Occurrence and Characterization of Fusarium Wilt of Salad Cucumber in Southern Kerala, India

Abstract

The present study investigated the occurrence, symptomatology, pathogenicity and characterization of the fungal pathogen associated with wilt of salad cucumber in selected agro-ecological units (AEU 8 and AEU 9) of Kerala. Survey conducted in Thiruvananthapuram, Kollam and Pathanamthitta districts revealed that the disease was most prevalent during flowering and fruit bearing stages. Typical symptoms included yellowing and wilting of foliage, starting from the lower leaves and progressing upwards, ultimately leading to the complete drying of the vines. The location Anchal recorded the highest disease incidence and severity of 81.00% and 55.67% respectively. The pathogen isolated from infected tissues fulfilled Koch’s postulates. Among the isolates, the isolate I3 from Kakkamoola was the most virulent producing symptoms within seven days of artificial inoculation. Cultural and morphological characterization revealed considerable variability among isolates. Mycelial growth among the isolates varied from cottony and fluffy to floccose, fibrous or sparse, with colony colours ranging from white to pale brown, lilac and pinkish red. Most isolates exhibited circular, radial growth with concentric or zonate rings, while a few showed uniform spreading or feathery margins. Pigmentation of the medium differed markedly, producing light salmon to reddish-pink or yellowish-brown hues, whereas some isolates showed no pigmentation. All the isolates produced macroconidia, microconidia, and chlamydospores. Macroconidia were fusiform to falcate shaped, with 2–4 septa, and microconidia were mostly oval shaped. Chlamydospores were globose and thick walled, with some having rough walls, produced terminally or intercalary in the mycelium, either singly or in pairs. Molecular identification of the most virulent isolate using ITS primers confirmed the identity of the pathogen as Fusarium incarnatum with 99.8% homology.