The Effects of Postharvest Treatments and Sunlight Exposure on the Reproductive Capability and Viability of Phyllosticta citricarpa in Citrus Black Spot Fruit Lesions

Citrus black spot (CBS) is caused by Phyllosticta citricarpa, which is classified as a quarantine organism in certain countries whose concerns are that CBS-infected fruit may be a pathway for introduction of the pathogen. This study evaluated the reproductive capability and viability of P. citricarpa under simulated conditions in which the whole fruit, peel segments, or citrus pulp with CBS lesions were discarded. Naturally infected ‘Midknight’ Valencia orange and ‘Eureka’ lemon fruit, either treated using standard postharvest sanitation, fungicide, and wax coating treatments or untreated, were placed into cold storage for 5 weeks (oranges at 4 °C and lemons at 7 °C). Thereafter, treated and untreated fruit were incubated for a further 2 weeks at conditions conducive for CBS symptom expression and formation of pycnidia. The ability of pycnidia to secrete viable pycnidiospores after whole fruit and peel segments or peel pieces from citrus pulp were exposed to sunlight at warm temperatures (±28 °C) and ±75% relative humidity levels was then investigated. The combination of postharvest treatments and cold storage effectively controlled CBS latent infections (>83.6% control) and pycnidium formation (<1.4% of lesions formed pycnidia), and the wax coating completely inhibited pycnidiospore release in fruit and peel segments. Pycnidiospores were secreted only from lesions on untreated fruit and peel segments and at low levels (4.3–8.6%) from peel pieces from pulped treated fruit. However, spore release rapidly declined when exposed to sunlight conditions (1.4% and 0% after 2 and 3 days, respectively). The generally poor reproductive ability and viability of CBS fruit lesions on harvested fruit, particularly when exposed to sunlight conditions, supports the conclusion that citrus fruit without leaves is not an epidemiologically significant pathway for the entry, establishment, and spread of P. citricarpa.

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The Effects of Nutrition and Environmental Factors on Conidial Germination and Appressorium Formation of Phyllosticta citricarpa, the Causal Agent of Citrus Black Spot

Phytopathology ◽

10.1094/phyto-10-18-0378-r ◽

2019 ◽

Vol 109 (4) ◽

pp. 650-658 ◽

Cited By ~ 1

Author(s):

Nan-Yi Wang ◽

Megan M. Dewdney

Keyword(s):

Black Spot ◽

Nitrogen Sources ◽

Conidial Germination ◽

Appressorium Formation ◽

Potato Dextrose Broth ◽

Citrus Black Spot ◽

Fungal Biology ◽

Biological Studies ◽

Phyllosticta Citricarpa

Citrus black spot, caused by Phyllosticta citricarpa, has been identified in Florida since 2010 and can reduce fruit yield and marketability. The conditions required for conidial germination have been poorly understood for P. citricarpa, limiting further biological studies. In this study, the effects of citrus juices, concentration, pH, various carbon and nitrogen sources, and environmental conditions were evaluated in vitro. All tested juices, especially ‘Valencia’ (>85%, P < 0.05), favored germination and appressorium formation, whereas sterile water rarely stimulated germination (<1%). The ‘Valencia’ juice effect was concentration and pH dependent, and the maximum rate was reached in 1.5% juice with pH of 3.4. Most carbon, nitrogen, or complex sources did not favor germination or appressorium formation, with the exception of potato dextrose broth. An incubation period of 18 to 24 h at 24°C was required for peak germination and appressorium formation. The further analysis of critical juice components using synthetic citrus juice revealed that sugars, salts, citric acid, and thiamine were most important for germination and appressorium formation (>80%, P > 0.05). These results provide a better understanding of fungal biology of P. citricarpa and a robust and convenient system for further applications such as screening for efficacious fungicides.

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Pathogenicity of Phyllosticta citricarpa Ascospores on Citrus spp.

Plant Disease ◽

10.1094/pdis-08-17-1331-re ◽

2018 ◽

Vol 102 (7) ◽

pp. 1386-1393 ◽

Cited By ~ 5

Author(s):

Nga T. Tran ◽

Andrew K. Miles ◽

Malcolm W. Smith ◽

Ralf G. Dietzgen ◽

André Drenth

Keyword(s):

Potential Model ◽

Black Spot ◽

Future Studies ◽

Citrus Black Spot ◽

Phyllosticta Citricarpa ◽

Sexual State ◽

Epidemic Development ◽

The Impact ◽

Troyer Citrange

Citrus black spot, caused by Phyllosticta citricarpa, is one of the most important fungal diseases in many citrus-growing regions with hot and humid summers. Ascospores and conidia are known to contribute to epidemic development of the disease. However, pathogenicity testing has never been done for pure ascospores produced from fully characterized P. citricarpa isolates, due to the inability to induce the sexual state in vitro. Recently, an in vitro mating technique was developed to readily produce pure P. citricarpa ascospores for use in host inoculation studies. To test the pathogenicity of P. citricarpa ascospores, we inoculated Troyer citrange leaves and Murcott tangor fruit with ascospores produced in vitro from characterized P. citricarpa isolates. Typical symptoms of citrus black spot occurred. Recovery of P. citricarpa isolates from symptomatic lesions and their characterization using genetic markers enabled us to identify recombinant genotypes among the isolates recovered from ascospore inoculations and, as such, fulfill Koch’s postulates for ascospores. We have also identified Troyer citrange seedlings as a potential model system for citrus black spot inoculation studies, because it allows typical symptoms of citrus black spot to be expressed with a much shorter latent period than on fruit. This will facilitate future studies of epidemiological aspects of P. citricarpa ascospores relative to conidia and improve our understanding of the citrus black spot pathosystem. The susceptibility of Troyer citrange seedlings will also facilitate experimenting with disease management methods, aimed at reducing the impact of citrus black spot.

