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PFD Post Bloom Fruit Drop

Postbloom fruit drop (PFD) is caused by the fungus Colletotrichum acutatum. It affects all species and cultivars of citrus. It must be controlled on processing and fresh market fruit. Navel and Valencia oranges do experienced the most severe damages.

The pathogen infects flower petals causing peach to orange-colored lesions on open flowers and flower buds. Subsequently, the fruitlet abscises leaving the calyx and floral disk attached to the twig. These persistent structures remain attached for the life of the twig, and the leaves around inflorescences are usually twisted and distorted.

Sensors: leaf wetness, precipitation

The PFD Model in FieldClimate.com shows disease incidence with a max. threshold for infected flowers on 20 trees. A fungicide application is indicated if these three criteria are met:

  • Sufficient bloom is present or developing to represent a significant portion of the total crop;
  • the model predicts a disease incidence of greater than 20%- shown as max. threshold for 20 trees (red line in fieldclimate.com') and minimum inoculum for infection (blue line)
  • No fungicide application has been made in the last 10-14 days
     

Factors affecting PFD Severity:

  • PFD severity on a given cultivar may vary according to the time of bloom in relation to rainfall.
  • Most spores of this fungus are produced directly on the surface of infected petals.
  • Spores are splash-dispersed by rains to healthy flowers.
  • Where they infect within 24 hours and produce symptoms in 4-5 days.
  • The fungus survives between bloom periods as resistant structures on the surface of leaves, buttons, and twigs.

The PFD Model has been developed to assist growers in determining the need and timing of fungicide applications (Equation 1). The model is based on:

  • The amount of fungal inoculum present (i.e., the number of diseased flowers on a 20-tree sample TD in the model, "minimum inoculum");
  • The total rainfall for the last 5 days in mm (R)
  • The number of hours of leaf wetness greater than 10 hours for the last 5 days.

The model predicts the threshold for infected flowers for 4 days in the future. If the number of infected flowers on 20 threes is higher than the given threshold (red line) an application is recommended.

CitrusPostBloomFruitDrop

Graph: Inoculum of the fungus is low, therefore threshold for infected flowers on 20 trees increase from 10 to 150 flowers/20trees on the 20th of February. Because of the rainy periode during 16 to 120th of february the treshold decreased again to about 10 infected flowers/20 trees. This threshold (if the number of infected flowers is higher than the threshold) should support the farmer to decide for an application against the disease.