As spring arrives in Northeast Ohio, trees and shrubs begin to break dormancy. With warmer temperatures and increased moisture, fungal pathogens become active—making this a critical time to take preventative measures. It’s important to understand that once symptoms of these diseases appear, it is often too late for effective treatment. Preventative fungicide applications are essential to stopping infections before they start. Waiting until signs of apple scab, dogwood anthracnose, or spruce needlecast become visible can result in irreversible damage for the current growing season.
Applying fungicides at the right time—based on the Growing Degree Day (GDD) calendar from the Ohio State University Extension—can significantly reduce disease severity and promote healthier trees throughout the season.
Apple Scab on Crabapples
Apple scab, caused by the fungus Venturia inaequalis, is a prevalent disease in crabapples. It leads to premature defoliation and reduces the tree’s aesthetic value. The fungus overwinters in fallen leaves and releases spores in spring, infecting newly emerging leaves and young fruit.
Preventative Strategy:
- According to the OSU GDD calendar, primary apple scab infection begins around 200–300 GDD (base 50°F). In Northeast Ohio, crabapple bud break typically occurs in mid to late April, depending on local weather conditions.
- Apply a protectant fungicide such as chlorothalonil, mancozeb, or myclobutanil at bud break and continue applications every 7–10 days through the primary infection periods.
- Rake and remove infected leaves in the fall to reduce overwintering inoculum.
- Select resistant cultivars when planting new crabapples.
Secondary Treatment Option:
In addition to fungicide applications, plant growth regulators (PGRs) such as paclobutrazol can be used to reduce excessive shoot growth and improve disease resistance in crabapples. By slowing vegetative growth, PGRs enhance air circulation within the canopy, making conditions less favorable for apple scab development. Applications should be made in early spring, following label recommendations for proper dosage and timing.
Need help protecting your crabapples this spring? Contact Tree Docs today to schedule a professional evaluation.
Dogwood Anthracnose
Dogwood anthracnose, caused by Discula destructiva, is a serious disease affecting flowering dogwood (Cornus florida) and Pacific dogwood (Cornus nuttallii). It thrives in cool, wet conditions and can cause leaf spots, twig dieback, and overall tree decline.
Preventative Strategy:
- In Northeast Ohio, dogwood bud break typically aligns with 100–200 GDD, which generally occurs in early to mid-April.
- Apply a fungicide such as propiconazole or chlorothalonil at bud break and continue applications every 10–14 days during wet weather.
- Improve air circulation by pruning surrounding vegetation.
- Avoid overhead irrigation, which can promote spore dispersal.
Tree Docs offers seasonal tree health programs designed to protect ornamental trees like dogwoods. Schedule your consultation today.
Spruce Needlecast
Spruce needlecast diseases, including Rhizosphaera needlecast (Rhizosphaera kalkhoffii) and Stigmina needlecast (Stigmina spp.), are common fungal issues that cause premature needle drop and canopy thinning—especially in Colorado blue spruce (Picea pungens).
Preventative Strategy:
- According to OSU Extension, needle emergence on spruce begins at approximately 400–500 GDD, typically falling in mid to late May for Northeast Ohio.
- Apply chlorothalonil or copper-based fungicides when new needles emerge. Repeat applications every 3–4 weeks if conditions remain wet.
- Maintain tree vigor through proper spacing and avoid environmental stress like drought or poor drainage.
- Prune and dispose of infected branches to limit disease spread.
Worried about needlecast on your spruce trees? Let Tree Docs help you stay ahead of the damage with a custom treatment plan.
The Role of Nutrient Applications in Disease Prevention
Fungicides alone aren’t always enough. Proper nutrient management plays a critical role in disease resistance and overall tree health.
- Potassium (K) strengthens cell walls, enhances disease resistance, and supports overall plant vigor. A sufficient supply of potassium can help trees better withstand infections such as apple scab, dogwood anthracnose, and spruce needlecast.
- In contrast, excessive nitrogen (N) fertilization can lead to lush, dense growth—creating microclimates that trap moisture and encourage fungal development.
Soil testing is recommended to determine the right nutrient balance. Avoid high-nitrogen fertilizers unless specifically recommended by a certified arborist.
General Best Practices for Fungicide Applications
To get the most out of your treatment plan:
- Timing is key: Use the OSU Growing Degree Day calendar to track optimal fungicide application windows.
- Follow label instructions: Always use appropriate rates, spray intervals, and observe safety guidelines.
- Rotate fungicide modes of action: Alternate products with different FRAC (Fungicide Resistance Action Committee) codes to prevent resistance.
- Incorporate cultural controls: Good site selection, tree spacing, and proper sanitation help reduce disease pressure and increase effectiveness.
Keep Your Landscape Healthy with Tree Docs
By implementing a well-timed preventative fungicide program, aligned with the Ohio State University Extension’s GDD calendar, and following smart cultural practices, you can keep crabapples, dogwoods, and spruces healthy and resilient throughout the growing season.
Not sure where to start? Tree Docs’ certified arborists are here to help you protect your landscape with expert advice and proven treatments. Contact Tree Docs today to schedule a spring tree health assessment.
