Podosphaera xanthii (Castagne) U. Braun & Shishkoff

Golovinomyces cichoracearum var . cichoracearum (DC.) VP Heluta

Powdery mildews of Cucurbitaceae

General

• Cosmopolitan leaf fungi observed in all Cucurbitaceae production areas in the world. Podosphaera xanthii seems more present in the overseas departments.
• Obligate parasitic fungi very specific to the aerial organs of Cucurbitaceae.
• Observed in the open field as well as under shelter.

• Susceptible botanical family(s)

• Production areas affected :

• Organs attacked

Symptoms and signs

• Symptoms :
  • Powdery to downy, circular, white spots appearing on or under the leaves, often older and shaded (Figures 1-16.
  • The blade may be completely covered by powdery mildew giving the impression of being covered with talc (figures 17 and 18).
  • The leaves eventually turn yellow, become more or less necrotic before drying out and shrinking.
  • The plants age prematurely and when the attacks are early and severe, they have a more limited growth.
  • Comparable spots can be observed on the stem (figure 19) and more rarely on the fruits figure 20), this varying according to the Cucurbitaceae.

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• Signs : whitish powdery felt (figure 21) consisting of a mycelial network surmounted by numerous conidiophores producing hyaline conidia chains of several spores (asexual form) (figures 22 and 23). Tiny globular structures (cleistothecia), brown to black and often unnoticed, are visible on the powdery spots (sexual reproduction) (figures 24 and 25).
• Possible confusion :

Biology

• Conservation : forms of survival still poorly known: probably via cultivated alternative hosts or weeds; their globose cleistothecia do not seem to play a major role in the parasitic cycles of these two fungi.
• Infection : the conidia, perhaps the ascospores, germinate in a few hours and directly penetrate the epidermal cells and emit specialized suckers (haustoria).
• Sporulation : in the days following infection and production of a very large number of conidia.
• Dissemination : conidia dispersed by the wind over long distances, air currents in shelters, incidentally by splashing water, workers' clothing, certain insects such as thrips.
• Favorable conditions : the presence of water on the leaves is not necessary for their development. Excessive hygrometry would lead to a reduction in the severity of powdery mildew. Temperatures have little influence on this disease. Their thermal optima are between 15 and 26°C. Excessive nitrogen fertilizers and low light would sensitize leaf tissue.

Protection

• Use resistant varieties .
• Carry out crop rotations of two to three years, and a sanitary vacuum of two to three weeks under shelter.
• Wash and/or disinfect the walls of the shelters.
• Manage fertilization so as not to obtain plants with excessive growth (reducing the aeration and luminosity of the plant cover) and leaves with excessively succulent tissues.
• Use healthy plants.
• Carefully choose the location of the future plot so that it is located in a fairly airy and sunny place. Avoid proximity to plots already affected by powdery mildew.
• Eliminate weeds from the plot and its surroundings, these can serve as relay plants for the two parasitic fungi.
• Avoid too high planting densities in order to promote ventilation and sunshine of the foliage.
• Eliminate the very first leaves attacked by putting them in a plastic bag and destroying them outside the plot.
• Eliminate plant residues fairly quickly, during cultivation following the various cultivation operations, and at the end of cultivation after uprooting the plants. They will have to be destroyed or buried deep.
• Spray biopesticides or fungicides taking into account authorized uses.