On FieldClimate we offer models for two Citrus diseases. Alternaria brown spot, caused by the fungus Alternaria alternata, affects a wide range of orange and mandarin varieties specially Murcott tangors. Where severe, the disease results in extensive fruit drop and must be controlled on processing and fresh market fruit. 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.

 

• ALTERRATER ALTERNARIA MODEL

• PFD POST BLOOM FRUIT DROP

ALTERRATER ALTERNARIA MODEL

Alternaria brown spot, caused by the fungus Alternaria alternata, affects a wide range of orange and mandarin varieties specially Murcott tangors. Where severe, the disease results in extensive fruit drop and must be controlled on processing and fresh market fruit. Spores of Alternaria are airborne. Most spores are produced by lesions on the mature leaves on the tree or recently fallen infected leaves on the grove floor. The ALTER-RATER is a weather-based point system to assist in the timing of fungicide applications. The system was developed based primarily on the amount of infection which occurred on susceptible trap plants placed in a heavily infested Minneola tangelo grove for 24-hour periods and supplemented with data from lab studies.

Factors Affecting the Disease Occurrence:

• Rainfall, but not the amount of rain
• Days with leaf wetness durations of more than 10 hours had more disease than days with fewer numbers of hours.
• Alternaria infection was high over a wide range of temperatures but did diminish if the average daily temperature was above 28 or below 21°C.

The practical Use of AlterRater: The AlterRater Values for every day are accumulated over 5, 7, 10 and 14 days and this results are displayed in FieldClimate.Com. Florida University Extensions service suggests 3 thresholds for spray application:

• 50 for heavily infested Orchards
• 100 for moderate infested Orchards
• 150 for weak infested Orchards

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PFD POST BLOOM FRUIT DROP

Post bloom 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 experience 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. the 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

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.

Graph: Inoculum of the fungus is low, therefore the threshold for infected flowers on 20 trees increase from 10 to 150 flowers/20trees on the 20th of February. Because of the rainy period during 16 to 120th of February, the threshold 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.