Biology, epidemiology

• Conservation, source d'inoculum

 Monosporascus cannonballus can survive for several years in the soil, for example it can survive for more than 3 years in the absence of a host culture. It is preserved thanks to its mycelium which has been able to maintain itself on plant debris or in the soil, but especially through its ascospores which have several walls giving them a rather significant resistance. Ascospores are in many situations the primary inoculum.

Note that perithecia have been observed in the roots of other plants; they could ensure the multiplication and conservation of this fungus.

• Penetration, invasion

The mycelium present in the soil or the germ tube resulting from the germination of ascospores (favored by root exudates and certain microorganisms of the soil microflora) directly penetrate the epidermis. Subsequently, the mycelium grows inside the cells, invades the root cortex and gradually that of the roots. Eventually, it locally gains vascular tissues (the xylem), inducing the phenomenon of tylosis in the parasitized plant.

• Sporulation and dissemination

M. cannonballus forms spherical and black perithecia (measuring between 40 and 50 µm in diameter) in root tissues (Figures 1 to 4 and 7), this late in the season after harvest or plant death. At maturity, these structures contain numerous asci each containing a spherical ascospore (Figures 4 to 6, 8 and 9). Several hundred thousand ascospores could be produced and released from a root system of a diseased melon plant. These ascospores are distributed vertically and horizontally in cultivated soils in a uniform manner. Their number is still higher in the superficial soil stratum.

The potential for dissemination of this disease seems rather limited because M. cannoballus does not have an asexual form and its ascospores are of a rather large size, which does not favor aerial dispersal. Despite this, this fungus can be spread by dust and contaminated soil particles, and soiled tillage implements. Plants produced in contaminated areas or plots should be able to transmit the disease. Farm workers and agricultural machinery spread the disease from one plot to another during their travels.

Only one cycle of this disease would take place per year.

• Conditions favorable to its development

This fungus appreciates high temperatures and preferentially hot or semi-arid climates. Its infections are favored in warm soils. The temperature of the soil in the days following planting seems to have an important influence on the incidence of the disease.

Under culture conditions in vitro , its mycelial growth would be optimum between 25 and 35 ° C.

The severity of this disease would also be influenced by certain biotic or abiotic stresses, in particular water stress. The repeated cultivation of melon on the same plot, the date of planting and therefore the production period, the use of plastic mulch and drip irrigation, as well as the fruit load of the plants would also influence the severity of this telluric mycosis.