LIBRO-CONGRESO-CITRUS

S15P05
Modelling of Guignardia pseudothecium maturation and ascospore dispersal in citrus orchards
Fourie P.H. 1 , Schutte G.C. 1 , Serfontein S. 2 , and Swart S.H. 2
1 Citrus Research International (CRI), Disease Management, South Africa; and 2 QMS-Agriscience (Letsitele), South Africa.
phf@cri.co.za
Ascospores are considered the most important inoculum source of Citrus Black Spot (CBS), caused by
Guignardia citricarpa, but pseudothecium maturation and ascospore dispersal are inadequately studied.
Guignardia ascospore trapping and concomitant weather data were obtained for three localities for three
seasons (July through March from 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 (vapour pressure deficit < 5 hPa; DDvpd)
were used in two Gompertz models to predict onset of ascospore dispersal: 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, while the temperature/
moisture model predicted a further delay in drier seasons or climates. 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 from the first seasonal ascospore discharge [PAT = exp(-4.096 × exp(-0.005 × DDwet2); R2
= 0.908]. These models can be used to predict the onset and dynamics of ascospore dispersal in climatically
diverse regions.
S15P06
Infectious period of Citrus Black Spot on sweet orange
Lourenço S.A. 1 , Gasparoto M.C.G. 1 , Spósito M.B. 1 , Amorim L. 1 , and Gottwald T.R. 2
1 Escola Superior de Agricultura Luiz de Queiroz (ESALQ), Fitopatologia e Nematologia, Brazil; and 2 Agricultural Research Service (ARS),
United States Department of Agriculture, USA. salouren@hotmail.com
Citrus Black Spot (CBS) is an important fungal disease, caused by Guignardia citricarpa. The risk of introduction
and spread of CBS to new unaffected citrus-producing areas is a major concern for those orchards attempting
to remain free of the disease. It has never been demonstrated that G. citricarpa-infected fruit have established
infections in commercial diseased-free areas. The period of production of spores in fruit lesions is an important
variable in a model for risk assessment. The objective of this study was to evaluate the infectious period of
CBS on sweet oranges. Twenty infected oranges treated with carnauba wax with hard spot symptoms of CBS
were kept in a humid chamber until freckle spot symptoms developed. When the first pycnidium formed,
50 µl drop of sterile water was placed on one freckle spot lesion per fruit. After four hours, the water drop
was transferred to 250 µl of sterile water with Tween-20 (1%). Every 48 hours over 40 days, the number
of pycnidia and the number of conidia formed in each lesion were counted. The experiment was repeated
twice. The first pycnidia were observed 2-7 days after the appearance of typical freckle spots. The number
of pycnidia increased until 30-40 days, and ranged from 1 to 31 per lesion. Conidia liberation started at 2-12
days and stopped 16-38 days after pycnidia formation. The number of conidia ranged from 0 to 109,875 per
lesion. The average infectious period was 15 days and ranged from 0 to 26 days.
S15P07
Characterization of Guignardia spp. from citrus fruit in Argentina
Kornowski M.V., and Agostini J.P.
Instituto Nacional Tecnologia Agropecuaria (INTA), Plant Pathology E.E.A. Montecarlo, Argentina. jpagostini@montecarlo.inta.gov.ar
Citrus Black Spot (CBS) is a fruit disease caused by Guignardia citricarpa while G. mangiferae is a nonpathogenic
species that also infects citrus fruit. Four kinds of fruit symptoms are associated with this disease,
viz. hard spot, freckle spot, false melanose, and virulent spot. Both species can be characterized by either
molecular means or growth on culture media. For molecular characterization with PCR, 25 isolates from
XII INTERNATIONAL CITRUS CONGRESS 2012 - 261
S15