By Megan Dewdney
It was a difficult winter for many citrus trees in Florida, with freeze damage and very dry weather stressing them. Trees will be flushing as much as they can to replace the damaged canopy lost over the winter. This means they will be drawing strongly on the bank of carbohydrates in the roots. The spring 2026 root flush will be very important to sustain for long-term tree health and future flush, flowering and fruit.
The spring root flush occurs as the spring flush hardens and is normally vulnerable to phytophthora in areas with high propagule counts from the previous season. This is because the resting spores of phytophthora are becoming active with the warm weather. While dry weather does not favor the pathogen, the need to irrigate may expose the roots to severe wet-dry cycles, leading to sugar leakage from the roots. These sugars are highly attractive to the swimming zoospores, which are released in wetting events.
ROOT SAMPLING
This means it is time to sample roots if there have been phytophthora problems historically or if there is a suspicion thereof. For how to sample for the most accurate propagule estimate, refer to the phytophthora root rot chapter in the Florida Citrus Production Guide and stay away from the worst trees.
Normally, 10 to 20 propagules per cubic centimeter is considered a concerning level and anything above 20 propagules per cubic centimeter problematic. However, after the stress of the freezes and the importance of this root flush, I recommend treatment if more than 10 propagules per cubic centimeter are detected.
TREATMENT CHOICES
Regardless of what product you choose for the first treatment of the year, mode of action rotation is essential. The Fungicide Resistance Action Committee (FRAC) codes help guide choices so that you do not need to be an expert in this area. If products have the same FRAC code, they are the same mode of action.
The choices for phytophthora root rot management include mefenoxam (FRAC 4), fluopicolide (FRAC 43), phosphite salts (FRAC P 07) or oxathiapiprolin (FRAC 49). The order of use will depend on what your last treatment the previous season was and if you are concerned about brown rot. You should also verify how many applications are allowed per season as several products are limited to one application.
Phosphite salts during the summer do triple duty for phytophthora root rot, brown rot and algal spot if used in June or July. For root rot applications, it is best to start right after the spring flush. These are followed with late-spring and mid-summer applications to protect the summer root flushes. The final application is in the autumn to protect the roots that are the carbohydrate reserve for the winter.
Megan Dewdney is an associate professor at the University of Florida Institute of Food and Agricultural Sciences Citrus Research and Education Center in Lake Alfred.
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