Diaporthe angelicae: the agent that causes umbel browning in carrot has been revealed

Since 2007, umbel browning has appeared in France on carrot seed crops, thus impacting their seed yield.  FNAMS (French Seed Growers Association), GEVES, the companies HM-Clause and Vilmorin and INRA scientists have been working in partnership to identify and characterise the causal agent of this disease: the fungus Diaporthe angelicae.

Diaporthe angelicae:the agent that causes umbel browning in carrot has been revealed

Carrot seed producers the victims of fungal attacks

Carrot is a species that is very widely grown for its seeds in France: 25% of fine vegetable seed producing land is dedicated to this crop. In 2014, 2450 hectares of carrot seed crops (Daucus carota) were cultivated in France. Symptoms of umbel browning have been observed since 2007, particularly in the Centre–Val-de-Loire region, which is the leading area for carrot seed production. The lesions appear first of all on the umbels and can spread to the stem. Diseased umbels dry off prematurely, thus compromising seed production. This rapidly-spreading fungal disease had not previously been clearly characterised.  

Identification and characterisation of the Diaporthe sp. fungus

To determine the fungal species responsible for umbel browning and stem necrosis on carrot seed crops, the scientists worked on isolates collected between 2010 and 2014 from several French regions: 99 isolates were taken from carrot and parsley plants or from crop debris. Based on phenotypic criteria and molecular analyses, the study enabled the characterisation of two main fungal species: Diaporthe angelicae and Diaporthe eres were identified in 83% and 17% of cases, respectively.

Controlling the fungus through knowledge of its life cycle

The pathogenic potential of Diaporthe sp. on carrot umbels was measured under controlled conditions. The artificial contamination of carrot umbels in a climate-controlled chamber showed that the plant was more susceptible to infection by D. angelicae during the S2 developmental stage (fall of stamens and complete opening of the flower). One of the portals of entry for the fungus is the flower, and more specifically the stigma. Infection of a damaged stem is also possible, but this is not the case for foliage. To ensure its growth, the fungus may develop pycnidia (asexual form) and perithecia (sexual form), both forms containing spores and being found on carrot crop debris. Occurrence of the sexual phase in the life cycle of D. angelicae in open-field carrot crops partly explains the polymorphism that is more pronounced in this species than in D. eres.

Protection of carrot seed crops by managing the use of fungicides and removing sources of the inoculum

The susceptibility of the fungus to several fungicides with different modes of action was also tested in vitro and during field experiments. Fungicide efficiency was determined from the limitation of mycelial growth or from umbel symptoms. Some fungicide inefficiency could be observed on a few isolates, suggesting that a resistance phenomenon might develop in the field in the future if these fungicides were used intensively. Different sources of inoculum need to be monitored or even eliminated: crop debris, contamination by wild species of Apiaceae near carrot crops, insect vectors. However preliminary results have shown that seed is not the principal source of inoculum.  Molecular studies have demonstrated that D. angelicae was generally present in seeds before they were cleaned, but it could be removed during seed cleaning provided that the level of contamination is low. Moreover, as D. angelicae was not detected in the hypocotyle of plantlets, this fungus may not be transmitted via seed.

Experiments have been ongoing in 2017 to refine the pathways via which the fungus penetrates into different parts of the flower. INRA is transferring the molecular tools used to characterise the fungi to its research partners. In addition, the ASPHODEL model (developed for sunflower) has produced results consistent with field observations in predicting spore projections and plant contamination. This decision-support tool will be of value to carrot seed growers in controlling the spread of this disease.

Contact(s)
Scientific contact(s):

  • Pascal Poupard Université d’Angers-AgroCampus Ouest- Research Institute for Horticulture and Seeds (Joint Research Unit: UMR 1345 IRHS), INRA et Université d’Angers, 42 rue Georges Morel 49071 BEAUCOUZE CEDEX 01
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Plant Biology and Breeding
Associated Centre(s):
Angers-Nantes

Sources

  • F. Bastide, I. Serandat, J. Gombert, E. Laurent, E. Morel, J. Kolopp, P. L. Guillermin, B. Hamon, P. Simoneau, R. Berruyer and P. Poupard, « Characterization of fungal pathogens (Diaporthe angelicae and D. eres) responsible for umbel browning and stem necrosis on carrot in France », Plant Pathology (2017), Vol. 66, p. 239-253.

THE ASPHODEL model

The ASPHODEL model was developed in 1992 by the Midi-Pyrénées Regional Plant Protection Service in order to control phomopsis stem canker, Diaporthe heliantii, in sunflower.

The design of this model was based on studies and observations of the pathogenic agent, such as the mycelial growth rate, ascospore survival on leaves and perithecial maturation, all as a function of climatic conditions. It comprises three modules (fungus, plant and fungicide) which interact to accurately determine the periods of crop contamination.

The fungus module can simulate ascus maturation within the perithecia and then the projection of ascospores which, depending on prevailing climatic conditions, will result in contamination. At least 50% of mature asci are necessary to trigger ascospore projection. The rate of ascus maturation is directly dependent on temperature and hygrometry. Once the threshold of 50% mature asci has been reached, it is rainfall that will be the determinant factor which governs ascospore projection. The perithecia become full of water and the pressure rises in these organs, thus causing the expulsion of ascospores. The recharging of asci is calculated using the ascus maturation compartment. Contamination by asci is evaluated using the same parameters involved in ascus maturation, in the knowledge that a minimum of 36 hours is necessary with hygrometry higher than 90% in order to detect any contamination (Delos and Moinard, 1997).

Sources: “Les grillures d’ombelles en production de semences de carottes – Mieux connaître le phomopsis pour mieux contrôler”, Julie Gombert, Bulletin Semences – n°251, September-October 2016, pp. 37-40.