Abstract
Ascosporic infection plays a major role in the epidemiology of citrus black spot (CBS) in South Africa, a disease caused by Phyllosticta citricarpa. Phyllosticta pseudothecium maturation and ascospore release models have been integrated in infection models to predict the availability of the primary inoculum source. However, these models have not been validated on a broader data set and this study aimed to validate and improve these epidemiological models. New pseudothecium maturation and ascospore release models for P. citricarpa were developed, based on weather and ascospore trap data from 13 locations and up to five seasons. From the 29 data sets analysed, 3775 3-hourly periods with ascospore events were recorded on 1798 days; 90% of these events occurred between 16.0 °C and 32.1 °C (daily Tmin and Tmax of 15.4 °C and 33.5 °C, respectively) and 75% occurred above a relative humidity (RH) of 55.9% (daily RH > 47.9%). Rain was recorded during 13.8% of these ascospore events and 20.0% of ascospore days. Using logistic regression, a Gompertz model that best predicted pseudothecium maturation, or the probability of onset of ascospore release, was developed and was markedly more accurate than the previously described models. The model consisted of DDtemp [cumulative degree-days from midwinter (1 July) calculated as (minimum + maximum daily temperature) / 2 – 10 °C] and DDwet (DDtemp accumulated only on days with >0.1 mm rain or vapour pressure deficit <5 hPa) as variables in the formula: probability of first ascospore event = exp(-exp(-(-3.131 + 0.007 × DDtemp - 0.007 × DDwet))). A Gompertz model [PAT = exp(-2.452 × exp(-0.004 × DDwet2))] was also developed for ascospore release; DDwet2 = DDtemp accumulated, from first seasonal ascospore trap day, only on days with >0.1 mm rain or vapour pressure deficit <5 hPa. Similar to the DDwet2 model described in a previous study, this model adequately predicted the general trend in ascospore release but poorly predicted periods of daily, 3-day and 7-day ascospore peaks.
 Significance:
 
 We developed a new pseudothecium maturation model from 29 data sets, comprising different climatic regions in South Africa, and validated previously published models. The new model was markedly more accurate in predicting the onset of ascospore release and can be used to improve existing CBS epidemiological models and improve risk assessment and management of CBS in South African citrus orchards.
Highlights
Citrus black spot (CBS), caused by Phyllosticta citricarpa (McAlpine) van der Aa, is the most important fungal disease of citrus in South Africa, due to the quarantine status of this pathogen in certain fruit export markets
The use of mathematical models to estimate the maturity of pseudothecia of P. citricarpa is important in the management of CBS because they predict the start of ascospore release, which is key in determining when fungicide applications need to begin in the field
The Phyllosticta ascospore availability models were published by Dummel et al.[26] and Fourie et al.[25], of which the models described by Fourie et al.[25] were subsequently used in CBS risk assessment studies[17,35] and in CRI-PhytRisk
Summary
Citrus black spot (CBS), caused by Phyllosticta citricarpa (McAlpine) van der Aa, is the most important fungal disease of citrus in South Africa, due to the quarantine status of this pathogen in certain fruit export markets. The disease does not affect the internal fruit quality, but rather causes cosmetic lesions that reduce the fruit quality standard.[1,2] Fruit lesions form largely on maturing fruit from latent infections that occurred when fruit was immature.[1,2,3,4] The critical period for fruit infection in South Africa and Australia is the first 4–5 months after fruit set, whereafter fruit becomes more tolerant to infection.[1,5,6] In South Africa, Australia and Argentina, protective fungicide sprays are only required during this critical fruit infection period for effective control[3,4,5,6,7,8], but longer periods of protection are required under the highly CBS conducive conditions in São Paulo, Brazil[9]. Leaves are susceptible to latent infection during the 10 months after unfolding, but rarely show symptoms.[10]
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