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Modeling the Effect of Temperature and Wetness on Guignardia Pseudothecium Maturation and Ascospore Release in Citrus Orchards

First author: Citrus Research International, P.O. Box 2201, Stellenbosch 7602 and Department of Plant Pathology, Stellenbosch University, Private Bag X1, Stellenbosch 7602, South Africa; second author: Citrus Research International, P.O. Box 28, Nelspruit 1200, South Africa; and third and fourth authors: QMS Agri Science, P.O. Box 416, Letsitele 0885, South Africa.

Ascospores are the most important inoculum source of citrus black spot (CBS), caused by Guignardia citricarpa, but pseudothecium maturation and ascospore release are inadequately studied. Guignardia ascospore trapping and concomitant weather data were obtained for three localities over three seasons (July to March 2006 to 2009) in the Limpopo province of South Africa. Degree-days accumulated until first seasonal ascospore discharge (>10°C with 1 July as biofix) (DDtemp), and DDtemp accumulated on rainy (rainfall >0.1 mm) (DDrain) and moist days (vapor pressure deficit <5 hPa) (DDvpd) were used in two Gompertz models to predict onset of ascospore release: a temperature model [Event = exp(–exp(–(–2.725 + 0.004 × DDtemp)))] and a temperature/moisture model [Event = exp(–exp(– (–3.238 + 0.008 × DDvpd + 0.004 × DDtemp – 0.009 × DDrain)))] (R² = 0.608 and 0.658, respectively). Both models predicted a delay in pseudothecium maturation in climates with colder winters and springs. A Gompertz equation was also used to predict the proportion of Guignardia ascospores trapped (PAT) per season from DDtemp data accumulated on wet or moist days (DDwet2) from the first seasonal ascospore discharge [PAT = exp(–4.096 × exp(–0.005 × DDwet2); R² = 0.908]. The PAT model predicted lag phases and 7-day peaks in ascospore release patterns with reasonable accuracy. These models can be used to predict the onset and dynamics of ascospore release in climatically diverse regions.