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Biology, epidemiology

  • Conservation, sources d'inoculum

Didymella bryoniae can be kept for more than a year on and / or in the soil, on more or less dry undecomposed plant debris, and on seeds. Its chlamydospores and dormant mycelium are resistant to cold and allow it to be maintained from one year to another, or even beyond. This fungus is also rather resistant to drought, which allows it to maintain itself on shelter structures. It should be noted that during its development in the fruits, accompanied by black rot, a not insignificant proportion of the seeds is contaminated externally and internally by the mycelium of D. bryoniae . Subsequently, they also ensure its conservation and dissemination.

Thus, depending on the cultures, the sources of inoculum may vary:
- seeds are frequently incriminated, rightly for extensive productions where we have the annoying habit of recovering seeds from spoiled fruits, or when they are produced in heavily affected areas. It should be noted that is found D. bryoniae more or less deep in the seeds of cucurbits, and that it can be stored there for several months. In this case, seedlings that are already contaminated can be produced in the nursery;
- senescent tissues present in the environment of plants, on which the pycnidia and perithecia present will respectively produce contaminating conidia and ascospores.
  • Penetration and invasion

This fungus takes advantage of all the opportunities offered by cucurbits (their environment and anthropogenic interventions) to contaminate them. Thus, after germination of the conidia or of the ascospores, the penetration of the different tissues can take place directly through the cuticle or the epidermis, or by the intercellular spaces around the base of the trichomes. It can also occur through pruning wounds related to fruit harvesting, or leaf stripping, but also various other injuries. In fruits, it can take place via various injuries natural or mechanical , via floral scars. For example, in the cucumber D. bryoniae invades the flowers through the stigmas and the style . Its initial mycelial growth is intercellular, subsequently it is able to penetrate the cell wall, and its development becomes intracellular, leading to tissue death.
U do once in place in the organs that its more or less receptive according to their age and maturity, the mycelium of the fungus invades tissue emits various enzymes (pectin lyase ...), and symptoms appear within a few days, between 3 and more than ten days depending on the cucurbits and the surrounding conditions.

  • Sporulation and dissemination
D. bryoniae quite rapidly forms perithecia (Figure 6) and pycnidia (Figure 8)  on the surface of damaged tissue (Figures 1 to 5). These globose, brown to black structures at maturity produce ascospores and conidia, respectively, in large quantities, especially during humid periods. These spores will essentially constitute the primary and secondary inocula encountered in cultures.

The ascospores (figure 7) of 14 to 18 µm by 4 to 7 µm, s are hyaline, bicellular, ellipsoid, with generally rounded ends, and slightly constricted at the level of the septum (septum). They are expelled from the perithecia as soon as the air is humid. Their dispersion is maximum after a rain, at temperatures between 18 and 25 ° C and during periods of dew or morning condensations. Under cover, D. bryoniae is a potential danger to cucumber all year round because contaminating ascospores can be produced at any time. Their release usually takes place 3 hours after humidification of the affected organs; light and dark do not seem to affect this one. They are then carried by drafts and wind.
The pycniospores (Figure 9) s have wet spores exiting pycnidia in the form of yellowish masses mucilaginous cirrhi (Figure 8). These conidia of 6 to 10 µm (sometimes up to 13 µm) by 3 to 4 µm (sometimes up to 5 µm), are hyaline, cylindrical in shape with rounded ends, and most of the time bicellular but a small percentage is unicellular. They are dispersed during splashing and water runoff and can be transmitted mechanically to other plants by workers and their tools during cultivation operations. Seeds, plants and some insects (leaf beetles) also help disperse this fungus.
  • Conditions favorable to its development
D. bryoniae is particularly damaging when plants show various injuries or when they are weakened as a result of stress or attacks from other parasitic microorganisms or pests.

Temperature and humidity are sometimes limiting factors in the spread of the fungus. It is capable of developing and fruiting at temperatures between 5 ° C and 35 ° C, with an optimum located around 23 ° C for cucumber. On watermelon, the optimum development is slightly higher, around 24.5 ° C, while on melon it is clearly lower: 19 ° C - 20 ° C. The latter host becomes much less sensitive as temperatures rise. In fact, humidity influences disease development much more than temperature. It is certainly the preponderant factor triggering epidemics. Infections are rare at relative humidity close to 60%. The disease becomes particularly damaging from 95% relative humidity, but especially when there is the presence of free water on the plants. The presence of a film of water for one hour makes it possible to initiate contamination. The development of black rot on stored fruits is severely limited at temperatures below 7 ° C. Note the preponderant role played by the phenomenon of guttation in greenhouse cucumber crops. In fact, the more it takes place, the higher the number of plants affected. The damage caused by powdery mildew also promotes the development of the disease.
Generally, plants grown in dry conditions are less susceptible than those grown in wet conditions.
Last change : 04/30/21
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