Didymella bryoniae 

Gummy canker

General

• Champignon is very widely distributed on all continents and in many countries, particularly in tropical and subtropical production areas, but also in more temperate regions.
• Mainly affects cucurbits (watermelon, melon, cucumber, winter squash and pumpkins, but also courgette, cristophine, calabash, bitter gourd or melon, Luffa aegyptiaca , etc.).
• Attacks all their aerial organs: leaves, stems and fruits. Feared in many countries for its gum cankers on the stem, and its rots on the fruits.
• Strains resistant to various old and newer fungicides are known in D. bryoniae .
• Observed in the open field, as under shelter.

• Susceptible botanical family(s)

* The gummy canker of Cucurbitaceae, would not be caused by a single fungus as was long believed, but by at least three: Staganosporopsis cucurbitacearum (syn. Didymella bryoniae ), Staganosporopsis citrulli , and Staganosporopsis caricae .
These 3 species reveal a common host range in Cucurbitaceae ( S. caricae also attacks papaya), but their global distributions seem to be different. It would be appropriate to identify more precisely the nature of the species or species rife in the DRO-COMs.  

Biology

• Conservation : remains on and/or in the ground on more or less dry undecomposed plant debris, and on seeds. Its dormant chlamydospores and mycelium (figure 1) resist the cold well and allow it to survive from one year to the next, or even beyond. Rather resistant to drought, it is maintained on the structures of the shelters.
• Infection : germination of conidia or ascospores on healthy tissues, then penetration of the various tissues by the mycelium this directly through the cuticle or the epidermis, through the intercellular spaces around the base of the trichomes, via wounds of pruning, floral scars, or various other injuries. D. bryoniae invades tissues, produces various enzymes; its symptoms appear after a few days, 3 and more than ten days depending on the Cucurbitaceae and the surrounding conditions.
• Sporulation : quite rapidly forms perithecia (figures 6 and 7) and pycnidia (figures 8 to 12) on the surface of injured tissue. These brown to black globose structures at maturity (Figures 2 to 5) produce ascospores and conidia in large quantities, respectively.
• Favorable conditions : Particularly damaging when plants show various injuries or when they are weakened as a result of stress or attacks by other parasitic microorganisms or predators. Able to develop and bear fruit at temperatures between 5°C and 35°C, its optimum is around 23°C. Humidity is preponderant for its development, such as rain and sprinkler irrigation. Grafting seems to favor the attacks of D. bryoniae in the greenhouse, in particular at the level of the grafting zone.

Protection

• There are differences in sensitivity between Cucurbitaceae. Some rootstocks resistant to this fungus would limit its development.
• Use healthy seeds and seedlings .
• Disinfect the seeds if necessary.
• Establish crop rotations not involving susceptible crops, at least for 2 or 3 years.
• Orient the cultivated plots in the direction of the prevailing wind so that the vegetation is well ventilated.
• Ensure good drainage of cultivated plots.
• Avoid too high planting densities in order to favor the aeration of the foliage.
• Avoid irrigation , prefer drip irrigation. If they are essential, carry them out in the morning so that the vegetation drains quickly during the day.
• Under cover, ventilate as much as possible.
• Do not allow workers to work while vegetation is wet.
• Harvest fruit carefully to avoid injury.
• Get out of the crop and destroy the affected plants and especially the diseased fruits. At the end of cultivation, eliminate all plant residues. Deep plowing can bury the remaining debris, this measure must be combined with a crop rotation .
• If necessary, spray fungicides taking into account authorized uses. Alternate the chemical families so as not to select resistant strains.