Development and Evaluation of Two Pecan Scab Prediction Models.

Area of Science:

• Plant Pathology
• Agricultural Meteorology
• Mycology

Background:

• Fusicladium effusum (syn. Cladosporium caryigenum) causes pecan scab, a significant economic threat to pecan (Carya illinoinensis) production.
• Current Oklahoma advisories for fungicide application rely on 'scab hours,' defined by specific temperature and relative humidity thresholds.

Purpose of the Study:

• To evaluate the accuracy of existing temperature and relative humidity thresholds used in pecan scab advisories.
• To develop and compare alternative predictive models for pecan scab development.

Main Methods:

• Disease severity data and hourly weather variables (temperature, humidity, dew point, solar radiation, rainfall) were collected over multiple growing seasons (1994-96, 2009-10).
• Logistic regression models using generalized estimating equations were developed, including a temperature/relative humidity model and a dew point/dew point depression model.
• Model performance was validated using historical data and field studies evaluating different threshold combinations.

Main Results:

• The temperature/relative humidity model indicated a 0.62 probability of pecan scab at 21°C and 90% relative humidity, supporting the viability of current thresholds.
• Field validation confirmed the general effectiveness of existing thresholds but suggested potential modification of the relative humidity component.
• A dew point/dew point depression model, incorporating rainfall, adequately explained scab events, highlighting the importance of these variables.

Conclusions:

• Existing temperature and relative humidity thresholds for pecan scab advisories are generally effective but may benefit from refinement.
• Future models for predicting pecan scab epidemics should integrate dew point, dew point depression, rainfall, and total solar radiation for enhanced accuracy.