Rhizosphaera needle cast, caused by the fungus Rhizosphaera kalkhoffii, is a disease that affects several species of spruce trees, especially blue spruce. The pathogen targets the trees’ needles, leading to discoloration, premature shedding, and ultimately, diminished tree health and even branch death.
Identifying Rhizosphaera Needle Cast Symptoms
Proper identification is key. Keep in mind that Rhizosphaera is just one specific type of needle cast. The other common needle cast disease, stigmina, has spores that appear fuzzy and noticeably protrude out. Rhizosphaera spores, on the other hand, will appear smooth and rounded with clean edges. Here are other signs of Rhizosphaera:
- Small black spores will be visible on infected needles.
- Fewer needles will be present near the trunk of the affected tree.
- Lower branches will likely see signs of infection first as the pathogen travels up the tree.
- Premature shedding will occur in late summer to early fall.
- Needles will be discolored, often with a purplish cast (as seen in the image below).
How Rhizosphaera Needle Cast Forms and Spreads
Rhizosphaera needle cast typically begins its lifecycle during periods of high humidity and moderate temperatures. Its fungal spores, known as conidia, appear on infected needles and within fruiting bodies called pycnidia. These spores grow on the surface of infected needles, often appearing as black dots or bumps.
Once established, Rhizosphaera needle cast spreads within the tree canopy through several mechanisms. During wet weather conditions, the pycnidia (fruiting bodies) release conidia (spores), which are dispersed by rainwater and wind. Rain facilitates the dispersal of fungal spores from infected needles to healthy ones, and wind will carry spores over short distances, promoting the spread of the pathogen within the same tree or to neighboring trees.
After landing on foliage, the spores germinate and penetrate the stomata of the needles or enter through open wounds, causing the infection to spread. The fungus then gradually causes discoloration and necrosis.
Rhizosphaera Needle Cast Treatment
You will need to make fungicide applications to control Rhizosphaera. One thing people often ask me is whether you can treat trees for a couple of years and then stop. For the most part, you’ll be treating them indefinitely because the disease is so prevalent and blue spruces are so susceptible to it.
Rhizosphaera isn’t known to go quietly, so it’s best to make multiple applications. Your first application should be made in the late spring or the first weeks of summer, when buds break and new needles start to emerge. Make a second application three weeks to one month after your initial application, and a third application, if necessary, three to four weeks after the second.
Product Recommendations
Several contact and systemic fungicides are effective against Rhizosphaera, such as:
- CLT 720 FL (active ingredient: chlorothalonil, liquid, contact)
- Tourney EZ (active ingredient: metconazole, liquid, systemic)
- TM 462 F (active ingredient: thiophanate-methyl, liquid, systemic)
- Kalmor (active ingredient: copper hydroxide, dry flowable, contact)
Rotating between chlorothalonil, which is widely recommended for needle cast, and Tourney will provide an excellent one-two punch. Refer to each product’s label for information on usage rates and application timing.
Cultural Practices
While making chemical treatments is the most important part of controlling Rhizosphaera, there are other measures you can take to reduce the risk of it appearing.
The soil’s pH and nutrient levels may be contributing factors. Consider an application of Grow-In or Armament Concentrate to provide trees with the nutrients that the pH may be tying up. This won’t prevent the disease, but it will boost the overall health of the trees.
Research at the University of Kentucky has shown that stressed trees are more susceptible to infection. To maintain the health of spruces during periods of drought stress, consider watering them.
Unfortunately, the threat of Rhizosphaera is not over when the infected needles fall. Its spores can persist in fallen needles and infected debris on the ground, continuing the cycle, particularly in areas with dense tree populations. Maintenance activities, such as pruning, can also inadvertently aid the spread of Rhizosphaera needle cast by creating wounds that provide entry points for the pathogen. Additionally, contaminated pruning tools or other equipment can transmit fungal spores from one tree to another. Removing fallen needles and thoroughly cleaning the tools you use on and near infected trees can go a long way.
Combating Rhizosphaera Needle Cast
Rhizosphaera needle cast poses a significant challenge to lawn care professionals. Not only does it negatively impact tree health and appearance, but it also spreads rapidly. Fungicide applications and proactive maintenance practices can help minimize the pathogen’s impact on spruce populations. To learn more about controlling Rhizosphaera and for treatment recommendations, reach out to your ATS rep.
