• Conservation, source d'inoculum

The forms of conservation of Podosphaera xanthi and Golovinomyces cichoracearum are still rather poorly understood. Some authors have incriminated their cleistothecia (poorly developed perithecia without ostiole ensuring their sexual reproduction and at the origin of their genetic variability) (Figures 1 to 3), but their ability to perpetuate the disease from one year to the next. 'has not yet been clearly demonstrated. In addition, let us not forget that these structures are only very rarely observed in the field, which tends to invalidate this hypothesis.

Their conservation in their conidial form ( Oidium ) (figures 4 to 7) and on alternative hosts seems the most credible option. It could be carried out either on late and early Cucurbitaceae cultures providing the relay, or on weeds that can harbor one or the other of the 2 fungi, and thus ensure their sustainability. For example, G. cichoracearum is very polyphagous. It is capable of attacking a very large number of hosts, more than 200 species belonging to 25 different families. In fact, this species would contain more or less specialized forms infecting ranges of different plants.

In greenhouses, conidia can also survive for short periods on the surface of various supports (walls of greenhouses, gutters, etc.) and infect new crops, especially cucumber, set up too quickly after the previous ones. Note that the conidia only survive a few hours.

• Penetration, invasion

Whatever the mode of preservation of these obligatory ectoparasitic fungi, ascospores or conidia are the source of primary contaminations. These, once in contact with their host, germinate quickly in two hours if the surrounding conditions are favorable (18 to 25 ° C, 95 to 98% relative humidity in the case of G. cichoracearum ). They can germinate at a relative humidity of 50% or less, but the incidence of infection increases with humidity. Note that the germination of conidia P. xanthi would require the presence of dew. Once germinated, the spores form an appressorium which ensures their fixation. Mycelial hyphae subsequently develop which locally and directly penetrate the epidermal cells, generating specialized structures: haustoria. The latter play the role of suckers allowing the removal of the elements necessary for the growth of the mycelium of these fungi. Subsequently, a more or less dense mycelial network extends and covers more or less the limbus.

• Sporulation and dissemination

Within 4-7 days of infection with these fungi, short conidiophores develop on secondary hyphae on the leaf surface and produce chain conidia. The sporulation can be more or less abundant depending on the host and the climatic conditions.

These conidia are very light and are therefore easily transported and dispersed by the wind over long distances (up to 200 km according to some authors) and, incidentally, by rain or sprinkler irrigation, during splashing. The disseminations are maximum at night, between midnight and 4 am. Certain insects would contribute to the local dispersion of conidia, such as thrips for example.

If conditions are suitable for sexual reproduction, which seems a rare event, cleistothecia develop within the mycelium over several weeks. At maturity and in the presence of water, ascospores are ejected and disseminated by air currents.

• Conditions favorable to their development *

Unlike many parasitic fungi of cucurbits, powdery mildew do not need the presence of a film of water on the leaves to develop. In addition, in contact with water, the conidia are more or less altered, which may explain the stagnation of epidemics during rainy periods.

Temperature is not a limiting factor in their development, which takes place between 10 and 35 ° C, the optimum being around 23-26 ° C. Their development cycle is relatively short: between contamination by conidia and the appearance of powdery mildew spots, it can take about 5 to 7 days. The distribution of the two powdery mildew species over the year, by region and crop type, indicates that they likely have slightly different climatic requirements. G. cichoracearum would have an optimum development between 15 and 26 ° C without necessarily requiring very high humidity, that of P. xanthi would be between 15 and 21 ° C in the presence of humidity. These trends are sometimes to be put into perspective depending on the production areas, the production methods used.

In addition, note that powdery mildew often appears more serious on plants and leaves located more in the shade or inside the plant cover, in particular if their growth is vigorous and under the influence of excessive nitrogen fertilization. Young plants seem less susceptible to this disease, as well as senescent tissues.
Direct light and high temperatures above 38 ° C limit the development of powdery mildew.


* Warning: The results of studies concerning the biology of these fungi, and in particular the conditions influencing their development, are probably tainted with errors. Indeed, it has not always been easy to differentiate these two fungi and therefore some biological data has probably not always been attributed to the right fungal species.