Studies on Pseudopernospora cubensis Causing Downy Mildew of Cucumber and in-vivo Assay for Management

Introduction

Cucumber (Cucumis sativus L.), belonging to the family Cucurbitaceae, represents one of the oldest and most economically vital vegetable crops globally (Tatlioglu, 1992) [11] . Valued for its high hydration capacity, antioxidant wealth, and medicinal properties, cucumber serves as a fundamental source of vitamins and minerals (Javid et al., 2024) [3] . India is the secondlargest global producer of vegetables, trailing only China (Mallick, 2022) [5] . However, the per capita availability of protective vegetables in India remains around 125-130g per day, falling significantly short of the recommended dietary allowance of 285g (Singh, 1991) [10] . This gap is primarily driven by low productivity per unit area, largely caused by a range of biotic stresses. Among the various biotic constraints, downy mildew caused by the biotrophic oomycete Pseudoperonospora cubensis (Berkeley & Curtis) Rostovzev stands out as a highly destructive foliar threat (Lebeda and Cohen, 2011) [4] . Historically recognized since 1868, this pathogen rapidly spreads via wind-borne sporangia, thriving in warm, highhumidity environments characterized by prolonged leaf wetness (Cohen et al., 2015) [1] . The infection typically appears as chlorotic, yellow angular lesions bounded by leaf veins on the upper surface, with grayish-purple sporulation underneath (Savory et al., 2010) [9] . Advanced infections lead to leaf desiccation, curling, and premature defoliation, which severely impairs photosynthetic capacity, stunts plant growth, and compromises fruit yield and marketability (Thomas, 1996) [12] . In Rajasthan, cucumber cultivation has steadily grown, reaching an estimated 3.686 thousand hectares in 2025. However, expanding cultivation has also increased vulnerability to epidemics of P. cubensis. Because downy mildew severity is highly dependent on regional weather patterns, localized microclimates, and cultural practices, a systematic field assessment is essential. Therefore, this study was undertaken to survey, map, and evaluate the contemporary distribution and severity of cucumber downy mildew across the key production belts of Udaipur, Chittorgarh, and Banswara districts in Rajasthan, establishing a baseline for targeted disease management models.

Material and Methods

Survey Locations and Execution

An extensive roving field survey was conducted during the Kharif season of 2024-2025 across major cucumbergrowing tracts within three southern districts of Rajasthan: Udaipur, Chittorgarh, and Banswara. A total of 44 operational farmer fields spread across 17 distinct tehsils were examined. The sample distribution comprised 16 fields across 6 tehsils in Udaipur, 13 fields across 6 tehsils in Chittorgarh, and 15 fields across 5 tehsils in Banswara.

Symptomatological Identification

Infected plants were monitored in-situ for diagnostic macroscopic symptoms. Leaves were evaluated for the presence of early chlorotic spots, angular boundaries confined by secondary veins, tissue necrosis, and the characteristic velvet-like grey-brown to black downy sporulation on lower leaf surfaces during early morning hours under high relative humidity.

Disease Quantification (Incidence and Severity)

At each survey site, fields were traversed in a zigzag pattern. Five micro-plots were selected at random, and five plants per micro-plot were evaluated. Disease incidence was calculated using the standard formula by Wheeler (1969) [14] :

Disease Incidence (%) = 𝑁𝑜.𝑜𝑓 𝑖𝑛𝑓𝑒𝑐𝑡𝑒𝑑 𝑢𝑛𝑖𝑡𝑠 Total no.of units observed × 100

Disease severity on the foliage was scored using the standard 0 to 5 visual rating scale described by Jamadar and Desai (1997) [2] , categorized as follows:

Data Analysis

The mean PDI values for individual tehsils and overall districts were aggregated and descriptive statistical comparisons were compiled using MS-Excel and basic analytical packages to interpret spatial variation against regional agro-climatic conditions.

Results and Discussion Symptomatological Profile in Surveyed Zones

Field inspections across the surveyed zones confirmed that the disease expression was highly characteristic of Pseudoperonospora cubensis. Early symptoms appeared as pale green to bright yellow chlorotic spots on the adaxial surface of older leaves. Due to internal structural boundaries, these lesions became sharply defined, angular, and restricted by the leaf veins. During humid mornings, a prominent dark grey-brown downy fungal growth (sporangiophores and sporangia) was consistently observed on corresponding abaxial surfaces. In advanced stages of infection, adjacent lesions coalesced, causing leaves to curl, dry out, and defoliate prematurely. This severe defoliation led to sun-scalded, smaller fruits and stunted vines. This progression matches the diagnostic patterns established by Savory et al. (2010) [9] .

Spatial Distribution and Severity of Downy Mildew

The extensive survey revealed that downy mildew is widespread across all three surveyed districts, though its intensity varies considerably by location (Table 1). The field-level PDI ranged from a minimum of 12.32% in Jhadol (Udaipur) to a maximum of 48.21% in Bagidora (Banswara). At the district level, the highest average disease pressure was recorded in Banswara (mean PDI of 30.86%), followed by Udaipur (mean PDI of 26.22%). The lowest overall disease index was observed in Chittorgarh (mean PDI of 25.20%). [13] . Variations in disease index across different tehsils also point to the influence of agronomic practices (Narayanasamy, 2013) [7] . , confirming that P. cubensis infestations are closely tied to regional weather variations and field management practices.

Conclusion

This baseline survey confirms that downy mildew, caused by Pseudoperonospora cubensis, is widely distributed and poses a significant threat to cucumber crops across Southern Rajasthan. Banswara district, along with specific humid tracts in Kotra and Mavli tehsils of Udaipur, are identified as major disease hot-spots due to favorable weather conditions and high crop density. The significant variation in Percent Disease Index (PDI) across the 44 surveyed fields highlights the urgent need for regional forecasting models.

To protect cucumber yields, future management strategies should focus on these high-risk zones, combining diseaseresistant cultivars with optimized plant spacing, improved field sanitation, and timely, weather-based fungicide applications.

References

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