Hurricane Ian caused a lot of flooding in Florida citrus groves, and this created ideal conditions for phytophthora, which caused diseases like foot, crown and root rot, and fruit brown rot. Phytophthora is caused by two fungus-like organisms, Phytophthora nicotianae and P. palmivora.
The spores of these organisms are attracted to roots, and the root damage caused by flooding gives these spores easy access to the tree. UF/IFAS recommends that Florida citrus growers consider taking measures against phytophthora —especially in those groves that have a history of infection.
Phytophthora in the Groves
UF/IFAS maintains that standing water that has remained for longer than 72 hours can cause acute root damage that makes citrus trees much more susceptible to phytophthora infection, so growers with groves that suffered flooding for three days or longer should consider treating for phytophthora.
Megan Dewdney, a UF/IFAS associate professor of plant pathology at CREC, shared that phytophthora is unlikely to become a problem after flooding if there is no history of phytophthora with high propagule counts of 10–20 propagules/cm3 of soil, but that for groves with a history of phytophthora, it is advised to apply treatments as the soil dries.
Additional phytophthora guidance from Dewdney for Florida citrus growers concerning phytophthora include:
- Watch toppled trees that have been uprighted because scion bark that has come into contact with the soil can develop lesions on scaffold limbs.
- Splashing that has occurred due to flooding in a grove with a history of phytophthora—particularly P. palmivora—can lead to brown rot on mid-season fruit.
- The Florida Citrus Production Guide has detailed treatment options for phytophthora.
- Product options for treating root rot include phosphite salts, Aliette, Ridomil and Presidio, and there is a newer product, Orondis, that holds promise for treating phytophthora, especially for groves with high populations of P. palmivora.
- Make sure to consult the labels for specifics, especially for those products that require irrigation after application.
- Only a product labeled as a fungicide is legal to use for disease management when choosing a phosphite product for root or foot rot.
- Check the label for concentration levels to ensure you are applying at a rate that gives the best efficacy for the cost.
- If already treating for phytophthora, continue with your existing treatment’s planned rotations of products to combat resistance development.
- Treatments for root rot should also help with foot rot, but scions tend to be more susceptible, so products may not work as on the root system.
- It is likely too late to apply a phosphite fungicide or Aliette for brown rot if it wasn’t done in August if enough fruit remains for an application; a better alternative is a foliar spray of a copper fungicide, or a newer fungicide, like Orondis, Revus and the premix Orondis Ultra, which can be applied 0 to 1 day prior to harvest and applied by air if a fast application is required.
This column is sponsored by Griffin Fertilizer Co., and the opinions expressed herein may not reflect those of CFAN or of its advertisers.
BIO: Mike Roberts is the Vice President of the Frostproof, Fla.-based Griffin Fertilizer Co. Roberts joined the company in November 2011. He has spent the majority of his career in the fertilizer/agchem industry. Roberts earned a Bachelor of Science degree in citrus production from Florida Southern College in Lakeland. For more information, visit griffinfertilizer.com.