Strategic Management of Alternaria Brown Spot and Citrus Scab

Fungal foliar diseases significantly threaten commercial citrus production by reducing fruit packout and overall crop yield. Tangerines and mandarins are highly susceptible to these pathogens, particularly Alternaria brown spot and citrus scab.

In a presentation at the recent Citrus & Specialty Crop Expo, Megan Dewdney, University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) plant pathologist, outlined proactive management strategies for Florida growers, emphasizing that these fungal threats cannot simply be “sprayed away” without careful planning.

SITE SELECTION AND CULTURAL CONTROLS

Effective management begins before trees are in the ground. Dewdney offered the following advice:

• Grove History and Soil Evaluation: Evaluate past disease or nematode pressures, soil pH, drainage and water table levels prior to planting.
• Cultivar and Rootstock Selection: If planting mandarin or tangerine cultivars that are highly susceptible to disease, avoid highly vigorous rootstocks. Increased vegetative growth creates dense canopies that trap moisture, fueling fungal proliferation. Growers should also consider widening planting distances to optimize airflow.
• Sanitation: Always start with certified disease-free nursery trees. If existing groves are badly affected, hedge and top trees to remove inoculum sources. For Alternaria brown spot, hedge in late March.
• Input Management: Limit irrigation and fertilizer to minimize vegetative growth.

FUNGICIDE TIMING

Chemical management of citrus scab requires precise seasonal timing based on specific tree growth phases. Dewdney suggested the following application timing:

1. First Application: Apply at 1/4 flush expansion using Enable (FRAC 3) or a strobilurin fungicide (FRAC 11, such as Gem, Headline, or Quadris) to clear out overwintering fungi from the previous year’s lesions.
2. Second Application: Apply at petal fall, ensuring a change in chemical chemistry from the initial spray.
3. Third Application: Apply three weeks post-petal fall. Copper formulations fit well into this window and simultaneously protect against Alternaria brown spot, citrus canker or melanose.

For Alternaria brown spot, Dewdney noted control programs typically span from March through June for tangerines. Strobilurin fungicides (FRAC 11) can be used, but premixtures — such as Amistar Top (FRAC 11 + 3) and Pristine (FRAC 11 + 7) — are preferred. She said premixtures are useful as an early spray, when disease severity is high and when the weather is hot.

OVERCOMING FUNGICIDE RESISTANCE

The deregulation of ferbam in 2025 has eliminated all multisite fungicides but copper. Dewdney recommended using copper later in the season as it is not as effective as other products.

She said growers must manage blocks as if fungicide resistance is already present. This requires strict rotation of modes of action. While single-year data shows Pristine maintains good efficacy, UF/IFAS requires multi-year data before issuing a formal recommendation.