove trees as they begin to intermingle lower branches.
    • Avoid planting young trees near old trees that may be harboring pathogens.
  • Proper irrigation practices:
    • If possible, do not water newly planted trees with sprinklers or overhead irrigation, which will keep foliage wet for long periods. Drip irrigation or hand watering is preferable.
    • Irrigate during morning hours so that plants will have a sufficient amount of time to dry.
  • Prevent and minimize stress to plant:
    • Conduct regular soil and plant tissue analysis tests to determine if plants require supplemental nutrients. Otherwise, do not fertilize woody plants.
    • Water tree(s) during periods of drought.
    • Keep weeds and other vegetation from encroaching around the base of trees to encourage air circulation among and light penetration to the lower branches.

Scouting:

  • Inspect trees, especially 5 to 10 year olds, for brown needles in spring.
  • Use a hand lens to observe pycnidia, which appear as tiny, black spots arranged in neat rows on infected needles.
    • If pycnidia are not present, test for disease by placing needles showing symptoms in a plastic bag with a wet paper towel. Keep the bag with the samples at room temperature. If the pathogen is present, pycnidia should develop in a day or two.
  • If you have a Christmas tree plantation, randomly select 20 or more trees scattered throughout the plantation. Remove three lower branches from each tree. If half the branches have fruiting bodies on at least 10% of the needles, consider treating the entire plantation in spring and summer.

Cultural control practices:

  • Avoid pruning, shearing, and/or other operations that would facilitate spore dispersal and infection during wet periods.
  • To avoid transferring diseases, prune or shear healthy blocks in plantations or healthy trees in the landscape before pruning or shearing unhealthy blocks or trees.
  • Sterilize tools after pruning or shearing trees known to be infected.
  • For high value trees, dislodge and destroy fallen needles that collect in branch crotches and on the ground. These needles may harbor large amounts of pathogen inoculum.
  • Once the pathogen has become well established in/on the host, the specimen may need to be removed from the landscape.

Chemical control:

  • The disease can be effectively controlled by applying fungicides containing chlorothalonil. New growth can be protected and the tips of branches will continue to grow and mask the older, defoliated stems and branches if the disease is detected early before branches have been killed. Older needles which are already infected may continue to be cast, thus it may take several years of treatment for the tree’s appearance to improve.
  • Two properly timed applications per year, for 2 to 3 consecutive years are often needed for control.
    • The first application should occur just after bud break, when new needles are half elongated relative to the previous year’s needle length.
    • The second application should occur 3 to 4 weeks after initial application.
  • Some locations have an additional infection period between September and October and will require another treatment. Consider an additional fungicide application if tree(s) begin producing new growth late in the season and environmental conditions are conducive for infection and disease development.
  • Read each fungicide label thoroughly for safety precautions and information on how to prepare, apply, store, and dispose of the product.
  • Check with your county or state extension specialist for an updated list of registered fungicides that are available.