Agent pathogène

Several fungi are responsible for powdery mildew in cucurbits, with Podosphaera xanthii et Erysiphe cichoracearum being the most significant pathogens. Historically, E. cichoracearum was regarded as the primary causal agent worldwide until 1958. However, P. xanthii has become the more prevalent species associated with cucurbit powdery mildew globally. E. cichoracearum typically thrives in cooler conditions, such as spring and early summer periods, while P. xanthii proliferates more effectively during the warmer months. Additionally, P. xanthii appears to be more aggressive than E. cichoracearum. Although distinguishing between the conidia of those two species can be challenging, the presence of fibrosin bodies in the conidia of P. xanthii serves as a key criterion in determining both.

Powdery mildew fungi reproduce through both asexual and sexual reproduction.

Asexually, conidia (asexual spores) land on a host plant, where they germinate and produce an appressorium and haustorium to extract nutrients from the plant. Conidiophores emerge from the hyphae and produce new conidia, dispersed by wind, spreading the infection. The incubation period usually lasts for 3~7 days and a significant number of conidia can be produced during a short time.

Sexual reproduction occurs when hyphae of opposite mating types meet and form cleistothecia. Cleistothecia contain several asci, each containing eight ascospores (sexual spores). They also serve as overwintering sources, enabling the pathogen to survive tough conditions. However, cleistothecia of P. xanthii has rarely been observed.

Symptômes

Although all cucurbits are susceptible, symptoms are less common in cucumbers and melons as many commercial cultivars have resistance. White powdery spots of mycelia and conidia are formed on leaf surfaces, petioles, and stems, making them easily recognizable. In the beginning, reddish-brown spots appear, turning white as hyphae and spores are produced later. These fungal colonies coalesce, covering the entire surface. Powdery mildew infection leads to reduced photosynthesis, premature senescence, and even plant death. While fruits are not directly or rarely attacked by the fungus, they may suffer deformation or early ripening. Such fruits exhibit poor storability (winter squash), low soluble solids with poor taste (melon), poor rind color (pumpkin), and shriveled, discolored handles (pumpkins). Disease stress can also cause imperfections on the fruit’s surface, including speckling, raised indentations, and edema.

FieldClimate Modèle

Melon/Cucumber/Zucchini/Pumpkin/Melon Powdery Mildew Risk

Infection can occur within a temperature range of 0°C to 32°C, with an optimal mean temperature between 20°C~27°C. Disease development is halted when daytime temperatures exceed 28°C. Favorable conditions for the pathogen include dense plant growth and low light intensity. High relative humidity also favors infection and conidial survival although infection can occur even as low as 50%. Rain and free moisture on plant surfaces are unfavorable for pathogen development. Therefore, dry conditions with limited leaf wetness are considered optimal for increasing the risk of powdery mildew outbreaks.

Littérature

• McGrath, M. T. (1997). Powdery mildew of cucurbits. Nuñez-Palenius, H. G., Hopkins, D., & Cantliffe, D. J. (2006). Powdery mildew of cucurbits in Florida. HS-1067.(Viewed 18.12. 2019). Available: ef283dfcacb140070eb63b12bbbb8d9a9452.Pdf.
• PÉREZ‐GARCÍA, A. L. E. J. A. N. D. R. O., Romero, D., FERNÁNDEZ‐ORTUÑO, D. O. L. O. R. E. S., LÓPEZ‐RUIZ, F. R. A. N. C. I. S. C. O., De Vicente, A., & Tores, J. A. (2009). The powdery mildew fungus Podosphaera fusca (synonym Podosphaera xanthii), a constant threat to cucurbits. Molecular plant pathology, 10(2), 153-160.