Phytophthora Treatments for Hurricane-Damaged Groves

Josh McGillDiseases, hurricane, Tip of the Week

By Megan Dewdney

A priority for the Florida citrus industry is evaluating groves for damage and determining the best course of action going forward after the destruction of Hurricane Ian. One of the many considerations is the effect of flooding on root health, particularly if standing water has remained longer than 72 hours.


The main diseases are phytophthora foot, crown and root rot and fruit brown rot, which are all related. To further complicate things, there are two fungal-like organisms, Phytophthora nicotianae and P. palmivora, responsible for the diseases. These organisms have spores that swim and are attracted to roots, particularly ones producing a lot of exudates, and fruit on the soil surface.

After a hurricane, we commonly see toppled trees which may be uprighted. Be cautious because scion bark in contact with soil can develop lesions on scaffold limbs. Additionally, brown rot may become problematic for mid-season fruit if a lot of splashing occurred in a grove with a history of phytophthora, particularly P. palmivora, which can move more readily within the canopy.

Root damage becomes acute after 72 hours of flooding and makes the root systems more vulnerable to phytophthora infection. Phytophthora is unlikely to become a problem after flooding if you do not have a history of phytophthora with high propagule counts (10–20 propagules/cm3 of soil). However, if you do have a history of phytophthora problems and flooding, you should consider applying treatments in the next 30 days as the soil dries.

There are several products to choose for root rot as detailed in the Florida Citrus Production Guide, including phosphite salts, Aliette, Ridomil and Presidio. There is also the newer product, Orondis, that looks promising particularly for groves with high populations of P. palmivora.

Keep in mind that some products require irrigation after application. Please consult the labels for specifics. Remember if you are choosing a phosphite product for root or foot rot, only a product labeled as a fungicide is legal to use for disease management. Check the concentration to make sure you are getting the best efficacy for your expense.

If you are already using a program, continue with your planned rotations of products to avoid resistance development. The treatments for root rot should also help with foot rot, although the scions tend to be more susceptible, so products may not work as on the root system.

If sufficient fruit remain for an application, it is probably too late to apply a phosphite fungicide or Aliette for brown rot if you have not already done so in August. Your better choices would be a foliar spray of a copper fungicide or some of the newer fungicides that show promising results. These are Orondis, Revus and the premix Orondis Ultra, which can all be applied 0 to 1 day prior to harvest and can also be applied by air if there is an urgent need for speedy application.

Megan Dewdney is an associate professor of plant pathology at the University of Florida Institute of Food and Agricultural Sciences Citrus Research and Education Center in Lake Alfred.

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