7955_xml
20201121100000
assa
nadiag@assaf.org.za
assa
South African Journal of Science
S. Afr. J. Sci
1996-7489
11262020
116
11/12
Models for predicting pseudothecium maturity and ascospore release of Phyllosticta spp. in South African citrus orchards
Providence
Moyo
Citrus Research International, Nelspruit, South Africa
https://orcid.org/0000-0001-8604-5629
Susan
du Raan
QMS Laboratories, Letsitele, South Africa
Paul H.
Fourie
Citrus Research International, Nelspruit, South Africa
Department of Plant Pathology, Stellenbosch University, Stellenbosch, South Africa
https://orcid.org/0000-0002-2210-6880
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.
11262020
1
10.17159/sajs.2016/crossmark
sajs.co.za
false
2020-02-12
2020-08-17
2020-11-26
Department of Science and Innovation, South Africa
http://dx.doi.org/10.13039/100016962
Citrus Research International
https://creativecommons.org/licenses/by/4.0/
10.17159/sajs.2020/7955
20201121100000
https://www.sajs.co.za/article/view/7955
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https://www.sajs.co.za/article/view/7955/10654
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https://www.sajs.co.za/article/view/7955/10654
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https://www.sajs.co.za/article/view/7955/10654
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https://www.sajs.co.za/article/view/7955/10654
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https://www.sajs.co.za/article/view/7955/10654
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https://www.sajs.co.za/article/view/7955/10654
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https://www.sajs.co.za/article/view/7955/10654