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