Biology, epidemiology

• Conservation, sources d'inoculum 

Fusarium oxysporum f. sp. melonis is a soil fungus capable of sustaining itself in the soil for many years. It can be preserved there in the absence of melon culture, on the most diverse plant debris, thanks to its chlamydospores (figure 3), conservation spores with thick and resistant walls which form on the mycelium or macroconidia. It seems endowed with aptitudes for saprophytic life which allow it to colonize and survive from various organic compounds; it would be able to colonize most of the crop residues entering in rotation with melon. It seems to keep on seeds: on the surface, more rarely inside melon seeds.

• Penetration, invasion

This fungus penetrates the melon via its root system, at the level of various wounds (insects ), but especially from the zones of emission of the new roots which constitute doors of entry favorable to the contaminations. Subsequently, it gains the xylem and can thus invade the vascular system of the melon, both in its aerial part and underground.

• Sporulation, dissemination 

Under humid conditions, F. oxysporum f. sp. melonis fruit abundantly on lesions formed on stems and fruits (Figures 1 and 2). These are covered with countless fruiting bodies and spores (Figure 3). This potential inoculum represents major sources of contamination at the origin of the very easy dissemination of this fungus. It is dispersed via wind, splash and runoff of water, equipment and tools. Note that during tillage, the inoculum is redistributed in the plot and disseminated by the tillage equipment.

The seeds can be contaminated and ensure the transmission and conservation of the disease.

• Conditions favorable to its development

Fusarium wilt is particularly rife in cold springs. It is also very favored in production areas where soil temperatures are around at certain times of the year 18 to 25 ° C. On the other hand, the expression of leaf symptoms is maximum when temperatures rise and the relative humidity of the air decreases. The incidence of wilts would decrease significantly above 30 ° C, especially when races 1 and 1-2 occur.

The mineral nutrition of plants influences their susceptibility to Fusarium wilt. These are more receptive when the nitrogen supply is important (especially in the presence of NH4 form); on the contrary, during higher intakes of potassium and calcium, fewer affected plants would be observed. Light, sandy and somewhat acidic soils (pH 5 - 5.5) appear to favor the disease.

The severity of Fusarium wilt increases during stress heat and when plants are heavily fruited or bear too large fruit. Light intensity would also play a role: low light and short photoperiods increase it.

The influence of other soil-borne pests (root-knot nematodes, other fungi, etc.) on Fusarium wilt is not yet fully established.