Rhizosphaera needlecast

Overview

Rhizosphaera needlecast is caused by the fungus, Rhizosphaera kalkhoffii. The disease causes death and casting (dropping) of needles in spruces, especially Colorado spruce and occasionally white spruce. It is usually first evident on the lower branches of the tree and can cause severe defoliation and death of branches if allowed to persist. It rarely kills the tree.

Symptoms and Diagnosis

Infections take place in the spring, and symptoms show up about a year later. Infected 2-year-old needles become spotted or mottled, some turning yellow and some developing a purplish brown color in late summer. Browning becomes general in late winter to early spring, and needles are prematurely shed during summer and fall, 12 to 15 months after initial infection. Black fruiting bodies are visible on discolored needles and even some green ones. The spores emerge from the stomates on the needles so appear in rows along the needles. Scattered lower branches are usually affected first and then browning progresses upward. Symptoms may be confused with spider mite damage. In the latter, mites and fine webbing may be visible; the lines of black fruiting bodies on the needles will also be absent.

Life Cycle

Rhizosphaera kalkhoffii overwinters in infected needles on the tree or ground. Spores are released from the infected needles in the spring, during wet weather. Spores are dispersed by splashing and dripping water. They infect newly emerging needles, as well as mature needles. Infection begins in spring, in April or May. The optimal temperature for fungal development on wet foliage is 77° F. Infection will occur in 48 hours if spores are present. Prolonged wetness can induce extensive infection.

Integrated Pest Management Strategies

1. Select healthy plants. If spruce trees are dropping their needles, avoid using these trees in the landscape.

2. Remove infected needles and branches during dry, sunny weather. Prune out diseased branches and rake up fallen needles. Burn or remove the collected debris. Disinfect pruning tools between cuts by dipping them in a 10% chlorine bleach solution. Oil tools when used to inhibit rust.

3. Avoid overhead watering. Water on the foliage can promote infection. Spores require moisture to germinate and infect.

4. Improve air circulation to allow needles to dry more quickly. Maintain open spacing when planting or by mowing grass or brush.

5. Use fungicidal sprays. Severe infections should be sprayed with chlorothalonil, thiophanate methyl, or copper-containing fungicides registered for use against this disease. Spray when the needles are half-elongated and again when fully elongated. A minimum of two years of treatment is recommended because of the organism's life cycle.

Organic Strategies

Strategies 1, 2, 3 and 4 are strictly organic approaches. Of the fungicides mentioned in Strategy 5, consult the Organic Materials Review Institute (OMRI™) for appropriate organic copper products.

Pesticide Disclaimer: 

Always follow the product's label and ensure the product is effective against rhizosphaera. Not following the pesticide label before usage is a violation of federal law.