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Biology and epidemiology of citrus black spot (Phyllosticta citricarpa) in Australia

10.14264/uql.2018.756 ◽

2018 ◽

Author(s):

Nga Tran

Keyword(s):

Black Spot ◽

Citrus Black Spot ◽

Phyllosticta Citricarpa

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Inoculum Dynamics and Infection of Citrus Fruit by Phyllosticta citricarpa

Phytopathology ◽

10.1094/phyto-02-20-0047-r ◽

2020 ◽

Vol 110 (10) ◽

pp. 1680-1692

Author(s):

Nga T. Tran ◽

Andrew K. Miles ◽

Ralf G. Dietzgen ◽

Timothy A. Shuey ◽

Stephen R. Mudge ◽

...

Keyword(s):

Leaf Litter ◽

Disease Incidence ◽

Citrus Fruit ◽

Black Spot ◽

Summer Rainfall ◽

Tree Canopy ◽

Economic Losses ◽

Citrus Black Spot ◽

Spore Trapping ◽

Phyllosticta Citricarpa

Citrus black spot, caused by Phyllosticta citricarpa, is characterized by fruit blemishes and premature fruit drop, resulting in significant economic losses in summer rainfall areas. The pathogen forms both conidia and ascospores during its life cycle. However, the occurrence of these spores and their contributions to infection of fruit in field conditions are not well understood. Our research using direct leaf litter monitoring and volumetric spore trapping in Queensland orchards revealed that pseudothecia and ascospores in leaf litter as well as trapped ascospores had low abundance, while pycnidia and conidia were highly abundant. Both P. citricarpa and endophytic Phyllosticta spp. were identified, with P. citricarpa being dominant. In replicated field trials, we determined that infection of Imperial mandarin fruit by P. citricarpa occurred from fruit set until week 20 of fruit development, with the key infection events taking place between weeks 4 and 16 in Queensland subtropical conditions. These results demonstrate that protecting fruit during weeks 4 to 16 significantly reduced P. citricarpa infection. We found no significant correlation between the disease incidence in fruit and P. citricarpa conidial abundance in leaf litter or ascospore abundance measured by volumetric spore trapping. Therefore, it is suggested that inoculum sources in the tree canopy other than those detected by spore trapping and direct leaf litter monitoring may play a major role in the epidemiology of citrus black spot. Improved knowledge regarding epidemiology of P. citricarpa and an understanding of propagules causing infection may aid in development of more effective disease management strategies.

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COPPER OXYCHLORIDE APPLIED ALONE TO CONTROL CITRUS BLACK SPOT

Revista Caatinga ◽

10.1590/1983-21252019v32n306rc ◽

2019 ◽

Vol 32 (3) ◽

pp. 616-624

Author(s):

ANTONIO EDUARDO FONSECA ◽

ANTONIO DE GOES ◽

FERNANDA DIAS PEREIRA

Keyword(s):

Control Measure ◽

Block Design ◽

Black Spot ◽

Additional Treatment ◽

Copper Oxychloride ◽

Concentrated Suspension ◽

Citrus Black Spot ◽

Progress Curve ◽

Randomized Block Design ◽

Phyllosticta Citricarpa

ABSTRACT Citrus black spot (CBS) is a disease caused by the Phyllosticta citricarpa fungus that causes lesions in fruits and, in more severe stages, fruit drops. The use of systemic fungicides is the main control measure for CBS; however, an alternative control measure is the use of cupric fungicides applied alone with short intervals. Therefore, the objective of the present work was to evaluate the effect of applications of copper oxychloride at different rates on the control of CBS. The experiment was conducted in Bebedouro, SP, Brazil, in the 2014/2015 crop season, in a randomized block design, using a Citrus sinensis variety (Valencia). The treatments consisted of copper rates (zero - control; 31, 24.5, 18.5, 12.2, and 9.1 mg of metal copper per cubic meter of canopy) using copper oxychloride (concentrated suspension - CS; 588 g L-1 of copper oxychloride or 350 g L-1 of metal copper), and an additional treatment with application of a cupric fungicide (cuprous oxide) and a strobilurin fungicide. The applications were carried out with 14-day intervals, starting when 2/3 of the petals of the plant's flowers were fallen, totaling 14 applications. The incidence and severity of CBS were evaluated five times with 30-day intervals up to mid-November, when the fruits were harvested. The data were used to calculate the area under the disease progress curve (AUDPC). Applications of copper oxychloride CS at rates of 31 and 24.5 mg m-3 with 14-day intervals are efficient for the control of CBS, with similar efficiency to the farm standard treatment.

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